The Eagle Owl 978-1472900661, 1472900669

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THE EAGLE OWL

Dedication: To our best friends Fabio and Francesco, who shared with us the beginnings of this long story, and to Stefano and Giulia.

THE EAGLE OWL

VINCENZO PENTERIANI & MARÍA DEL MAR DELGADO

T & AD POYSER Bloomsbury Publishing Plc 50 Bedford Square, London, WC1B 3DP, UK BLOOMSBURY, T & AD POYSER and the T & AD Poyser logo are trademarks of Bloomsbury Publishing Plc First published in the United Kingdom 2019 This electronic edition published in 2019 by Bloomsbury Publishing Plc Copyright © Vincenzo Penteriani and María del Mar Delgado, 2019 Illustrations © Tom Björklund and Giulia Bombieri, 2019 Photographs on pp.189–193, 281, 311, 312, 322 and 327 © Vincenzo Penteriani, 2019 Figures on pp. 20, 24, 44, 45, 48, 49, 67, 96, 126, 130, 206, 209, 210, 211, 213, 214, 215, 216, 218, 228, 229, 230, 243, 268, 286, 287, 288, 290, 308 and 325 © Julian Baker (JB Illustrations), 2019 Vincenzo Penteriani and María del Mar Delgado have asserted their right under the Copyright, Designs and Patents Act, 1988, to be identified as Authors of this work For legal purposes the Acknowledgements on pp. 334–335 constitute an extension of this copyright page All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage or retrieval system, without prior permission in writing from the publishers Bloomsbury Publishing Plc does not have any control over, or responsibility for, any third-party websites referred to or in this book. All internet addresses given in this book were correct at the time of going to press. The authors and publisher regret any inconvenience caused if addresses have changed or sites have ceased to exist, but can accept no responsibility for any such changes A catalogue record for this book is available from the British Library ISBN: HB: 978-1-4729-0066-1; ePDF: 978-1-4729-4245-6; ePub: 978-1-4729-0067-8 To find out more about our authors and books visit www.bloomsbury.com and sign up for our newsletters

Contents Preface

6

1. The Eagle Owl

8

2. Distribution and population estimates

34

3. The nesting site

97

4. Nest spacing and breeding density

115

5. Feeding ecology

122

6. Interspecific interactions

154

7. The Eagle Owl calendar

173

8. Breeding performances

203

9. Home range behaviour

223

10. Natal and breeding dispersal

241

11. Mortality and threats

265

12. Vocal communication

280

13. Visual communication

310

Plate Section Acknowledgements

334

Appendix 1. List of prey species found in the diet of the Eagle Owl

336

Appendix 2. List of abbreviations

339

References

340

Index

378

Preface Today is a great day. It is the day when I am sitting in front of my laptop writing the final words of this book. I have waited for this moment for more than four years, the time it took to prepare and write this book…or more accurately, I have waited more than 30 years, since I first started to be interested in Eagle Owls. I was only 16 years old. Together with my first field work projects on brown bears, otters and Goshawks, Eagle Owls have represented a passion of mine for as long as I can remember and which, later in life, became my work. It is a passion that my grandfather Oscar helped me to develop when he encouraged me to read the Felix Rodriguez de la Fuente encyclopaedia of the animals of the world and when he brought me a plastic figurine of an animal each week. And similarly, when my aunt Tina took me to the field in her car because I was still a minor and could not drive myself. These acknowledgements are very important to me because these ‘eagle owl years’ have not only been years of exciting and stimulating research, but have also afforded the opportunity to meet many great people. And all of them have made this work even more wonderful and interesting. Some of these people have become lifetime friends. Two of them are Fabio Liberatori and Francesco Pinchera, who both shared with me the unforgettable beginnings in the Abruzzo Apennines, in Central Italy. At that time, when every day was a new, significant experience, the long walks in remote beech forests, the cold winter nights spent hoping to hear a call in the dark and the long talks which we thought would change the world forged unbreakable friendships. Without Fabio and Francesco this period would not have been the same. Mario Melletti, who shared with me many cherished moments all through these ‘eagle owl years’, was always ready to join me in the different countries of Europe where I was living and working with Eagle Owls. Every day was more entertaining when Mario was there. My SROPU (Stazione Romana Osservazione e Protezione Uccelli) friends have always been among the nicest people I have ever met; a special and warm thanks to, among others, Massimo Brunelli, Fulvio Fraticelli, Arianna Aradis, Alessandro Montemaggiori, Claudio Carere, Enzo Savo, Fabrizio Bulgarini, Alberto Sorace, Fulco Pratesi and Francesco Petretti. During these first few years in the Apennines, I had the chance to meet some special friends including Marina Cerasoli, Mario Pellegrini, Massimo Pellegrini, Angela Natale, Paolo Barrasso, Mario Chiavetta and Michele Cogliati. It was in Abruzzo that I first received support for my research at the Centro Studi Ecologici Appenninici of Pescasseroli, where Cinzia Sulli, Franco Tassi and Giorgio Boscagli were crucial people for me at that time. During this period, I also had the opportunity to spend an incredible time in Abruzzo and in Germany with Wilhelm Bergerhausen and Oliver Krischer. Wilhelm has been one of the most important people with regard to Eagle Owl studies of the past century. And I still remember my meeting with the extremely kind Viking Olsson, another very important pioneer in Eagle Owl studies. The ‘French years’, mainly spent in Burgundy, Central Massif and Provence, allowed me to strike up a lasting friendship with Gilbert Cochet, from whom I learned many of those Eagle Owl ‘secrets’ that were later essential during subsequent steps of my career. Our nightly 6

observations of bears and wolves in the Apennines made our friendship even stronger. Thanks to my research in Provence, today I have a bond of friendship with Max Gallardo, who kindly shared with me his vast knowledge of the Luberon Eagle Owls, Hervé Magnin, who was an indispensable supporter of my research in the Parc du Luberon, and Claudine and Pierre Horisberger. When I started working with Eagle Owls in the Luberon, Claudine and Pierre allowed me to stay for many years in their ‘Roquerousse’ house, an old house dating from the 17th century lost in the middle of the Provence ‘garrigues’, located only a few hundred metres from an Eagle Owl cliff. During my stay in France I also met other extraordinary people such as Camille Ferry, Luc Strenna, Odíle Malbec, Otello Badan, Soisick de Champsavin and, above all, my best friend Marie de Saint Jacob, who will always have a special place in my heart. My long stay at the Estación Biológica de Doñana in Spain represents one of the most amazing, fruitful and unforgettable periods of my life. That is where I met my co-author María del Mar Delgado and where we both created an ‘improbable’ research team which we used to call ‘the night ecology group’. During almost fifteen years of Eagle Owl research in Spain we met many people with whom we shared extraordinary experiences. Thank you to Rui Lourenço, Chiara Bettega, Letizia Campioni, Joan Real, Mario León-Ortega, Fabrizio Sergio, Paola Bartolommei, Miguel Ferrer, Eva Casado, Carlos Alonso-Álvarez, Manuela de Lucas, Márton and Zsuzsa Horvath, Carlotta Maggio, Giulia Bastianelli, Eleonora Caferri, Lindy J. Thompson, Michael Thornton, Renato Stigliano, María Sánchez, Pedro José Garrote and Giulia Bombieri. My daughter Giulia was a ‘special guest’ of this group and she bravely joined us on most of our field trips, which were not always easy and safe for a little girl. A special thanks to my friend José María Fedriani (Cani), and to Bruno Caula, Pier Luigi Beraudo, Paolo Marotto and Massimo Pettavino for the nice moments we shared in the Sierra Norte. But none of this would have been possible if, at each step, I had not been supported and accompanied by María del Mar, who has warmly watched over me all these years. I will always fondly remember my friendship with Gary Bortolotti, which was abruptly interrupted by his premature passing away. Finally, the years of research in Finland have, in particular, provided the opportunity to meet extremely kind people, who are now very special friends. Thanks to the friendship and support of Pertti Saurola, Heidi Björklund, Jari Valkama and Heikki Lokki, my nearly three-year stay at the Finnish Natural History Museum of Helsinki has, day-to-day, been an unforgettable experience. During that period I also had a wonderful time with two other friends: Raimo Seppälä, with whom I spent a pleasant time searching for Eagle Owl nests on the roofs of downtown Helsinki, and Patrik Byholm, with whom I shared nice walks in Finnish forests. Jere Toivola and Eino Salo made field work an amazing experience and my collaboration with Erkki Korpimäki has been a very rewarding time. I will always remember when I first became enchanted by the Eagle Owl. I had just finished reading the paper that Jacques Blondel and Otello Badan published in 1976 on the Alpille’s Eagle Owl, in southern France. The black and white pictures of owls at the nest, as well as the deep caves in the Provence cliffs that were used by Eagle Owls to breed, haunted me for months, until the day Francesco Pinchera and I decided to start our first research project on Eagle Owls in the Central Apennines of Abruzzo. For over three decades since then I have worked on Eagle Owls every year, until 2014, when I decided to stop. But not before writing a book on the animal that helped me transform the dreams of a child into a wonderful profession. Vincenzo Penteriani, February 2019 7

CHAPTER 1

The Eagle Owl In this chapter we describe the main features of the Eagle Owl Bubo bubo (Linnaeus 1758), i.e. field characteristics, size dimorphism, moult, taxonomy and population genetics. Because there are several specific books on owls describing in detail the main features of this group of birds (e.g. Mikkola 1983, 2012, Voous 1988, Burton 1992, Duncan 2003, König & Weick 2008, Toms 2014), here we will only focus on the most peculiar and typical characteristics of this species, avoiding (or reducing, at least) more general descriptions of those aspects that are common to other owls. The geographical variation in plumage colours and patterns (e.g. bars, bands, spots and vermiculations of the feathers) of Eagle Owls is rather pronounced, predominantly clinal (i.e. individuals of adjacent populations show gradual change in their characteristics) and correlated with climatic factors (Cramp & Simmons 1980): (i) individuals from humid areas are generally darker and browner; whereas (ii) those living in open or arid regions are paler and more yellowish. In addition, Eagle Owls are gradually smaller and paler southward, as well as paler from central and northern Europe east to Siberia. Because of these large geographical variations, we do not pay particular attention to colour patterns (but see The nominate Bubo and its subspecies, page 17).

The Eagle Owl in a nutshell The Eagle Owl can be considered the biggest owl in the world, although small males may overlap with females of the Snowy Owl Bubo scandiacus and Great Grey Owl Strix nebulosa in Europe (Cramp & Simmons 1980), and body length, weight and wingspan are similar to the Blakiston’s Fish Owl Bubo blakistoni, distributed in south-eastern Siberia, South and North Korea and northern Japan (Hokkaido; Mikkola 2012). Eagle Owls may also be 8

The Eagle Owl

considered the most eclectic owl in terms of inhabited biotopes (they are almost ubiquitous), nest sites and diet. They look huge and heavy, almost barrel-shaped, and are characterised by their long ear-tufts (these head ornaments may serve a communication function, Galeotti and Rubolini 2007, see also The function of head ornamentation in owls in Chapter 13, page 332), a marked dark but warm brown plumage with buff and ochre ground-colour, a relatively dark facial disk (which is, however, quite poorly developed compared to other owls) and large eyes. The tawny-buff ground colour is mainly influenced by bleaching and wear, grading to off-white, especially on scapulars, upper wing-coverts and feather tips of the breast and belly (Cramp & Simmons 1980). Cochet (2006) reports the possibility of a grey phase in central France, which seems to be extremely rare. The bill is greyish-black or black, the cere is dark olive-grey or slate-grey, and the claws are black. The tarsus and toes are densely covered with hair-like feathers down to the base of the claws. Hidden by the feathers of the throat, Eagle Owls have a peculiar white patch only visible when an individual is calling (see Chapter 13 for more details on the characteristics and social functions of this throat badge). The changes in size of Eagle Owls are remarkable, generally increasing to the north and east (Table 1 and Figure 1). This is conventionally explained by ‘Bergmann’s rule’, which states that larger warm-blooded individuals have a smaller surface area to volume ratio than smaller ones and, consequently, they lose a lower proportion of their body heat through their skin in a given time, an advantage in cold northern climates. It seems that Eagle Owls have slightly better daytime vision than humans, which is made possible by an exceptional range of aperture sizes (pupil size) controlled by the iris (Mikkola 1979, 1983, 2012). The diversity of morphology and habits of owl species might extend to their physiological optics, as in the case of the ability to change focus (i.e. accommodation). When looking for correlations between accommodative range and relative size, age and diurnality of different owl species, only relative size was found to have a significant inverse correlation with accommodative range (Murphy & Howland 1983). That is, different species of owls have different accommodative abilities: the smaller the body, the larger the accommodative range. This may be due to the fact that small owls often locate, pursue and consume small prey items and, as a result, they must be able to focus on objects at closer range than bigger owls. Individual variation in eye colour in Eagle Owls is important, ranging from reddishorange to pale yellow, but it is generally assumed that taiga Eagle Owls have more orange and darker eyes than individuals living at more southern latitudes (e.g. deserts; Cramp & Simmons 1980). Hearing sensitivity recorded by van Dijk (1973) for the Eagle Owl showed that this species has very good hearing compared to other owls, within the frequency range of 0.5–4 kHz, which is of the same order as for the Tawny Owl Strix aluco and the Longeared Owl Asio otus (see also Görner 1982). In the Hann (1953) listed species of birds with authenticated records of old age in captivity, the oldest was an Eagle Owl that lived for 68 years, whereas the oldest ringed bird was at least 21 years old (Olsson 1979). Flying Eagle Owls look powerful and they are relatively fast, with rather shallow, regular wingbeats; wings are raised more above the body than depressed below it. Bursts of wingbeats are combined with quite long, straight glides on slightly arched wings. Eagle Owls generally fly in lower airspace, except for over long distances and when soaring on rising air currents (Cramp & Simmons 1980, Pukinsky 1993, König & Weick 2008). Eagle Owls’ wings are broad, with a large area in comparison to the weight; their wing-loading (i.e. total weight in g per cm2 of wing area; the higher the wing-loading, the more effort is required by the bird to support itself in the air) is one of the highest (0.71 g/cm2) among owl species (e.g. Great Horned Owl Bubo 9

The Eagle Owl

virginianus = 0.80 g/cm2; Tawny Owl = 0.40 g/cm2), but relatively low if compared with that of some other birds (e.g. Golden Eagle Aquila chrysaetos = 0.65 g/cm2; Peregrine Falcon Falco peregrinus = 0.63 g/cm2; Black Grouse Tetrao tetrix = 1.34 g/cm2; Mikkola 1983, 2012). Owls are able to fly almost noiselessly, and their wing feathers are considered to play an important role in silent flight. Gliding and flapping owls are thought to generate noises at such low frequencies that they are below the hearing range of prey. Features of the owl’s feathers, rather than the owl’s relatively slower flight speed, account for silent flight in this species (Geyer et al. 2009). The serrated leading edges, fringe on the trailing feathers and velvet-like surface of wing feathers (all of which are absent in diurnal raptors, Chen et al. 2012) are most likely responsible for the owl’s silent flight. Indeed, some experiments have shown that owls fly more noisily and their aerodynamic flight performance decreases when the serrated leading edge or the trailing edge fringe is removed (Gruschka et al. 1971, Kroeger et al. 1971, Neuhaus et al. 1973). Recently, Chen et al. (2012) added new insight into the acoustic properties of Eagle Owl wing feathers and their effects on sound suppression. Firstly, the serrations of the leading edge seem to behave as a set of closely spaced streamwise vortex generators that can reduce flow separation and boundary layer thickness, stabilising air flow on the wing surface. This suppresses the generation of large-scale vortex noise. Secondly, the fringes are helpful for suppressing turbulence and vortex, thus preventing the trailing edge of wings from becoming a noise source. Finally, the dorsal feathers of Eagle Owls have a downy and velvet-like surface that seems to function as a suppressor of high frequency noise. Additionally, by comparing the sound absorption properties of Eagle Owls and Buzzards Buteo buteo, Chen et al. (2012) demonstrated that the wing feathers of the Eagle Owl have better sound absorption properties than the Common Buzzard, as the absorption coefficient at higher frequencies is higher than that at lower frequencies. Moreover, the Eagle Owl generates lower sound intensity than the Common Buzzard in flapping flight. These results prove that the special sound suppression characteristics of the Eagle Owl’s wing feathers play an important role in silent flight in this species. Haematological studies on owls are relatively rare but important for: (i) a greater awareness of blood values in order to improve our understanding of the pathology and biochemical disorders of this group of species, as well as seasonal modifications and shifts in blood components (e.g. during the breeding cycle; Gerlach et al. 1984); and (ii) taxonomic and ecological conclusions from groups of owls that are phylogenetically related. For example, knowledge of the existence of circadian rhythms in several blood values may be highly relevant. García-Rodríguez et al. (1987) studied the daily variation of several blood chemistry values in Eagle Owls, primarily showing that: (a) the variations of glucose, urea, uric acid and triglyceride concentrations in the blood showed significant, high-amplitude rhythms, with the highest glucose value occurring at dawn; (b) urea, uric acid and triglyceride concentrations exhibited an important circadian variation, characterised by an increase in their levels at dawn (although urea and uric acid also increase at night); and (c) the values of calcium and amylase were also different between twilight and daylight periods. Concentrations of total proteins were the most homogeneous values over the light–dark cycle. Under natural conditions, Eagle Owls are subjected to different and variable environmental factors, like photoperiod and food availability, as well as temperature and their own rhythms of activity, and the light–dark and eat–fast cycles may play a crucial role in shaping blood values. Thus, the respective patterns of variation seem to be affected by particular activity times and consequent physiological activities, e.g. decrease in the renal function during the central phase of the night, when metabolic activities should be low, as well as energy requirements during the phases of activity. 10

B. bubo buboo

B. bubo bubon

B. bubo bubom

B. bubo bubol

B. bubo bubok

B. bubo buboj

B. bubo bubo i

B. bubo bubo h

B. bubo bubo g

B. bubo bubo f

B. bubo bubo e

Sub-species

421–468

466

479

472

460–490

447

435–460

448

473±7

460–510

433–470

439–505

479±15

454±16

453±5

463–513

430–453

402–480

Female

482±14

Male

444±8

Wing-lengtha (mm)

2500–3200 2556±228 2182–3000

2000–2500

1880±139

1710–2077

450–485

2600

2100

3300

2554

2100–2950

2583±437

2250–4125

1890

1880

1295–2690

2779±47

3025 2200–4000

2225

1620–3000

2203±84

2270–4200

2588±180

Female

1835–2810

2062±165

Male

Weightb (g)

74–82

79±3

Male

95

76–88

82±4

Female

Tarsus length (mm)a

231–252

240±7

Male

248–288

266±13

Female

Tail (mm)a

49–53

51±2

Male

52–58

55±2

Female

Bill F c (mm)a

Owl Bubo ascalaphus and the Rock (or Indian) Eagle Owl Bubo bengalensis, previously considered subspecies of the nominate Bubo.

30–34

32±1

Male

33–40

36±2

Female

Bill C d (mm)a

Table 1. Size measurements (mean ± SD and range) for both males and females of the nominate Bubo and subspecies, as well as two similar Bubo species, the Pharaoh Eagle

The Eagle Owl

11

12

466–487

431–453

B. bubo interpositus

B. bubo interpositus

B. bubo hispanusw

B. bubo hispanusv

B. bubo hispanus

480

468–502

428–463

440–485

425–475

445

466

450–495

451

420–470

449±6

445–470

440±9

420–461

453±18

445–470

422±15

420–450

475–520

430–470

1900

1500–1850

1670±95

1220–1770

1543±164

2100–2750

2458±238

485

B. bubo bubou

453

1550

1500–1950

1230–1775

1485±260

1570–2010

Male

2300

1920–2600

2233±179

1750–2390

2027±163

3075–3260

3164±76

1820–2650

2276±246

2299–2525

2395±103

2322–3000

Female

Weightb (g)

B. bubo bubot

B. bubo bubo

s

474±7

444±10

B. bubo bubor

478

450–500

Female

445

440–480

Male

Wing-lengtha (mm)

B. bubo buboq

B. bubo bubo

p

Subspecies

76–97

85±6

81–103

95±7

91–96

93±2

70–73

Male

63–99

87±9

94–111

102±5

98–106

101±3

74

Female

Tarsus length (mm)a

240–290

261

240–290

261

160–224

234–286

260±15

235–300

260±22

236–254

249±8

27–30

Male

188–233

241–300

276±15

265–310

290±15

266–282

271±6

29–31

Female

Tail (mm)a

41–50

46±2

Male

40–52

48±4

Female

Bill F c (mm)a

29–38

31±2

30–32

31±1

33–35

Male

21–36

32±4

34–36

35±1

34–35

Female

Bill C d (mm)a

The Eagle Owl

B. bubo yenisseensis

B. bubo yenisseensis

B. bubo tarimensis z

(or turcomana)

B. bubo turcomanus

(or turcomana)

B. bubo turcomanus

B. bubo sibiricus p

B. bubo sibiricus

B. bubo sibiricus

B. bubo ruthenus

B. bubo ruthenus

B. bubo nikolskii

B. bubo interpositus y

B. bubo interpositus x

443–468

456

435–470

450–455

473–518

487

473–518

465–475

470–492

420–468

471

482

445–512

482

490–550

443

440–470

450

470–510

492

472–515

451

472–515

435–480

438–465

491

471–515

430–468

456

485

446

482

471–490

453

440–468

438

394–465

419

440–460

420–430

378–430

448

425

75–76

76–78

300–330

240–265

242–250

300–315

254

247

35–36

35–36

The Eagle Owl

13

14

B. bubo tibetanus ab

B. bubo hemachalana

B. bubo hemachalana

B. bubo gladkovi aa

B. bubo omissus

B. bubo omissus

B. bubo kiautschensis

B. bubo ussuriensis

B. bubo ussuriensis

B. bubo jakutensis

B. bubo jakutensis

Subspecies

470–502

430–465

425–460

402–424

499

490–505

461

450–485

486

473–508

451

433–466

492

470–505

462

450–472

495–500

445

415

440–470

425–460

404–450

440–485

483

410–480

460–502

448

480–503

452–468

430–475

493

480–503

Female

458

455–490

Male

Wing-lengtha (mm) Male

Female

Weightb (g) Male

Female

Tarsus length (mm)a Male

Female

Tail (mm)a Male

Female

Bill Fc (mm)a Male

Female

Bill Cd (mm)a

The Eagle Owl

375–433

350–430

335–380

358–391

395

390–391

344–366

359

390

374–390

330–345

355

383

338

1100

175

185–195

160–203

186

160–190

203

205–227

190–220

207

201–206

Data are rounded to the nearest 10mm; b Weight data are rounded to the nearest 10g; c Bill to forehead; d Bill to cere; e Central and northern Europe together; f Norway; Leif Gunleifsen, Karl-Otto Jacobsen, Torgeir Nygård, Roar Solheim & Ingar Jostein Øien unpublished data; g Finland; h Finland; Jari Valkama, Jere Toivola & Mikko Honkiniemi unpublished data; i Sweden; j Denmark; Lars Bo Jacobsen unpublished data; k the Netherlands; l Germany; m Western Germany; n Eastern Germany; o Czech Republic; p Romania, Nicola Pârlog unpublished data; q Switzerland; r France, Christian Riols unpublished data; s Italy; t Greece; u North-eastern Russia; v South-western Spain (Sierra Morena and Doñana National Park); Penteriani & Delgado unpublished data; w Atlas Mountains of Morocco and Algeria (extinct?); x Middle East; y Middle East; z Reported as tarimensis in Piechocki 1985, but considered a synonym of turcomanus in König & Weick 2008; aa Reported as gladkovi in Piechocki 1985, but considered a synonym of omissus in König & Weick 2008; ab Reported as tibetanus and auspicabilis in Piechocki 1985, but considered synonyms of hemachalana in König & Weick 2008; ac North Africa only; ad North-western Africa; ae Egypt; af Middle East.

a

Notes: When not specified below, the measurements are intended for the whole geographical range. When the same subspecies is shown more than once and there is no note to denote a different location, this indicates that various authors have reported different data with no exact geographical location provided.

Sources: Vaurie 1960, Piechocki 1984, Piechocki & März 1985, Cramp & Simmons 1980, Pukinsky 1993, Estafiev & Neifeld 1999, Martínez et al. 2002, Hudec & Štastný 2005, Brichetti & Fracasso 2006, König & Weick 2008, Mikkola 1983, 2012 and unpublished data of other authors (see below).

Bubo bengalensis

B. ascalaphus af

B. ascalaphus ae

B. ascalaphus ad

367

340–390

346

324–368

B. ascalaphus ac

483

455–508

415–482

465

484

451

B. bubo borissowi

B. bubo auspicabilis ab

The Eagle Owl

15

The Eagle Owl

Sexes look similar, but with reversed sexual dimorphism (females are larger than males; see Size dimorphism and sexing (page 22) and Table 1). Both the bill and claws are visibly wider and longer in females: generally, the female’s bill protrudes from the head bristles more than in males. Some authors (Blondel & Badan 1976, Pukinsky 1993) have also suggested that the sexes may be dimorphic in coloration, with females being darker than males, and in the appearance of the face mask (Mysterud & Dunker 1983), in particular by the usual position of ear-tufts and the size of the eyebrows. Although males and females have different voices (Chapter 12), the morphology of their vocal organ (syrinx) does not show any sexual dimorphism (Cevik-Demirkan & Ozdemir 2011). The syrinx is located at the base of the trachea and the sound is produced by vibrations of some or all of the walls of the syrinx and the pessulus caused by air flowing through the syrinx. Eagle Owls breeding pairs are sedentary and strongly territorial throughout the year. However, nomadism has been reported in the former USSR (especially the Asiatic part) due to very harsh winter conditions (Cramp & Simmons 1980), where individuals could make more or less regular southward movements. Occasionally, vertical (altitudinal) displacements (e.g. from the mountain range to the lowlands) may be observed during harsh winters, with breeders from high-altitude breeding places moving to warmer and more food-abundant, low-altitude areas. During the daytime Eagle Owls are generally hidden behind or in the middle of trees and bush foliage, as well as rock fissures and caves, but sometimes they also perch in daylight on more visible places (e.g. rock pinnacles, dominant trees and bushes, branches projecting from surrounding vegetation). Although principally crepuscular (i.e. dusk and dawn activity) and nocturnal, another facet of the eclecticism of Eagle Owls is their ability to adapt to diurnal conditions (even outside the boreal range). Indeed, some individuals (e.g. those that inhabit areas with high availability of diurnal prey) during specific periods of the year (e.g. when feeding young) may start their activity during the day. In particular, Pukinsky (1993) reported some cases of Eagle Owls hunting during the daytime in Russia and Moldavia, which accords with our occasional observations from both southern France (Alpilles and Luberon, Provence) and southern Spain (Sierra Morena, Andalusia). Eagle Owls have been considered a ‘vulnerable’ species (according to Annex II of the CITIES Convention on International Trade in Endangered Species of Wild Fauna and Flora and Annex I of the Council Directives 2009/147/EC on the conservation of wild birds, Birds Directive), but they are now classified as ‘Least Concern’ according to IUCN criteria, mostly as a result of the recent (both natural and human-induced) population recoveries and range expansion.

16

The Eagle Owl

The nominate Bubo and its subspecies

Table 2 provides the current classification and distribution of the nominate Bubo and subspecies. Presently, 13 subspecies are officially recognised, whereas nine other subspecies are considered synonyms of true subspecies (Penhallurick 2002, König & Weick 2008). The current classification represents a more precise taxonomy of the Bubo genus, for which 24 subspecies were recognised in the middle of the last century (Peters 1940); such a large number of subspecies was considered greatly overdone, especially in view of the large amount of individual variation and the comparatively small number of available individuals from given localities. In fact, ten years later, Dementiev & Gladkov (1951) reduced this number to ten subspecies for the Soviet Union and 17 for the species as a whole, whereas Vaurie (1960) only recognised 16 subspecies within the Bubo bubo species. The area in which the greatest number of subspecies has been reported to occur at the same time is central Russia, where up to five subspecies have been recorded: B. b. bubo, B. (b.) interpositus (see below for more information on its current taxonomic position), B. b. ruthenus, B. b. sibiricus and B. b. turcomanus (Karyakin 1998). In Central Asia the most widespread subspecies are (Mitropolskiy & Rustamov 2007): turcomanus, omissus and hemachalana. Moreover, Eagle Owls that have been classified as gladkovi (probably a synonym for omissus) appear along the coast of the Mangyshlak Peninsula (western Kazakhstan). In winter time, particularly along the Caspian Sea coast and the Aral Sea, individual representatives of sibiricus are observed, while in the Tarbagatai (between Xinjiang, China and Kazakhstan) the appearance of yenisseensis, which mainly breeds in the Altai region, is expected. In Central Asia, in the mountain regions of the Pamir (Tibet), Eagle Owls may reach 4,000–4,500m a.s.l. (sub-alpine zone). Eagle Owls living in arid conditions are virtually sedentary, but the majority of individuals migrate from northern regions to somewhere south for winter. Birds breeding in the high mountain range of the Tien Shan and Pamir-Alai descend in winter to lower mountain belts and adjacent valleys, but some individuals of the Pamir, for instance, lead a sedentary life even at high altitudes. In winter, individuals of the more northern populations (Siberia, Altai) exhibit nomadic movements into lowland areas of Central Asia. 17

The Eagle Owl

Table 2. Current classification and distribution of the nominate Bubo and subspecies (both recognised and synonyms of true subspecies). Subspecies Bubo bubo bubo

Bubo bubo meridionalis Bubo bubo hispanus

Current distribution Europe from the Pyrenees and Mediterranean east to the Bosphorus and Ukraine, north to Scandinavia, Moscow and NW Russia. In NE Russia it is distributed in the east up to Kandalaksha, Arkhangelsk, Pinega and the Vashka, Volga and Mezen River basins (up to Leshukonskoe), the borders of the Nizhny Novgorod Region (Oblast), up to the Tambov and Voronezh Regions (Oblast), the breeding range covering the whole taiga zone; during winter it appears in the Timan tundra. The southern border virtually lies on the 50th parallel. Considered a synonym of B. bubo bubo.

Source of the first description Linnaeus, C. 1758. Tomus I. Systema Naturae, ed. 10: 92. Holmiae, Laurentii Salvii

Orlando, C. 1957. Contributo allo studio delle forme europee del Bubo bubo (L.). Rivista Italiana di Ornitologia 27: 42–54 Rothschild, W. & Hartert, E. 1910. Novitates Zoologicae 17: 110–111 Rothschild, W. & Hartert, E. 1910. Novitates Zoologicae 17: 111

Iberian peninsula and (formerly?1) Atlas Mountains of Morocco and Algeria (N Africa); it seemed to overlap with Bubo ascalaphus in N Algeria2. B. bubo interpositus Bessarabia (part of Moldova and Ukraine), Crimea, (or possibly Bubo Caucasus, Asia Minor, Palestine, Syria and NW Iran. interpositus following Following Cramp & Simmons (1980), it might replace B. more recent genetic b. bubo in E and S Romania and S Russia. In particular, the analyses3,4) subspecies inhabits Russian territories from the Carpathians up to the Caspian Sea and the Mangyshlak coast; to the north, nearly up to the 50th parallel, up to the middle reaches of the Don; to the south, up to Trans-Caucasus area and the state border of the former USSR, across the coast of the Mangyshlak up to the southern parts of the peninsula. B. bubo armeniacus Considered a synonym of B. bubo interpositus. B. bubo nikolskii From S and E Iraq through central and southern Iran to Sarudny, N. 1905. Zwei Afghanistan and Pakistan. ornithologische Neuheiten aus West-Persien. Ornithologisches Jahrbuch 16: 142 B. bubo ruthenus East of a line from Moscow to Pechora (Komi, N Russia), to Zhitkov & Buturlin 1906. Data the Ural River and the Uzha River valley, south to the mouth on the avifauna of the Simbirsk of the Volga River and along the middle course of the Don Region. Transactions of Russian River, Manych-Gudilo Lake and the lower reaches of the Geographical Society, 41 (2), Volga River. In the Udora River basin, along the Vashka River 270–272, St. Petersburg tributary, there is a transition between B. b. bubo and ruthenus. B. bubo sibiricus W Siberia and Bashkiria (or Baskortostán, SW Russia) to Gloger, C.L. 1833. Das the middle Ob River (W Siberia) and W Altai Mountains. Abändern der Vogel durch Winter records are reported in the west up to Karelia, the Einflufs des Klimas: 142 Mezen River basin. B. b. bashkiricus Considered a synonym of B. bubo sibiricus3. B. bubo turcomanus Between Volga, Semipalatinsk, Zaysan, the upper reaches of Eversmann, E. 1835. Addenda (or turcomana) the Irtysh River and upper Ural, Caspian coasts and Aral Sea, ad celeberrimi Pallasii east to Transbaikalia (east of Lake Baikal, Russia) and the Tarim Zoographiam Rosso-asiaticam. basin (NW China) to W Mongolia. Aves: 3–4 B. bubo eversmanni Considered a synonym of B. bubo turcomanus3,5. B. bubo tarimensis Considered a synonym of B. bubo turcomanus3. B. bubo yenisseensis Central Siberia, between the Ob River, Lake Baikal, Altai Buturlin, S.A. 1912. Bubo Mountains and N Mongolia. In particular, the Russian range bubo yenisseensis subsp. nov. includes the territory from the Ob River to the east up to Ornithologisches Mitteilungen the Nizhnyaya Tunguska and Baikal; to the north, up to the 2: 26 border of the species’ geographical range; to the south up to the Saur, Altai, Khangai and Kentei Mountains (Mongolia).

18

The Eagle Owl

Subspecies

Current distribution

Source of the first description

B. bubo zaissanensis

Considered a synonym of B. bubo yenisseensis .

B. bubo jakutensis

It inhabits the territory from the Nizhnyaya Tunguska (Siberia) and the upper reaches of the Vilyui River to the east, up to the upper reaches of the Kolyma River and the Sea of Okhotsk; to the north, up to the border of the species’ geographical range; to the south, up to the Udskaya Guba and the Stanovoy mountain range (SE parts of the Russian Far East).

Buturlin, S.A. 1908. Bemerkungen über die geographiske Verbreitung der Vögel im nordöstlichen Sibirien. Journal für Ornithologie 56: 287

B. bubo ussuriensis

SE Siberia and N China, Sakhalin (off the E coast of Russia and north of Japan) and Kuriles (Russia, northeast from Hokkaido).

Poljakov, G.J. 1915. Zur ornithologischen Fauna des Ussuri-Gebietes. Ornithologisches Mitteilungen 6: 44

B. bubo dauricus

Considered a synonym of B. bubo ussuriensis3.

B. bubo borissowi6

Considered a synonym of B. bubo ussuriensis3.

Hesse, E. 1915. Journal für Ornithologie, 63: 366

B. bubo kiautschensis

Korea and China, south to Sichuan and Yunnan.

Reichenow, A. 1903. Ornithologische Monatsberichte 11: 85

3

B. bubo setschuanus, Considered synonyms of B. bubo kiautschensis3. inexpectatus, tenuipes, jarlandi, swinhoei, yamashinai B. bubo omissus

Turkmenistan (Central Asia), adjacent Iran and Chinese Dementiev, G.P. 1932. Alauda Turkestan. In Russia, it inhabits the territory from the Bolshoy 2, 4 (30 Jan., 1933): 392 Balkhan and the lower reaches of the Atrek to the east up to the Gurkhandarya and Karfirnigan; to the north, presumably up to the Ustyurt, the Amur Darya River valley and the Zerafshan; to the south, up to the state border.

B. bubo gladkovi

Considered a synonym of B. bubo omissus3.

B. bubo hemachalana

From Tian Shan (Central Asia) to Pamir Mountains (formed by the junction of the Himalayas with Tian Shan), north to Kara Tau, south to Baluchistan (SW Pakistan) and the Himalayas.

B. bubo tibetanus

Considered a synonym of B. bubo hemachalana3.

B. bubo auspicabilis

Considered a synonym of B. bubo hemachalana3.

Hume, A.O. 1873. Stray Feathers 1: 315

Sources: Cramp & Simmons 1980, Pukinsky 1993, Estafiev & Neifeld 1999, Mitropolskiy & Rustamov 2007, Sieradzki et al. 2007, König & Weick 2008, Mikkola 2012. 1

Several authors consider that B. b. hispanus is now extinct from the Atlas Mountains (Morocco and Algeria), but there is no information on the possible causes or when this event may have occurred. Authors of the 19th century stated that bubo (together with ascalaphus) inhabited Algeria (Malherbe 1855, Loche 1867), and the last collected specimen of a bubo individual in Algeria occurred in 1917 (Rothschild 1918). 2 Bubo bubo hispanus and Bubo ascalaphus have interbred on several occasions in Israeli zoos, where their fertile hybrid offspring also interbreed freely with each other as well as with both ‘pure’ species (Duncan 2003). Vaurie (1960) also reported the possibility that bubo and ascalaphus may interbreed. These hybrids were described as aharonii by Rothschild & Hartert (1910), probably a hybrid of ascalaphus and interpositus. 3 Mitropolskiy & Rustamov 2007, König & Weick 2008. 4 Wink et al. 2009. 5 However, Karyakin et al. (2009) consider that this is the subspecies inhabiting the Aral–Caspian region (Kazakhstan), assuming that this subspecies intergrades with B. b. turcomanus along the entire northern border of the breeding range in the Aral–Caspian region and with B. b. omissus along the southern border of its distribution range in Turkmenistan. 6 Pukinsky (1993) reports that B. bubo borissowi could inhabit the territory of Sakhalin and, perhaps, the Kuril Islands to the north up to Urup Island. However, he also says that only the insular confinement of some individuals might provide some evidence for the existence of this subspecies.

19

Femalewing-length wing-length(mm) (mm) Female

Malewing-length wing-length(mm) (mm) Male

20 440 440

420 420

400 400

380 380

360 360

340 340

320 320

300 300 bubosibiricus sibiricus B.B.bubo bubobubo bubo B.B.bubo

bubosibiricus sibiricus B.B.bubo

buboyenisseensis yenisseensis B.B.bubo bubohemachalana hemachalana B.B.bubo bubojakutensis jakutensis B.B.bubo

buboyenisseensis yenisseensis B.B.bubo bubohemachalana hemachalana B.B.bubo bubojakutensis jakutensis B.B.bubo

bubobubo bubo B.B.bubo

bubobubo bubo B.B.bubo

bubobubo bubo B.B.bubo buboruthenus ruthenus B.B.bubo

buboruthenus ruthenus B.B.bubo

460 460

bubobubo bubo B.B.bubo

buboussuriensis ussuriensis B.B.bubo

bubobubo bubo B.B.bubo

bubobubo bubo B.B.bubo buboturcomanus turcomanus B.B.bubo

bubobubo bubo B.B.bubo

bubobubo bubo B.B.bubo

bubointerpositus interpositus B.B.bubo

bubobubo bubo B.B.bubo

Subspecies Subspecies

buboussuriensis ussuriensis B.B.bubo

bubobubo bubo B.B.bubo

bubobubo bubo B.B.bubo buboturcomanus turcomanus B.B.bubo

bubobubo bubo B.B.bubo

bubobubo bubo B.B.bubo

bubohispanus hispanus B.B.bubo

bubonikolkskii nikolkskii B.B.bubo

440 440

bubointerpositus interpositus B.B.bubo

bubohispanus hispanus B.B.bubo

380 380

bubonikolkskii nikolkskii B.B.bubo

400 400

buboomissus omissus B.B.bubo

420 420

buboomissus omissus B.B.bubo

Buboascalaphus ascalaphus Bubo

460 460 Buboascalaphus ascalaphus Bubo

480 480

Buboascalaphus ascalaphus Bubo

500 500 Buboascalaphus ascalaphus Bubo

300 300

Buboascalaphus ascalaphus Bubo

320 320

Buboascalaphus ascalaphus Bubo

340 340

Buboascalaphus ascalaphus Bubo

360 360

Buboascalaphus ascalaphus Bubo

The Eagle Owl

500 500

480 480

Subspecies Subspecies

Figure 1. Trends of male and female wing-lengths for Eagle Owl subspecies and Bubo ascalaphus. The increase in size is significant for both males (F = 14.85, p = 0.001) and females (F = 9.29, p = 0.006), with the subspecies living in Scandinavia, Siberia and Central Asia being the largest. Multiple measurements for a subspecies occur when various studies have measured the same subspecies in different geographical areas.

The Eagle Owl

As a general colour patterning, the darkest individuals are the ones belonging to the nominate Bubo, whereas the subspecies inhabiting southern Russia and the northern Middle East are increasingly yellower towards the south of their ranges, in particular (Vaurie 1960, Cramp & Simmons 1980, König & Weick 2008): (1) B. b. hispanus is the most similar to the nominate Bubo, although paler (in the mid-1800s A.E. Brehm was inclined to consider it a synonym of sibiricus); (2) B. (b.) interpositus is darker and more rusty in colour than B. b. ruthenus, the latter being paler, greyer and less buffish than the nominate. The subspecies interpositus has been considered intermediate between B. b. bubo and B. b. turcomanus, although it is more similar to the latter but generally darker; (3) B. b. nikolskii is more rusty than B. b. omissus; the latter is a small, typical desert form with a general pale ochre coloration and less prominent dark markings on the underparts. B. b. nikolskii is closely allied to B. b. turcomanus although smaller and with a more rusty upper side; (4) B. b. sibiricus is another very pale subspecies, the general coloration being creamy-white with dark markings and dark primary coverts that contrast with the rest of the wing. It is notable for its larger size and heavier mass than B. b. bubo and B. b. ruthenus (Estafiev & Neifeld 1999); (5) again, B. b. turcomanus is very pale and yellowish, resembling nikolskii and omissus, but less darkly streaked and vermiculated, with less contrasting brown feathers. Although some specimens are barely distinguishable from interpositus, they are generally lighter, more yellowish and more sandy in colour than interpositus; (6) B. b. yenisseensis is a bit darker and greyer than the previous, paler subspecies, with a more yellowish ground colour than sibiricus; (7) B. b. jakutensis is much darker and browner above than yenisseensis, and more distinctly streaked and barred below than sibiricus; (8) B. b. ussuriensis is darker above than jakutensis, with a more ochre wash on underparts; (9) B. b. kiautschensis is smaller, darker, more tawny and rufous than ussuriensis; and (10) B. b. hemachalana has a general pale brown coloration, with ear-tufts more brownish than blackish, characterised by a more intense and varied coloration than the Siberian subspecies. The subspecies living in Scandinavia, Siberia and Central Asia are significantly bigger than the other subspecies (Figure 1). For 97 owl taxa from 15 of the larger genera, a molecular phylogeny was inferred from a combined dataset of nucleotide sequences of mitochondrial cytochrome b and nuclear RAG-1 genes (Wink et al. 2009). Strigiformes are divided into two families, Tytonidae and Strigidae. Tytonidae consists of two subfamilies and two genera, whereas Strigidae has a more complex structure characterised by three subfamilies subdivided into six tribes: (1) subfamily Striginae with tribes Otini, Bubonini and Strigini; (2) subfamily Asioninae; and (3) subfamily Surniinae with tribes Surnini, Aegolini and Ninoxini. Within the tribe Bubonini, recent analyses performed by Wink et al. (2009) showed that: (i) Bubo ascalaphus can be considered a distinct species, supporting the classification proposed by Sibley & Monroe (1990); and (ii) B. (b.) interpositus is genetically distinct from B. bubo and, thus, it should be treated as a different species or, at least, as a subspecies of B. ascalaphus (Wink et al. 2009). Recent analyses (Penhallurick 2002, Wink et al. 2009) have also demonstrated that: (a) the Snowy Owl (formerly Nyctea scandiaca) shares a common ancestry with the genus Bubo, especially with the New World Great Horned Owl B. virginianus, and should thus be called Bubo scandiacus; and (b) because of genetic relationships, the genus Ketupa should be merged in Bubo. The two subspecies (Bubo ascalaphus ascalaphus and Bubo ascalaphus desertorum) of the Pharaoh Eagle Owl (or Desert Eagle Owl) Bubo ascalaphus (formerly Bubo bubo ascalaphus) are distributed in (König & Weick 2008): N and NW Africa, from the southern slopes 21

The Eagle Owl

of the Rif and Atlas Mountains to most of the Sahara (south to Chad), Mauritania, Mali, Niger, N Egypt, Sudan and NW Ethiopia, Arabia, Syria, Israel and Palestine to W Iraq. Locally, the Pharaoh Eagle Owl may be sympatric with B. (b.) interpositus and, at least in the past, it was sympatric with B. b. hispanus in the Algerian Atlas Mountains: to date, it is not known if this latter subspecies is now extinct from the southern slopes of the Atlas Mountains. Vaurie (1960) was the first to draw attention to the fact that desertorum may not be a Bubo Bubo subspecies; although he considered that those individuals described as desertorum were the palest individuals of the former Bubo (Bubo) ascalaphus. The Rock Eagle Owl (or Indian Eagle Owl) Bubo bengalensis (formerly Bubo Bubo bengalensis) is distributed in (König & Weick 2008): W Himalayas, Pakistan to India, Kashmir, Nepal and Assam. The subspecies B. b. turcomanus shows range overlap and lives sympatrically with the Rock Eagle Owl in Kashmir.

Size dimorphism and sexing Eagle Owls, as is the case with most of the other owl species (and diurnal raptors), exhibit reversed sexual size dimorphism (hereafter, RSD), i.e. females are larger than males. This form of sexual size dimorphism is referred to as reversed because it is contrary to the general rule observed in other animal classes (i.e. reptiles, mammals and birds), in which males are usually larger than females. Natural and sexual selection are considered the main forces shaping such sexual size differences. Numerous hypotheses have been put forward to explain such an inverse pattern in sexual dimorphism (Andersson & Norberg 1981), with most of these hypotheses stressing the significance of female largeness (instead of male smallness) as a cause for the evolution of RSD. Yet there is no consensus given the lack of understanding regarding the relative importance of the various factors determining RSD and whether one or more different factors may explain RSD in different species (Korpimäki & Hakkarainen 2012). RSD is probably not simply the outcome of a single selective force acting on one of the sexes. Many factors may be involved and some of them are not necessarily mutually exclusive; in addition, their relative importance is likely to change from species to species, from year to year and as humans modify the environment (Kenward 2006). The complexity of the topic warrants a quick look at the principal hypotheses: (a) the ‘starvation hypothesis’ (e.g. Korpimäki 1986, Lundberg 1986), which contends that large females with extra body reserves should have (i) a lower probability of starvation and (ii) higher fecundity under unpredictable food conditions or when food is scarce. This hypothesis seems to apply mainly to boreal owls, due to the inter-annual variation in food resources and harsh conditions at the beginning of the breeding period; (b) the ‘reproductive effort hypotheses’ or ‘big mother hypothesis’ (e.g. Selander 1966, Reynolds 1972), which proposes that large females (i) may produce more offspring, (ii) possess better egg production and incubation efficiency and/or (iii) provide better parental care; 22

The Eagle Owl

(c) the ‘female dominance hypothesis’ (e.g. Mueller 1986), which suggests that females choose small males because they are easier to dominate than large males. This may play an important role at the time of mating and during pair formation, resulting in easier maintenance of the pair bond and an increase in food provisioning by the male; (d) the ‘dietary divergence hypothesis’ (e.g. Reynolds 1972, Opdam 1975), which proposes that a difference in size would confer each sex with the ability to take different prey and, consequently, reduce competition during periods of food shortage and when food demands are high for the young. This could be a beneficial consequence of a size difference, although it does not explain why females are the larger sex; (e) the ‘sexual selection hypothesis’, directly related with female competition and/or preference for small males, which may consequently result in RSD. Sexual selection for small males (very successful in provisioning food to incubating females and nestlings, which is energetically costly; Masman et al. 1989) may be related to the important investment of males in breeding attempts, which may determine female–female competition for males (e.g. Newton 1986). Thus, both female choosiness and the importance of male foraging ability may contribute to RSD (Korpimäki & Hakkarainen 2012); (f ) the ‘small male hypothesis’, which assumes that smaller males are more efficient at foraging (e.g. shorter wings; Lundberg 1986) or territorial defence (Hakkarainen & Korpimäki 1991, Massemin et al. 2000). In the Eagle Owl, as in other birds of prey, males do most of the hunting for the whole family from before egg-laying (when females could be particularly at risk of injury during prey capture while eggs are in the oviduct, and eggs and egg reserves also give a female the disadvantage of higher wing-loading) until, and including most of, the nestling phase. During this period, females spend most of the time in the nest or its close surroundings. Thus, the different roles of the sexes during the breeding period may cause divergent and complementary selection pressures on morphological characteristics depending on sex. An explanation based on divergent energetic costs between the sexes during breeding seems to be one of the most comprehensive and valid hypotheses for other species showing RSD (Korpimäki & Hakkarainen 2012). For example, in the case of the Tengmalm’s Owl Aegolius funereus, the small male hypothesis appears to provide a reasonable explanation for RSD patterns (Hakkarainen & Korpimäki 1991, Sunde et al. 2003, Korpimäki & Hakkarainen 2012): small male size is probably of great importance in determining RSD, although it is not possible to completely discard those factors favouring large female size. Actually, there is increasing support for the hypothesis that small males are more successful when food is difficult to catch (Temeles 1985) or scarce (Hakkarainen & Korpimäki 1991, 1995), and the small male hypothesis has also been supported by one review study on the origin and maintenance of RSD (Krüger, O. 2005). In Eagle Owls, sex role partitioning seems important over rather a long period of time, starting from egg formation and laying, and males may do better by specialising as hunters while females remain incubating and guarding 23

Female wing-length (mm)

Male wing-length (mm)

The Eagle Owl 490 480 470 460 450 440 430 420 410 400 490 480 470 460 450 440 430 420 410 400

E

F

CH

S

E

CZ

S

F

F

GR

E

NED

E

I

D

FIN

N NE Russia

N

CH

FIN

D NE Russia

Male weight (g)

3500 3000 2500 2000 1500 1000 500 0

D

FIN

N NE Russia

Female weight (g)

3500 3000 2500 2000 1500 1000 500 0

F

D

DK

FIN

N NE Russia CZ

Figure 2. Trends of wing-length and weight for European Eagle Owls (B. b. bubo and B. b. hispanus) depicting the increase in size towards northern countries. The difference between the two subspecies is more evident for wing-length (B. b. hispanus has the shortest wings) than for weight. However, these trends are never significant: male wing-length with (linear regression; F = 3.24, p = 0.12) and without B. b. hispanus (F = 0.28, p = 0.62), female wing-length with (F = 0.79, p = 0.40) and without B. b. hispanus (F = 0.72, p = 0.43), male (F = 0.11, p = 0.76) and female (F = 0.01, I = 0.91) weight (always tested jointly with B. b. hispanus). E = Spain; F = France; CH = Switzerland; S = Sweden; D = Germany; N = Norway; NE Russia = North-eastern Russia; Fin = Finland; CZ = Czech Republic; NED = the Netherlands; GR = Greece; I = Italy; DK = Denmark.

24

The Eagle Owl

the nest. If so, selection may act more strongly on males for foraging skill, and on females for reproductive efficacy (Lundberg 1986). Recently, Sonerud et al. (2014) showed that size dimorphism is positively related to the extent of parental role asymmetry, represented as female confinement to the nest as a sedentary food processor for nestlings, which would leave greater potential for differential selection on male and female body size. This may explain the female-biased size dimorphism among raptors, which becomes more pronounced as diet changes from insects, reptiles and mammals to birds, as well as when relative prey size increases. In this ‘family conflict scenario’ (Sonerud et al. 2013), we expect that females, confined to the nest for prey partitioning and, thus, dependent on food provided by males, need to control the allocation of prey between offspring and themselves. This would select on the one hand for larger female body size and, on the other, for smaller body size in males, which counteracts the female’s interception of their food provisioning to their offspring by selecting smaller prey. That is, males would maximise their control of food allocation by capturing smaller prey, which favours selection for smaller males. The differences in size between the sexes are reported in Table 1 (see also Table 3 for specific morphological differences between sexes for individuals from Spain), for both the nominate Bubo and subspecies. On the basis of these available data, mean weight of female Eagle Owls is ~30% larger than males, the latter also having on average 6.5% shorter wings than females. The range of variation is relatively similar both across the distribution range of the species and among subspecies, which indicates that RSD is not affected by the locality or the typical size of each subspecies. Despite the evident increase in size of both wing-length and weight from southern to northern European countries and Russia, which is typical of Bubo bubo bubo and of bubo vs. hispanus subspecies, such a trend is not statistically significant (Figure 2). Although genetic analysis has been the best and most reliable method to sex Eagle Owls for a long time (Herzog 1985), sex determination is also possible via morphology (i.e. body measurements; Delgado & Penteriani 2004). When analysing 13 morphological characteristics, i.e. length of the four claws (measured from the hallux claw (toe number one) to the outer claw (toe number four)), tarsus, bill including cere, exposed culmen without cere, bill depth, wing chord, tail, the two ear-tufts and forearm (the length from the front of the folded wrist to the proximal extremity of the ulna), we demonstrated that females were significantly larger than males in all the measured variables except tail, wing chord and ear-tufts (Table 3). Second claw, forearm, length of exposed culmen without cere and bill depth were the most dimorphic variables. A correct classification was obtained for 90.5% of males and 90.9% of females, or 90.7% of all cases were classified correctly. The measured variables are easy to record in the field and, in comparison with other proposed morphometric criteria for gender determination (e.g. wing and body mass), are not influenced by moulting or the condition of specimens or their feathers. In particular, the length of the forearm: (i) has been successfully used for gender determination in other raptors (e.g. Ferrer & De Le Court 1992, Balbontín et al. 2001); (ii) is the best predictor of sex for Eagle Owls, as reported by Martínez et al. (2002) and Mikkola & Lamminmäki (2014) (Table 3 shows that there is little overlap between sexes for this morphological characteristic, as also reported by Risto Tornberg for Finnish Eagle Owls: males = 171–197mm, females = 197–205mm); and (iii) is easy to measure, and repeated measurements taken by both the same and different observers show little variation (Ferrer & De Le Court 1992). Females possess higher brightness values for the white patch of the throat compared to males (Penteriani et al. 2006; see also The white badge of adults in Chapter 13, page 312), but this distinction is only appreciable by spectrophotometric analyses. 25

The Eagle Owl

Table 3. Morphometric data of studied skins of Eagle Owl males (n = 22) and females (n = 28) from Spain. Claws are numbered according to toe number (hallux = 1, outer claw = 4). All measurements are in mm.

Females

Males

Mean

SD

Range

Mean

SD

Range

t1

df

p

Claw of toe 1

34.6

3.6

26.7–40.1

30.6

3.3

21.7–34.0

−3.512

37

0.001

Claw of toe 2

34.9

2.4

27.7–28.5

31.3

2.2

27.9–38.8

−5.252

44

100 (von Haslinger & Plass 2008, C. Leditznig pers. comm. and Figure 15). Recently, Eagle Owls have started to appear on unusual nest sites (for example roofs, silos) and the number of observations of individuals living in towns is increasing (e.g. every year there are observations from Vienna; Hans Frey pers. comm.). On the other hand, the number of traditional breeding sites along some rivers seems to have decreased, probably because of shooting by local hunters. Trend: stable

Poland Formerly widespread, the Eagle Owl reached its lowest population level between the 1920s and 1940s; during the 1950s the Polish population was estimated at 60–70 pairs (although this was probably an underestimate) and between the end of the 1980s and the beginning of the 1990s it was estimated at 130–150 breeding pairs (Tomiałojć 1990, Profus 1992, Wójciak et al. 2008). At that time the population was increasing slightly, mainly in the Silesia (Śląsk) region (SW Poland), with the core distribution areas of the species located in the north (Pomeranian and Masurian lakelands), east (Podlasie and Polesie) and south (West Sudetes, as well as the West and East Beskid and Bieszczady mountains; Ruprecht & Szwagrzak 1988, Tomiałojć 1990); more scattered in central Poland, the southern distribution in lowlands was mostly concentrated in Lower Silesia. Currently, the Polish population is estimated at 250–280 pairs (Wójciak et al. 2008, Sylwester Aftyka pers. comm., Romuald Mikusek pers. comm., Paweł Mirski pers. comm.), with the highest density recorded in Parczew Forest (Wójciak et al. 2008) and the Pieniny Mountains (mean distance between pairs ca. 2,200m, range 1,700–2,400m; Ciach 2005). High densities are also noted in marshy forest complexes in the Biebrza Valley (Pugacewicz 1995) and Augustowska Forest (NE Poland; Zawadzka et al. 2009), as well as in the Stołowe Mountains (SW Poland; Mikusek 2004). Most of the breeding sites in mountains are concentrated in the Carpathians and its foothills (southernmost Poland, ca. 40 pairs, with 5–10 pairs in Slone Mountains, 6–8 pairs in Gorce, 6 pairs in Pieniny Mountains, 4–7 pairs in the Tatras, mainly in the lower montane zone), the Pomeranian Region (40–45 pairs) and Masuria (N Poland, 30–35 pairs; Mikusek 2004, Ciach 2005). In the lowlands Eagle Owls breed in highest numbers in Silesia (30–35 pairs; 20–25 pairs are located in the region of Kłodzko, over 70% of them between 400–700m a.s.l.), Północnopodlaska Lowlands (NE Poland, 35–40 pairs; the population here has been relatively stable since the 1990s, with most of the breeding area – ca. 25 pairs – located in the Biebrza River Valley, where is located the most important population of North Podlasian Lowland; Paweł Mirski pers. comm.) and the Lublin Region (SE Poland, 60 pairs; Pugacewicz 1995, Cichocki et al. 2004, Wójciak et al. 2008). The former decline of the Eagle Owl was principally due to persecution (shooting and the taking of nestlings), whereas more recent threats are electrocution and nest disturbance (Profus 1992). Between 1993–2004 a release project was carried out in Wolin National Park, situated on the island of Wolin in the far northwest of the country (West Pomeranian Voivodeship): 42 Eagle Owls were released and, throughout the project, these individuals formed 2–3 breeding pairs and established several other nesting sites with at least one bird permanently occupying the breeding area (Dylawerski 2006). Trend: stable or increasing 79

The Eagle Owl

Czech Republic From the end of the 19th century to the beginning of the 20th century, the Eagle Owl was the subject of intense persecution (e.g. there are records of at least 350 nest destructions, 100 shootings, 50 trapped individuals and 30 nest despoliations) in the former Czechoslovakia (W. Bergerhausen pers. comm., Frey 1981, Šťastný et al. 2006): at the beginning of the last century there were only 20–25 pairs remaining. Starting in 1929 the species was protected and, since then, it has increased relatively rapidly: in the 1940s the breeding population was estimated at 75 pairs, in the 1970s at 400–600 pairs, at the end of the 1980s at about 600– 1,000 pairs (W. Bergerhausen pers. comm., Zemanová 2009) and, currently, the number of breeding pairs is estimated at 600–900 pairs. The first figures obtained from the recently constituted Czech Republic revealed that a total of 115 and 95 Eagle Owl nests were found in the country in 1993 and 1994, respectively (Voríšek 1995a,b). Specific information comes from: (1) an area of the Czech-Moravian Highlands (S Czech Republic; Kunstmüller 1996), located at an altitudinal range of 340– 800m a.s.l., in which 81 breeding sites were found: the number of pairs varied from 6.0–8.2 pairs/100km2, with an average distance between breeding sites ranging from 2.4–7.5km; (2) the western part of the Nízký Jeseník and southeastern sector of the Hrubý Jeseník Mountains, where a population had been followed starting in the mid-1950s (Suchý 2001). The population size began to increase in the 1970s, peaked in the 1980s and remained relatively stable between the end of the 1980s and the beginning of the 2000s, when the recorded nesting density was 1.5 pairs/100km2; a relatively large number of pairs (44.6%) did not breed successfully because of an insufficient food supply and unfavourable weather conditions, whereas failures during incubation were mostly due to human disturbance. Electrocution is the major threat in this area; (3) between 1983 and 2003 the Czech Ornithological Society collected data on the status of the Eagle Owl in western Bohemia (Plzeň and Karlovy Vary region; Schröpfer et al. 2005). According to estimates, the population has declined from 150 breeding pairs at the beginning of the 1990s to 50–100 in 2000–2003; at the same time, a continuous decline in fecundity was recorded: between 1996–2003 reproduction declined by an average of 32%. Most of the losses are suspected to be the result of human persecution; and (4) recently, the species has colonised lowland floodplain forests in south Moravia (Břeclav and Znojmo districts; Horal & Škorpíková 2011): in this habitat, Eagle Owls have bred in large, wooden nest boxes originally installed for Saker Falcon Falco cherrug, as well as in raptor and Black Stork Ciconia nigra nests. Current breeding population is estimated at 600–900 pairs (Šťastný et al. 2006, Hora et al. 2010): the average density calculated for 22 areas was 4.1 pairs/100km2 (range = 0.9–10.5 pairs/100km2). In six river valleys, the density per length of water course averaged 1.8 pairs/10km of river (range = 0.9–3.2 pairs/100km2). Nowadays, Eagle Owls are mainly killed by power lines and vehicles, but illegal shooting and taking of nestlings still occur. In addition, reproduction frequently fails mainly because of human disturbance (Hora et al. 2010). Trend: stable

Slovakia The Eagle Owl is widespread throughout almost the whole country, with the exception of the lowlands of western Slovakia, the northwestern part of eastern Slovakia, and a part of the Východoslovenská rovina Plain (Darolová 2011); the population is estimated at 300–400 80

Distribution and population estimates

pairs (Danko & Karaska 2002, Dravecky & Guziová 2012). Similar to what has occurred in the woodlands of the Czech Republic, the Eagle Owl has been found breeding in raptor stick nests (Šotnár 2007) and nest boxes (Noga, pers. comm.). Additionally, there are some records of breeding around man-made buildings, e.g. on a railway bridge located on a disused railway line at the end of the 1980s (Karaska 1995), in the tower of Bojnice Castle (Danko et al. 2000), on the walls of a ruined castle and on a gamekeeper’s hide (Hudec & Šťastný 2005), as well as in a water tower (Hrtan 2010). Trend: increasing

Hungary Historical data from the 19th century suggests that the apparently recent occupation of floodplains recorded in some countries of Central and Eastern Europe might be a recolonisation of ancient breeding areas from which Eagle Owls disappeared, for some reason (e.g. ease of access by men), several centuries ago. Actually, historical data from Hungary relates that Rudolph, the son of Emperor Franz Joseph, went hunting along the Danube by boat and, according to his memoirs, during the 15 days he spent hunting on the islands and floodplains of the Hungarian Danube, Eagle Owls were shot relatively often (Mátyás Prommer, pers. comm.). This indicates that two centuries ago this species already inhabited those riparian habitats. Some time ago the species was widespread throughout Hungary, but from the middle of the 20th century to the 1980s it steadily declined, mainly as a result of persecution and egg/young robbery by collectors and taxidermists (Gorman 1995), until only 15 pairs remained (Márkus 1998). Due to this marked population decline, a restocking attempt was undertaken in the 1980s (Solti 2006). However, only 100 individuals were released in the mountainous areas and due to the lack of financial support there was no followup of this project (Mátyás Prommer, pers. comm.). Starting from the late 1990s a slow population increase was observed, and the population was estimated at 15–17 pairs at the beginning (Gorman 1995, Firmanszky et al. 2006) of the 1990s and 27–31 at the end of that decade (Firmanszky et al. 2006). Depending on the available sources and date of the estimation, the current population is approximated at 34–40 pairs (Bagyura 2003, Mátyás Prommer, pers. comm.), 39–50 pairs (Firmanszky et al. 2006), 46–60 pairs (Petrovics 2006, 2009a, 2010) or 70 pairs (Kovács et al. 2012), primarily located in the northern mountains and the western part of the country (Solti 2006). The main areas inhabited by Eagle Owls include the mountain ranges of Zemplén, Bükk, Mátra, Bakony and Dunazug, as well as the Szatmár-Bereg area; recently, due to population increase in Austria, the species has expanded its range to the east (Miklós 2008) and has started to spread along the Danube and Tisza Rivers, with at least one pair nesting in a heron colony (on an island of the Danube; Mátyás Prommer, pers. comm.). Today, the main risks are electrocution, motor vehicle traffic and wire fencing (Petrovics 2007). Following the ‘modern’ trend of urban Eagle Owls, they have also recently been observed in Budapest (Lendvai 2010). Trend: increasing

Romania In Romania there are two distinct subspecies, Bubo bubo bubo (the breeding subspecies) and Bubo bubo sibiricus, which is a wintering visitor only (mainly in the Dobruja Province and 81

The Eagle Owl

the Black Sea area; Nicola Pârlog, pers. comm.). Eagle Owls seem to be quite widespread in Romania, where the breeding population is estimated at 1,000–1,500 pairs (Nicola Pârlog, pers. comm.). Main areas of presence are the Carpathian Mountains, the Danube River and Delta, as well as the Măcin Mountains (Dobruja). In the last decade, some individuals have been also observed in towns (Nicola Pârlog, pers. comm.). Trend: unknown

Slovenia The Eagle Owl in Slovenia was already reported in the 18th and through the 19th centuries as a breeder of mountain forests (Scopoli 1769, Freyer 1842). At the end of the 19th and beginning of the 20th century it was considered as common in most of Slovenia, but scarce in northeastern sectors of the country (Ponebšek 1917). However, heavy persecutions determined the decline of the species during the first half of the 20th century (Beuk 1920). At the end of the 20th century the population size was estimated at 50–100 breeding pairs with the distribution confined mainly to the western part of the country (Geister 1995). However, more recent surveys estimated the current population size at 120–140 breeding pairs, and many breeding places were also found in E Slovenia (Tomaž Mihelič, unpublished data). The discovery of breeding pairs in new sectors of the country was due to both more intensive researches (which took place in the last two decades, e.g. Lipej 1995, Mihelič & Marčeta 2000, Mihelič 2002, 2008) and recent population increase. Nowadays, the species is not only confined to the most remote regions, as reported in the past, but is frequently breeding in lowlands and quarries close to human settlements. Disturbance of breeding sites (mainly by rock climbers; Mihelič & Marčeta 2000) and electrocution (the main cause of mortality) are the main conservation problems, which seem to be especially high in SW Slovenia (Mihelič 2008, Mihelič & Denac 2010). Trend: increasing

Croatia At the end of the 1980s, Tutiš et al. (1990) reported that, for the whole former Yugoslavia, Eagle Owls were mainly concentrated in the Mediterranean and mountainous sectors of the country, although direct persecution and habitat changes led to the decline of the population, mainly in the north. In Croatia the species was reported to breed more commonly on the Adriatic coast (where it can reach relatively high densities; Vesna Tutiš, pers. comm.), whereas inland it was heavily persecuted (Kralj 1997). Indeed, at the end of the 19th century, the Eagle Owl was also widespread in Lowland Croatia (Pannonian and Peri-Pannonian areas) but in the mid-20th century it disappeared as a breeder from that area. During the 1980s, the presence of the species was confirmed in both Mediterranean (Istria, Kvarner and Dalmatia) and interior (the area of northwestern Dinaric Alps) Croatia, giving a population estimate of 1,000–1,500 pairs. The species is quite abundant in the islands of the central part of the eastern Adriatic coasts in N Dalmatia, e.g. 2.4 breeding pairs/10km2 at Dugi otok (Bordjan 2002) and from 4.5–26.2 pairs/100km2 (Vesna Tutiš pers. comm., Barišić et al. 2016). The most recent estimate for the Croatian breeding population is 800–1,200 pairs (Vesna Tutiš, pers. comm.). Trend: unknown 82

Distribution and population estimates

Bosnia-Herzegovina At the end of the 19th century, the Eagle Owl was considered as the most common owl species in Bosnia and Herzegovina (Reiser 1939). Nowadays, the Eagle Owl is considered a regular breeder in this country, with preliminary estimates of the current population at 400–500 breeding pairs (Kotrošan & Hatibović 2012). Trend: unknown

Serbia The most recent assessment of the breeding population estimated 330–450 pairs (Milan Ruzic, pers. comm.), mostly located in the eastern parts of the country, especially in limestone gorges, which are among the most favourable habitats. The species has recovered for the most part from the historical decline, which was mainly due to persecution and illegal killing by hunters and farmers. It might also be the case that the use of some pesticides also contributed to this past decline. Nowadays the species is spreading more to the north and west and it is slowly settling in the Panonnian Plain, usually along large rivers such as the Danube. Recently, Eagle Owls have also colonised the Deliblato Sands Nature Reserve, which is home to the largest inland sand dunes in Europe, where they can find abundant prey (Milan Ruzic, pers. comm.). Trend: increasing

Bulgaria In the 19th century and the beginning of the 20th century the species was fairly common and very widely distributed, predominantly (Hristov et al. 2007): in the Provadiya Valley; near Burgas, Ravda and Nesebar; in Dobrudzha; on the rocks in the city of Plovdiv; in the Western Rhodopi to the South of Pazardzhik (S Bulgaria), Varvara and Perushtitsa; in rocky areas around the Sofia Plain (as well as the periphery of the town itself ); in the gorge of the Yantra River; in the Central Balkan region, at the foot of the Rila Mountains (SW Bulgaria), but also higher up, in the Sitnyakovo region; and along the Danube River (Oryahovo and on an island near Vidin). Around 1950 the Eagle Owl was still a quite common and widespread species, in both the plains and the mountains (Hristov et al. 2007). During subsequent decades the population started to decline (Simeonov & Michev 1985), owing to the mass extermination of birds of prey. After 1975 some increase in the number of pairs was observed, with the highest densities recorded at Iskar Gorge, Lomovete, the valleys of the Provadiyska, Rositsa and Suha Reka rivers, the middle current of the Struma River and the eastern Rhodopi (Hristov et al. 2007). The total number of breeding pairs during the 1980s was estimated to be 120–150, most of them in northern Bulgaria and especially in Ludogorie and Dobrudzha (Simeonov & Michev 1985), with the highest densities of the species recorded in Strandzha, along the Black Sea coast, and around Burgas and Karnobat (Hristov et al. 2007). Pesticides appeared to represent one of the major threats at that time, together with persecution (Simeonov & Michev 1994, Nankinov 2002). After 1990 the species benefited from the collapse of economic activities in many parts of the country, and breeding in abandoned buildings has been recorded since the beginning of the 2000s (Miltschew 2003). The current population size has been estimated at 420–490 pairs (Hristov et al. 2007), which exhibit a patchy distribution throughout the whole country; more 83

The Eagle Owl

dense concentrations of breeding pairs are recorded in SE Bulgaria, e.g. Strandzha, Burgas, Karnobat, Lomovete, as well as in the eastern Rhodopi, eastern Balkan, western Balkan and the mountainous areas to the south of it. High densities have also been recorded along the northern Black Sea coast (sea cliffs and rocky shores). To some extent, the distribution of Eagle Owls is influenced by the dynamics (opening and closing) of quarries, which allows the species to breed in otherwise unsuitable areas (Hristov et al. 2007). Trend: increasing

Montenegro At the end of the 19th century Reiser & Führer (1896) considered the Eagle Owl as common and abundant in Montenegro; later, in the 20th century, Matvejev & Vasić (1973) confirmed this information, although there is no recent information available on population numbers and trend. Trend: unknown

Kosovo No available information, except for a single estimate of 40–80 pairs (Milan Ruzic, pers. comm.). Trend: unknown

Albania No available information

Macedonia A recent assessment of the breeding population estimated 100–300 pairs (Velevski et al. 2010). The species has been recorded from the lowest parts of the country (100m a.s.l.) up to 1,500m (although the presence of breeding Eagle Owls at higher altitude is possible), and in a variety of rocky habitats (gorges, ravines, cliffs and sand cliffs). Trend: unknown

Greece In the 19th century the Eagle Owl was regarded as common inland and scarcer on the islands (Reiser 1905). Later, in the 20th century, it still appeared to be more common in the interior and generally absent on islands, except for Lesbos and Kalamos (Handrinos & Akriotis 1997). Today, although the population is estimated at 200–500 breeding pairs, it might be that it exceeds 500 pairs. Trend: unknown

Turkey No available information 84

Distribution and population estimates

Portugal The Eagle Owl is the only owl species in Portugal that appears to be increasing, most likely as the result of land abandonment and, perhaps, a decrease in human persecution (Cabral et al. 2005, Catry et al. 2010). The recent increase of the species in Spain may also have favoured its possible increase in Portugal (by ~25%; Lourenço et al. 2015a). Nevertheless, because some local populations may have decreased due to habitat loss and prey decline, it is also possible that the recorded population increase may partially result from an increasing monitoring effort (Lourenço et al. 2015a). The Eagle Owl occurs throughout most of continental Portugal, having a more regular distribution inland, especially in the south (Figure 16): the increase of the breeding population between the 1980s and today is certainly also due to greater monitoring effort in comparison to that undertaken for the first breeding bird atlas (Rufino 1989), during which time the Eagle Owl may have passed unnoticed in several areas where it occurred. However, the enlargement of the distribution area may also be a consequence of the possible population increase mentioned above, namely in Ribatejo and Estremadura (central Portugal; Lourenço et al. 2015a). Indeed, the size of the breeding population was roughly estimated at 100–1,000 pairs at the end of the 1980s (Rufino 1989), at 250–500 pairs by Cabral et al. (2005) and, more recently, at 380–580 pairs (Lourenço et al. 2015a). The mean density calculated for the whole country is 1.8 breeding pairs per 100km2 (Lourenço et al. 2015a). Currently, the Eagle Owl is nearly threatened in Portugal. Between the Douro and Minho rivers the species inhabits the massifs of Arga, Amarela and the Castro Laboreiro area (Peneda Massif ) at very low densities. In the Trás-os-Montes region the most commonly occupied locations are Douro Internacional Natural Park and the Sabor River. In the central coastal half of the country the species is relatively rare and has been recorded in the massifs of Freita, Sicó, Aire and Candeeiros. In the Beira Interior region the species has been found along the rivers Côa and Águeda, Penha Garcia hills, in Tejo Internacional Natural Park and Portas de Ródão. In the region of Lisbon and the Tagus

1978–1984

1999–2005

2005–2013

Figure 16. The evolution of the breeding population in Portugal since the end of the 1970s to 2013 (data from Rufino 1989, Equipa Atlas 2008, Lourenço et al. 2015a; the variously sized circles denote different data sources).

85

The Eagle Owl

Valley the Eagle Owl inhabits several hilly areas near Tomar and Entrocamento, and it is also present between Montejunto and Sintra, as well as in Espichel and Arrábida. The Alentejo region has a high density of breeding pairs, mainly located in the São Mamede massif, Cabeção, in the Monfurado hills, and along the Guadiana River and its tributaries, especially in the surroundings of Moura, Mourão, Barrancos and Mértola. Finally, in the Algarve region the species is present in the tributaries of the Guadiana River (near Alcoutim), in the Rocha da Pena area and near Sagres (Lourenço et al. 2015a, Rui Lourenço pers. comm.). Trend: increasing

Spain Starting from the end of the 1970s, the studies conducted in many different areas of Spain have largely shown a high heterogeneity in breeder densities: (1) Donázar & Ceballos (1984) estimated the population in Navarra (N Spain) at 40 pairs in the 1980s, mainly distributed in the southeastern part of the region and along the lower sections of massifs, i.e. between 400–800m a.s.l.; (2) between the end of the 1970s and the beginning of the 1980s, a study in Catalonia (Vallès and Bages, NE Spain; Real et al. 1985) demonstrated that this species suffered a serious decrease in the more mountainous areas, in which the average distance between two neighbouring pairs was 8km, whereas the density in the lower part of the region remained relatively high, i.e. the average distance between two pairs was 2.4km and the minimum was 1km; however, fecundity was in general relatively low (1.5 juveniles/pair), especially in the mountainous sectors. A subsequent study (Beneyto & Borau 1996) estimated the population for the whole of Catalonia at 740–860 pairs, most of them being concentrated in the Mediterranean areas located in central and southern Catalonia (ca. 375–400 pairs); although the species may inhabit the Catalonian Pyrenees up to 1,600m a.s.l., most of the pairs were concentrated in the pre-Pyrenean sectors, where 250–300 pairs were estimated. Thus, the species was distributed from sea level (Garraf, Costa Brava, Islas Medes) to the Axial Zone of the Pyrenees, being absent from the lowlands of Lleida, Ebro delta, Empordà, as well as from the most urbanised areas (e.g., Vallés, Barcelona, Camp de Tarragona), where some isolated pairs may still be found. In the county of Penedés (central coast of Catalonia; Beneyto & Borau 1996), some sectors exhibited a high density of breeding pairs (1 pair/10km2; mean distance between pairs = 2.4km), although the density for the entire study area (40 pairs/700km2) was 1 pair/17.5km2. Here, the shortest distance between neighbouring pairs ranged between 1 and 1.7km, and mean productivity was two young per pair per year; (3) at the beginning of the 1990s, a study on Eagle Owl distribution in the region of Murcia (southeastern Spain, Martínez et al. 1992) assessed the population at 200–250 pairs, following the regular distribution of natural cliffs and with a density of 1 pair/50km2. The mean distance between nests, estimated for two local subpopulations, was 5.1km (range = 2.6–6.5km) and 4.2km (range 2.3–10.6km), and mean breeding success was 2.9 young fledged per laying pair; however, the appearance of a new viral disease, viral hemorrhagic fever, affecting the local Rabbit population (the main prey of Eagle Owls in this area) strongly affected breeding performance, with the number of pairs that laid eggs and fecundity falling by 48.4% and 60.2%, respectively. Subsequent assessments estimated the number of pairs at 182–220 during the mid-1990s (Sánchez-Zapata et al. 1996) and 140–170 ten years later (Martínez & Calvo 2006), showing a slightly decreasing trend of the species in this region, probably as a consequence of Rabbit decline, which negatively affected the Eagle Owl population 86

Distribution and population estimates

together with electrocution, persecution and car collisions; (4) in the area of Burgos (Alava and Treviño, N Spain), Fernandez (1993) reported a relatively low density (probably due to the rather scarce food resources and the reduction of Rabbit populations), with a mean distance between pairs of 5km (range = 1.8–13km); (5) in the eastern Sierra Morena (S Spain), RuizMartínez et al. (1996) reported a density of 1.7 pairs/100km2, which is markedly lower than the very high density reported for the western Sierra Morena (Sierra Norte of Seville), i.e. 40 pairs/100km2, mainly due to Rabbit abundance (Campioni et al. 2013, Penteriani et al. 2015); (6) 49 breeding sites were found for the whole of Alicante Province (SE Spain, Martínez et al. 1996a), although at the beginning of the 21st century Martínez & Zuberogoitia (2003a) estimated the population at 100–120 pairs. In the south of the province of Alicante (e.g. Sierra de Escalona), a very high density of breeding pairs was detected by Pérez-García et al. (2012): altogether, 99 breeding sites were detected with a mean density of 22 pairs/100km2 (mean distance between pairs = 0.9km, range = 0.1–2.5km), but when analysing the densest sector of breeders, the density increased to 46 pairs/100km2 and the mean distance dropped to 0.7km. This is probably the highest density of breeding pairs ever reported for the species, which is probably due to a high local abundance of Rabbits (>6 Rabbits/ha); (7) at the end of the 1990s, three breeding pairs and five areas occupied by lone individuals were detected in Biscay (Basque Autonomous Community, N Spain; Zuberogoitia & Campos 1997). Owing to the low density, during a five-year period a total of 64 Eagle Owls were released (Zuberogoitia et al. 2003): the individuals came from wildlife rehabilitation centres located in Madrid, Cádiz and Huelva (central and S Spain). After 101 days 19 of these individuals were found dead (their mean dispersal distance was 9.6km), ten Eagle Owls had settled in new breeding sites (the average distance between the release point and these sites was 8.1km) and three new cases of reproduction were recorded. In the mid-2000s, the results of a specific study in Álava (a province of the Basque Autonomous Community) recorded the presence of 15–20 breeding pairs and a density of 0.7 pairs/100km2, separated by a mean distance of approximately 6km (although the distance between some pairs was around 1km; Martínez de Lecea et al. 2006, Illana et al. 2010). However, in some sectors the density was much higher and reached 5.1 and 7.5 pairs/100km2 in the Rioja Alavesa (nearest neighbour distance between pairs = 2.3km) and Portillo-Toloño areas, respectively; however, the species appears to be completely absent from the Cantabrian slopes, in most of the central area (Llanada of Álava) and most eastern sectors (Izquiz). Subsequently, a study in Alavesa Mountain (SE of Alava Province; Illana et al. 2012) reported 0.9 pairs/100km2 and a mean distance between breeding sites of 6km (max = 11km, min = 2km); (8) in the area of Toledo (central Spain), a very high density was reported by Ortego & Calvo (2004), i.e. 20 pairs/100km2, with a mean nearest neighbour distance between pairs of 1.3km. Again, similar to the western Sierra Morena, the high availability of Rabbits most likely explains the recorded high density; (9) eight pairs have been found in Galicia (NW Spain) between 2000 and 2005, most of them inhabiting Orense Province (Epifanio et al. 2006); (10) in the Community of Valencia Urios et al. (1991) estimated 250– 300 pairs, whereas more local assessments found 33 pairs in the municipality of Requena and Chera (Muñoz 2001) and 65 pairs in Vega Baja County (Pérez-García et al. 2007). More recently, in another county of Valencia, the district of Vall d’Albaida, 11–13 breeding sites were found, corresponding to a density of 4.1 pairs/100km2 (Pérez-García & Botella 2010); and (11) there are between 6–20 pairs in Asturias (N Spain), where the species is relatively scattered throughout the region and some breeding sites are located along the easternmost coast, as well as along the Cantabrian Cordillera and the Picos de Europa (Silva González 2014). 87

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The first estimate of the Eagle Owl population in Spain reported 400 breeding pairs in the 1970s (Garzón 1977), which may represent the result of both a decrease of the species due to persecution and the lack of specific knowledge for many sectors of the country. At the end of the 1990s the Spanish population was estimated at 520–600 pairs, although it was recognised that a better survey could increase this number to >1,000 pairs (Elósegui Aldasoro 1997), e.g. Andalucía 250, Aragón 50, Castilla y León 30, Castilla-La Mancha 250, Cataluña 150, Extremadura 50, Madrid 50, Murcia 20, Navarra 35, La Rioja 25 and Valencia 250 pairs. At that time, persecution, electrocution, collision with fences and the generalised decline of Rabbit populations were the main threat to the species. Following more recent estimations, the Eagle Owl is largely widespread in the whole of Spain, with the exception of Baleares and Canarias, and Ceuta and Melilla; the last assessment considers that there are at least 2,350 pairs, although this estimate represents the minimum number of pairs for the whole country (Martínez & Zuberogoitia 2003a). In fact (Martínez & Zuberogoitia 2003a, Epifanio et al. 2006, Penteriani & Delgado 2010, Penteriani et al. 2012): (i) new areas have recently been recolonised or occupied for the first time due to recent expansions of the species distribution range in Spain (Cantabria, Asturias, Galicia, Doñana National Park and surroundings, e.g. Guadalquivir and Guadiana River valleys); (ii) the settlement of urban pairs in towns like Córdoba, Jerez de la Frontera, Madrid and Seville; and (iii) the extreme eclecticism of the species, which can breed almost everywhere and, thus, may be easily overlooked during conventional censuses and estimates. Power lines are responsible for the highest number of deaths in Castilla-León (54.5%), Castilla-La Mancha (22.3%), Catalonia (22.2%) and Andalusia (21.3%), whereas persecution is the main cause of death in the Community of Madrid (27.0%), Community of Valencia (24.4%) and Region of Murcia (24.3%). In the Basque Country power lines and persecution are responsible for 47.1% of deaths (Martínez et al. 2006). Status: increasing

France Similar to what occurred in Spain, many specific studies have been carried out in different areas since the 1970s, which helps to understand the evolution of the species over the last few decades (see also Figure 17) in the different French regions and departments: (1) Auvergne region (central France) and central Massif Central: in the Puy-de-Dôme and neighbouring departments, only seven couples were present between the end of the 1960s and the beginning of the 1970s, which may have represented half the number of pairs breeding in this same area 15 years before (Choussy 1971). After the successive expansion of the Eagle Owl in the Puy-de-Dôme (Brugière et al. 1989), the Cantal (Brugière 1995), and the Creuse, where the species started to appear at the beginning of the 1990s (SEPOL 1993, Brugière & Duval 1993), 15 pairs were estimated to be located along the Dordogne River and its tributaries in the Corrèze department (Limousin region; Defontaines 2004a). More recent research (Martin 2008, 2009, 2013) revealed the presence of 100–110 pairs in the department of Puy-de-Dôme, which represents an increase of the population since the beginning of the 2000s, when the number of pairs was estimated at 80–100. In less than 40 years the breeding population of Eagle Owls has thus grown from fewer than ten to more than 100 pairs in the Puy-de-Dôme, providing an interesting example of the species’ ability to recolonise ancient breeding grounds; in fact, at the beginning of the 21st century, the 88

Distribution and population estimates

species seemed to be relatively common in this department (Cochet 2006). The most common local threats are electrocution and collision with cables, fences and motor vehicle traffic, whereas the percentage of deaths from persecution is relatively low (9–10%; Martin 2010a, 2011). Martin (2010b) reports 250–300 pairs for the whole of Auvergne, with the lowest density recorded in the Allier department, where the species is mainly concentrated in the south; however, due to a recent increase in the number of breeding pairs, the Eagle Owl population is increasing in the north of this department (Yvan Martin pers. comm.). The highest concentrations in the Allier department are in the Puy-en-Velay and in the south of Livradois, in the valley of the Allier River (after Langeac) and in the lowest sector of the Loire River Valley (ca. 40 pairs). Both the Allier and Loire Rivers have thus played the crucial role of population sources for the Eagle Owl in the NE of the Massif Central, especially during the periods of population decline (Patrick Balluet pers. comm.). The Eagle Owl was relatively abundant during the 1970s (Blondel & Badan 1976) in the departments of Loire, Haute-Loire and Ardèche (approximately 40 pairs), as well as around Lozère (30 pairs). However, a later estimate of the Ardèche population only (about 100 pairs; Cochet 1999) showed that, due to limited knowledge, these figures probably still underestimated the actual number of breeding pairs in most of the areas. Finally, at the end of the 1990s, 50–100 pairs were estimated for the Rhône department (central eastern France); the Rhône department has been recolonised by Eagle Owls coming from the west (Loire; Patrick Balluet pers. comm.), as also happened for the Saône-et-Loire department, where the species had been absent since the 1970s (Gaget et al. 1999); (2) the lastest information for the region of Limousin comes from Haute-Vienne, where in 2013 the Eagle Owl started breeding again after an absence of 200 years (Goursaud 2013); (3) Provence (SE France): the first study on one of the highest density populations in France was conducted in the 1970s by Blondel & Badan (1976), who reported 8.5 pairs/100km2 and short distances between breeding sites, generally less than 2km (subsequently Bergier & Badan 1979, 1991 reported an even higher density of 20 pairs/100km2 for this area). At that time, population density was also high in the neighbouring departments of Drôme, Alpes de Haute Provence, Var and Alpes Maritimes. At the beginning of the 1990s, Bayle (1990) presented a synthesis of the knowledge of the species in the SE of France (19 departments), demonstrating that: (i) with the exclusion of five departments (Pyrénées Orientales, Aude, Drôme, Alpes de Haute-Provence and HautesAlpes), as well as the departments of Var and Alpes Maritimes (for which there was no reliable information at the time), the number of pairs in the area is estimated to range from 445 to more than 650; and (ii) the areas with the highest densities overlap with the distribution of Rabbits, i.e. Ardèche, Vaucluse, Hérault, Gard and Bouches-du-Rhône. However, when considering the whole of SE France, it was possible to estimate the breeding population at 750–800 pairs, despite direct persecution and inadvertent electrocution, which were considered the main threats at that time. More recently, a study in the Luberon Massif revealed a density of 15.3 pairs/100km2 and a mean distance between pairs of 1,770m (range = 700–4,300m; Penteriani et al. 1999, 2001, 2002a, 2003) for the whole area, although important differences exist between the interior areas of the massif (20 pairs/100km2, mean nearest neighbour distance between pairs = 1,808m, range = 800–3,500m) and the borders close to the Durance River (32 pairs/100km2, mean nearest neighbour distance between pairs = 1,353m, range = 700–3,000m). At the beginning of the 1990s approximately 40 pairs inhabited the Alpilles Massif, and recent research allowed for the estimation of 40–46 (Zenasni 2009) or 58–75 (Jérémie Demay, pers. comm.) breeding sites; (4) S-SW of 89

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the Massif Central (Tarn-et-Garonne and Hérault, between the Midi-Pyrénées and Languedoc-Roussillon regions): around 100 breeding sites were located at the beginning of the 1980s; 70–75 of them were located in the Hérault (although this number probably underestimates the number of breeding pairs here), 11 in Tarn, 10 in Montagne Noire Audoise and one in Tarn-et-Garonne (Cugnasse 1983). The highest densities were recorded in the sectors closest to the Mediterranean (~1 pair/100km2, average distance between breeding pairs = 3.3km). Again, in Mediterranean habitats, the highest densities were related to the presence of Rabbit populations; (5) Causses and Cévennes massifs: during the 1980s, Cochet (1985) highlighted the presence of a very low density population of 22 pairs/5,000km2, i.e. 0.4 pairs/100km2, probably due to low food availability. At that time, this population exhibited the lowest density in all of southern France. Again, at the end of the 1980s, Cochet (1989) depicted the situation of almost the whole of Haute-Loire, Lozère and Ardèche, where 190 breeding sites were found. In particular, the distribution of the pairs in the Massif Central was extremely scattered and principally limited to the lowest altitudes, with some areas having a relatively high density (5 pairs/100km2) and others nearly vacant or with a very low density (0.3 pairs/100km2) and mean annual productivity (1.4 young per pair); taken together, there were 1.3 pairs/100km2 in Haute-Loire, 1.5 pairs/100km2 in Ardèche and only 0.6 pairs/100km2 in Lozère. Thanks to these extensive surveys it was possible to estimate the breeding population at 450–500 pairs for the whole Massif Central, Causses and Cévennes. Further studies (Pialoux 1994) showed that the Haute-Loire breeding population, which was estimated at 33 pairs at the end of the 1970s, comprised 93–100 pairs in the 1990s; (6) Burgundy: the last recorded breeding of Eagle Owls occurred in 1933, whereas the last observation was made in 1939 (Abel 2007); thus, it is possible to consider that the species disappeared from this region at the beginning of the 1940s, mainly because of persecution. The Eagle Owl reappeared almost 50 years later, with the first observation dating back to 1996 in Saône-et-Loire, and in 1998 13 breeding sites were recorded in this department (Penteriani & Strenna 2000). In Côte d’Or the first calling individual was heard in 1997 and the first breeding attempt was recorded in 1999 (Abel 2007). As Burgundy is located between the abundant Eagle Owl populations of southern France and the German, Swiss and NE French (Lorraine, Ardennes, Meuse, Moselle, Vosges and Jura) areas where several release projects have been carried out, the recolonisation of Burgundy may be the result of both an expansion of the natural southern population and/ or the arrival of released birds. By 2009, 25–31 pairs inhabited the Saône-et-Loire department (most of them located in the Mâconnais), one pair the SE of Nièvre and three pairs the E of Yonne, whereas eight occupied breeding places were recorded for Côte d’Or (Abel 2007). The last estimation for the Burgundy region is almost 40 pairs (Strenna 2013). Most of the reproductions have occurred in quarries; (7) in some areas of Franche-Comté (E France), the species has been followed since the end of the 1970s (Michel 2010): the initial information was only represented by the recoveries of ringed individuals that had been released in Germany and Switzerland, and the first breeding occurred in Haut-Doubs in 1981; in 1995, 7–10 breeding sites were estimated for Doubs and 2–6 for Jura, where in 2009 there were a minimum of 34 pairs. In Jura, the first breeders were the result of the Swiss and French restocking projects, and the first pair resulting from these releases was observed in 1985 (Dronneau 1985); (8) in the department of Ardenne (NE France), after having been persecuted until its disappearance during the 1970s, the Eagle Owl appeared again at the end of the 1980s (probably as a direct consequence of the German releases) and, at the end 90

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1970-1975 1970–1975

1985-1989 1985–1989

2014 2014

Figure 17. The evolution of the breeding population in France (by department) from the 1970s to 2014 (data from Bayle & Cochet 1994, LPO 2012; Patrick Balluet, Pascal Demarque, Alain Leduc and Renaud Nadal, pers. comm.). In some departments the appearance of Eagle Owls may have been the result of an increase in surveying efforts only, but at least for northeastern departments this has been due to actual expansion of the population.

91

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of the 2000s, it was still rare and mainly concentrated in the Meuse Valley (Cochard 2008); (9) in the Nord-Pas-de-Calais Region the first record of an Eagle Owl was a dead individual ringed in Namur (Belgium), 111km from the French recovery site (Demarque et al. 2014). Since the beginning of the 21st century, the Eagle Owl has been found breeding in several quarries in the Nord-Pas-de-Calais Region and this growing population was estimated at no fewer than 12 pairs between 2011 and 2012 (N France; Dubois et al. 2011, Delgranche et al. 2011a,b, Dominique Douay pers. comm., Pascal Demarque, Alain Leduc & Bruno Stien pers. comm.). More recently, Demarque et al. (2014) estimated the number of breeding sites at 17–24 for the whole of Nord-Pas-de-Calais, an increase probably due to an influx of individuals from the healthy population (more than 100 pairs) of neighbouring Wallonia (Belgium), with most of the pairs breeding in quarries and one on a cliff along the shoreline; and (10) the Alps and Pyrenees mountains are the least well-known areas, although densities do not seem to be very high and most of the pairs are concentrated at the lowest altitudes (Bayle & Cochet 1994), e.g. in a sample of 25 pairs in Isère (a department in the RhôneAlpes Region in the east of France), 20 pairs bred at less than 700m a.s.l. Thus, the French population is mainly native (Pyrenees, Massif Central, Languedoc, Provence and Alpes), the areas reached by released individuals being prevalently Jura, Vosges, Lorraine, Ardennes, Nord-Pas de Calais and Burgundy (Patrick Balluet pers. comm.). Since the beginning of the 20th century, it appears that the Eagle Owl has been widely distributed (Bayle & Cochet 1994), e.g. (i) in the Ardenne Massif, it was common in the Meuse Basin until 1985, (ii) it was breeding in the Vosges Mountains until 1914, and (iii) at the beginning of the 1930s, the species was still present in Burgundy (Côte-d’Or and Avallonnais). The estimate of the French population (ca. 100 pairs; Terrasse 1964, Yeatman 1976) for the beginning of the 1960s to 1970s is not reliable due to the absence of specific studies on the species. One decade later, the first studies on the species provided an estimate of the French population at several hundred pairs (Terrasse 1977), still an underestimate due to lack of knowledge. Since the end of the 1980s and the beginning of the 1990s, the French population can be considered partly native (mainly in the south) and partly (in the NE of the country) the result of the restocking projects carried out in neighbouring countries (Bayle & Cochet 1994): the estimated number of pairs at that time was 1,000. By 2009, the French population was estimated at >1,600 pairs by Zenasni (2009) and, more precisely, at a minimum 1,649 pairs by Cochet (2006): four in the Ardennes, 17 in Lorraine, 10 in the Vosges, 60 in the Jura Massif, 28 in Burgundy, 700 in the Massif Central, 300 in the Alps, 250 in Provence, 180 in Languedoc-Roussillon and >100 in the Pyrenees (because already 60 pairs have been located in a small sector of the Pyrenees, i.e. Ariège and HauteGaronne, Thomas Buzzi pers. comm.). Both these assessments are similar to the 950–1,500 pairs estimated at around the same time by Martin (2010b). The species is not breeding in Corsica, even though some occasional individuals have been observed (Bayle & Cochet 1994). Today, direct persecution is relatively rare, the main threats being electrocution, disturbance due to outdoor activities and traffic collisions. Status: increasing

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Italy During the 19th century the Eagle Owl was a relatively common species throughout almost the whole country (Sicily included, but with the exception of Sardinia; Salvadori 1872, Giglioli 1889) and it even inhabited cities like Florence and Rome (Temminck 1835). At the end of the 20th century the species was considered rare and locally endangered, but this perception was solely the result of a lack of specific information and research on Eagle Owls in Italy: the estimate of 100–150 pairs (Brichetti & Meschini 1993, Chiavetta 1988) is clearly a substantial underestimate of the population. Although at the beginning of the 21st century Brichetti & Fracasso (2006) estimated the Italian population at 250–340 pairs, the most precise information was more recently offered by the Gruppo Ricerca Gufo reale Italia (GIERREGI, Italian Research Group on the Eagle Owl), which estimated the population of Italian Eagle Owls at 417–565 pairs (GIERREGI unpublished data). Most of the group activity is concentrated in the north of the country and, thus, this figure should be considered the minimum number of pairs. In fact, for some regions in the centre and south (e.g. Latium, Umbria and Molise), the situation is still poorly known or, as in Tuscany, the species seems to have recently disappeared or, at least, the density is so low that the bird is extremely difficult to find (Rigacci 1993, Liberatori et al. 1997). More accurate estimations of breeding pairs exist for Lombardy (109–134 pairs), Trentino-Alto Adige (60–90), Veneto (53–62), Liguria (43–65), Piedmont (48–75, with 21–26 in the Turin province, Marotto 2012, and 14–25 in the Cuneo province, Caula & Beraudo 2014), Friuli Venezia-Giulia (20–25) and Aosta Valley (10–15). In the Alps and pre-Alps, where the species is more common in the central and eastern sectors, the distribution seems more homogeneous than in the Apennines. Although we cannot discard the possibility that this may also be due to the more limited knowledge of the status of the species in the latter region, it is also true that some specific studies in central and southern Italy have effectively shown that the density of the species in the Apennines is low (Penteriani & Pinchera 1990a,b, 1992, Muscianese 2006). The Apennines population has two main problems, one due to human activities and the other determined by its geographical position. Firstly, in the best breeding areas (from the bottom of the valley to the beginning of beech forests) the density of dangerous medium-voltage power lines is extremely high and, over time, it has created a sort of selection for the highest-altitude (but also the lowest-quality) breeding areas (leading to a scattered, low-density and high-altitude population). Electrocution on medium-voltage pylons and cables also represented the most important cause of unnatural mortality for Eagle Owls in the Italian Alps (Sascor & Maistri 1996, Sergio et al. 2004a), as demonstrated for the period 1960–1999, when it accounted for over 50% of recorded casualties (Rubolini et al. 2001). Secondly, the Apennines (and especially their southernmost part) are relatively isolated from other important breeding nuclei of Eagle Owls, being completely surrounded by the sea except in the north. Thus, we can hypothesise that the relatively short dispersal distances that are typical for the species (see Chapter 10) hinder the arrival of new individuals able to replace deaths or reoccupy empty breeding sites (where, nevertheless, most of the new breeders will be under a high risk of electrocution; Penteriani & Pinchera 1990a,b, 1992, Sergio et al. 2004a). For some areas, relatively recent and detailed information exists for density and trend: (1) in the Imperia province (Liguria, NW Italy), Toffoli & Calvini (2008) highlighted an irregular distribution of breeding sites (most of them located in the lower sectors of the valleys, up to 500–600m a.s.l.), which are generally separated from each other 93

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by approximately 5km (range = 1.5–23.9km, 1.8 pairs/100km2), although in some westernmost sectors the density increases (6.1 pairs/100km2) and the distance between pairs may decrease to 2.4km (range = 1.5–3.7km); in some areas, the density may be even higher, with 7.7 pairs/100km2 (Calvini & Valfiorito 2012). The availability of both food and nesting sites seems to explain the recorded differences in density within the study area; (2) in the Sondrio province (low Valtellina, Lombardy), the density is 5.2 pairs/100km2 (mean distance between pairs = 4.3km, range: 3.7–5.3km; Bassi et al. 2011, Brambilla et al. 2012); (3) in some sector of N Italy pair density is highly variable, e.g. less than 1 pair/100km2 and a minimum distance of 10km between breeding sites for the Alto Adige (Sascor & Maistri 1995, 1996), about 1 pair/100km2 in the Brescia Province (mean distance between breeding sites = 9km; Leo & Bertoli 2005), on the western valleys of Piedmont (Fasce et al. 1988) and the Verbano-Cusio-Ossola (the northernmost province in the Piedmont; Bionda 2003), or up to 3.9 and 4.5 pairs/100km2 in the Trento Province (Marchesi et al. 1999a) and a particular area of Verbano-Cusio (Bionda 2002), respectively; (4) in the central-eastern Italian Alps and pre-Alps (Province of Trento; Marchesi et al. 1999a, 2002), most of the breeding sites are located at low altitudes (around 450m a.s.l., as also highlighted for other alpine populations, e.g. Tormen & Cibien 1993, Sascor & Maistri 1995, 1996, Toffoli & Bionda 1997) and near villages and valley floors, with a density of 1.8 pairs/100km2 and a mean distance between pairs of 3.5km; (5) in a large area of the Abruzzi Mountains (3,500km2), a very low density (0.3 pairs/100km2, mean distance between breeding sites = 21.5km, range: 10–29km) was found between 1987 and 1990. When the census of breeding sites was repeated in a sub-sample of the original area 25 years later (2012–2014), the density was practically the same: landscapes, prey availability and, above all, the main threat to the species in this area (the amount of dangerous power lines) have remained almost unchanged (Penteriani & Pinchera, unpublished results). Similarly, Cattaneo (in press) reports that the current distribution of the Eagle Owl in NW Italy is comparable to the distribution described more than 120 years ago by Giglioli (1889), which could indicate that in some areas breeding sites can be extremely stable over time; (6) in Apulia (S Italy), the species was considered relatively common during the 19th century, but since then it has probably started to decline, e.g. recent information seems to confirm that the species has disappeared from the Gargano (Rizzi et al. 2010), where it was recorded breeding since the 1960s. Moreover, some pairs still breed in the ‘Gravine’ areas, Mount Aquilone, as well as the Mattinata and Rignano Garganico valleys; and (7) the Eagle Owl is currently extinct in Sicily (Sarà et al. 1987), where it probably disappeared at the end of the 1970s, and it has never bred in Sardinia, although a few observation records do exist, e.g. an individual flying over the sea 8km from the southern coast (Grussu et al. 2001). Status: unknown

General overview The estimated minimum and maximum number of breeding pairs (20,130–28,952; Table 4) represents a relatively accurate assessment of the European population, even considering the fact that (i) there is no information of any kind available for Crimea and (ii) some estimations at the country level may be the result of simple extrapolations from local studies or relatively subjective assessments. The positive aspect of the current estimate is 94

Distribution and population estimates

that it took into consideration the most recent information (sometimes still not published) on the status of the species for nearly every European country. Evidently, the assessment we presented here still represents an underestimate of the global Eagle Owl population because we lack reliable information for the whole of Russia and Central Asia, and several thousands of pairs may inhabit these regions. However, we preferred not to attempt any subjective estimation of the number of pairs in that part of the distribution range of the Eagle Owl to avoid degrading the quality of the quantitative data we presented here. In the past, estimates for the whole population have been attempted, e.g. the 2,000–20,000 pairs for Russia reported in Tucker & Heath (1994), but we consider that this kind of range is much too large to allow reliable assessments. Previous, older estimates are not really comparable because, as in the case of Mikkola (1983) who presented data on an even smaller sample of countries (16 countries), we lacked specific studies on the species in many countries and, regardless, the possibility we have today of successfully collecting the most recent information from everywhere in the world is greater than 30 years ago. Thus, the estimate provided by Mikkola (4,571–6,472) is most likely even more biased than the one presented here and does not allow for comparisons: the huge difference between the maximum 6,000 pairs reported in the 1980s and the almost 30,000 pairs reported here does not reflect, in the least, a real increase in Eagle Owl numbers in Europe (although, since then, the species has been increasing in some areas). More than ten years after Mikkola (1983), Penteriani (1996) reported a more detailed estimate (10,079– 12,362), but, again, while some information was not available for several countries, the results of a number of specific studies were not available or still in progress. One year later, Hagemeijer & Blair (1997) published a very similar estimate (10,353–12,926) that, although it did not include Russia at all, confirmed the knowledge of the species at the end of the 1990s. Hagemeijer & Blair also stated that altogether some 60% of the European population was in decline. Within the 43 countries/areas analysed, the status of the Eagle Owl population was unknown in 39% of them, increasing in 30.2%, decreasing in 21% and stable in 4%: compared to the data offered by Hagemeijer & Blair (1997), the percentage of the European population that is in decline today is lower than at the end of the 1990s, as only around 30% of the population is currently decreasing. Although the species is generally decreasing in the northernmost countries (Scandinavia and European Russia), which represents one of the strongholds of the European populations, Eagle Owls are now showing positive trends in Germany, France and Spain, where the numbers are between 1,000 and more than 2,000 pairs per country. On the basis of the numbers reported in Table 4, the most severe declines have been observed in Norway and Finland, whereas the most notable increases are the ones occurring in Denmark, the Netherlands, Belgium, Luxembourg, Germany and Poland. More difficult to evaluate are the apparent increases observed in France and Spain: probably half of the actual population represents a real increase, whereas the other half comes from the improvement in the acquisition of information. The largest numbers of breeding pairs in Europe are now located in Spain, France, Finland and Germany (Figure 18); however, when correcting the number of pairs for area of the country, the highest densities represented as pairs per 1000km2 are in several Eastern Europe countries and Luxembourg (Figure 19). Probably, smaller countries with more homogeneous landscapes allow for more homogeneous distributions of the species that, ultimately, leads to populations more widely spread across the entire geographic area than in bigger countries. 95

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Number of breeding pairs/1,000 km2

Number of breeding pairs 0

1,250

2,500

3,750

5,000

0.0

Spain

Croatia

France Germany

Czech Republic BosniaHerzegovina Slovakia

Romania

Luxembourg

Finland

Croatia

30.0

Portugal

Sweden

Serbia

Greece

Bulgaria

Italy

Finland

Belarus BosniaHerzegovina Bulgaria

Greece Austria

Portugal

Germany

Serbia

Romania

Slovakia

Estonia

Austria

Serbia

Serbia

Belgium

Montenegro

Albania

Poland

Switzerland

Ukraine

France

Estonia

Belarus

Switzerland

Macedonia

Belgium

Slovenia

Albania

Italy

Kosovo

Norway

Macedonia

Sweden

Hungary

Denmark

Denmark

Poland

Latvia

Latvia

Slovenia

Hungary

Britain Luxembourg

Ukraine

Lithuania

The Netherlands Lithuania

The Netherlands Minimum Maximum

Figure 18. Absolute number of estimated breeding pairs in Europe by country, shown in decreasing order.

96

22.5

Spain

Norway

Liechtenstein

15.0

Kosovo

Czech Republic

Moldova

7.5

Moldova Britain

Minimum Maximum

Figure 19. Number of estimated breeding pairs in Europe corrected for the area (km2) of the country, shown in decreasing order.

CHAPTER 3

The nesting site This chapter could have been the shortest of the book. Just a couple of lines: Eagle Owls may breed almost everywhere, use many and diverse natural or artificial structures to lay eggs, are able to exploit most nest orientations and can breed from sea level to relatively high altitudes. Indeed, people from different countries and landscapes may have very different feelings on where Eagle Owls live. For many decades Eagle Owls have been almost exclusively associated with cliffs and boulders, undoubtedly because nearly all of the scientific literature in the most common languages (English, Spanish, French and German) has consisted of studies carried out in rocky areas. The difficulty of reading Finnish or Russian works, for example, has prevented many people outside of the boreal range from associating this owl with flat terrain, forests and clear-cuttings; although we have to bear in mind that most of the Eagle Owl distribution range overlaps with boreal forests. Finally, we should also remember that dozens of years of persecution have forced Eagle Owls to live on the most inaccessible and remote cliffs, where they are relatively safe from humans. Or, at least, only those pairs that were living in remote areas and breeding on unreachable cliffs survived human persecution. Thus, breeding on remote cliffs may have mostly been the result of heavy persecution rather than a real preference for these places. As a clear example of the perception we have had of Eagle Owl nesting habitats at the beginning of the 1980s, we can cite what Cramp & Simmons (1980) wrote about Eagle Owl breeding places: ‘Surviving vestiges of wilderness, immune from human exploitation and from significant human disturbance…where such a powerful predator can find…sites for secure nesting and roosting. Such sites exist only in regions of sparse human settlement or in topographically inaccessible or forbidding terrain’. Are Eagle Owls birds of sunny, open landscapes dotted by rocks and Rabbits or are they forest-dwelling inhabitants preying on voles, Hares and grouses in foggy and frozen winters? We should say that they are both, without forgetting those Eagle Owls reproducing in trees 97

The Eagle Owl

or marshlands with a diet based on ducks and herons, as well as the urban Eagle Owls living among buildings and eating rats, crows and our pets. Indeed, nowadays, some dozens of years after the most intense persecutions, Eagle Owls have started (or restarted) to breed in many different nesting sites, frequently very close to humans. We cannot exclude that, even during periods of persecution, Eagle Owls were already breeding in some less ‘classical’ sites but, because of the peculiarity of such places, they were not pursued and eliminated there. And, consequently, we did not have any information of such ‘odd’ occupancies. Nevertheless, the heterogeneity in nest site choice, together with high fecundity (especially when compared to the fecundity of other big birds such as eagles or vultures), probably explains why this species is still well represented today and, if not persecuted, is able to show rather astonishing spreading in almost every habitat.

Nest characteristics The most typical nest of an Eagle Owl is a rounded depression (scrape) dug in the ground, and its dimensions generally depend on the thickness and softness of the soil. Indeed, such a depression can have a depth of only a few centimetres or can be a deep hole of more than 15cm; the depth of the nest cup is generally more variable than its diameter. For example, Mysterud & Dunker (1983) showed that such scrapes can range in length from 35–125cm and in breadth from 28–65cm, whereas the depth of nest scrapes can range from 5–13cm (average 9.7 ± 3cm). Görner (1983) described nest scrapes as hollows 30–35cm in diameter with a depth of 8–12cm, whereas Ryabtsev (1991) reported scrapes 20–35cm in diameter and 3–6cm deep. Kunstmüller (1996) reported an average dimension of 36.5cm in diameter by 5.3cm in depth. Finally, Gritschik & Tishechkin (2002) described the nest scrape during the early stage of reproduction (during the nestling stage the scrape frequently becomes larger and more flat) as having an average diameter of 33cm and a depth of 9cm. Fremming (1983) and Mysterud & Dunker (1983) reported that two to more than seven nests may be present in a single breeding site, and Leditznig et al. (2001) reported one to six nests per breeding site, whereas Olsson (1979) cited an average of 4.6 nests per breeding site. Olsson also found that Eagle Owls changed scrapes every third year on average, though variations were great. For example, in a nesting site one scrape was used 15 out of 16 years, while in other nesting sites four scrapes were used for five or six years. Disturbance or predation, weather conditions, individual preferences and/or experience and the result of the previous breeding season (e.g. after a successful breeding, individuals tend to return to the same scrape) are some of the reasons for nest changes. Incidentally, the presence or absence of terrestrial predators (which may target eggs and chicks, but rarely adults) might also determine the placement of the nest scrape (e.g. more or less hidden, and difficulty of access). Frequently, as also observed by Olsson (1979), Eagle Owls are strongly attached to one or two specific nests. The nests of the same breeding site may be very close to each other (2–3m), as also cited by Ryabtsev (2005). Nests are mainly dug with the help of both feet and bill, but the sides of the body are also used to rearrange the nest cup. Karyakin et al. (2009) reported that there were no scrapes dug in 23.3% of nesting sites, and thus the egg was laid directly on the ground. However, this situation is rare. Although the nest can be placed almost anywhere from the bottom to the top of a cliff (e.g. Leditznig et al. 2001, author’s pers. obs.), some authors (e.g. Karyakin et al. 2009) have reported that Eagle Owls 98

The nesting site

may avoid breeding on the top of cliffs. This may be true for most natural sites, but breeding on clifftops is relatively frequent in quarries, where Eagle Owls often place their nests just along the top edge of an artificial cliff, below a bush or small tree. This may primarily occur when quarries do not offer convenient locations (e.g. quarries with few cavities and smooth walls, sand quarries) in which to dig a better-protected nest cup (Robitzky 2007, Wassink 2011, author’s pers. obs.). The Eagle Owl, by depositing faeces, pellets and prey remains in the nest cup and the vicinity of the nest, enriches the topsoil in the immediate surroundings of its nest, which, in turn, enhances seedling growth (Fedriani et al. 2015). This means that the activity of Eagle Owls at nesting sites has the potential to locally increase topsoil nutrients and, consequently, may have an important effect on the surrounding vegetation by enhancing seedling emergence, plant growth and survival, as well as by altering patterns of plant distribution. Eagle Owls, like several other local ground-nesting birds (e.g. partridges, larks, wheatears, nightjars), are likely to markedly increase the spatial heterogeneity in nutrients (P, N, organic matter) at a landscape scale, particularly when the density of breeding pairs is high. Spatial heterogeneity results in microhabitat diversity and influences spatial patterning of different plant species in a community. Consequently, these birds may represent an overlooked yet widespread mechanism generating patchiness in nutrient distribution across landscapes in many different habitats, from deserts to boreal forests. Finally, from an animal perspective, nutrient patchiness enhances the growth and establishment of plants around the nest, which may provide greater protection from predators and/or amelioration of climatic factors. Thus, the interaction between ground-nesting territorial birds and vegetation might also generate a number of direct and indirect effects on both the plant and the animal side (Fedriani et al. 2015). When the nest is not dug in a cave, a deep cavity or under a bush or root, it has at least one of the sides protected by a sort of ‘wall’ (or an overhanging rock), which can be a rock or boulder, tree trunk or artificial wall (when in a human building). This sort of covered side is generally located in such a way that an incubating female has one of her sides (flanks) in immediate proximity to the wall. In some cases, this structure surrounding the nest may form an angle around the nest scrape, so that half the body of an incubating female is against it (generally one side of the body and the tail). Nests within a cave do not necessarily rely on such a structure in their vicinity, and they may also be found farther from the walls of the cavity (e.g. Blondel & Badan 1976). Nest scrapes at the base or on a terrace of a cliff have this typical aspect, in which one side is covered, as do nests dug beneath a tree, where the scrape generally starts at the base of the trunk. This sort of side protection can be considered the main characteristic of most Eagle Owl nests, as it is the most recurrent element associated with a nesting site, as also observed by Olsson (1979) and Görner (1983). Several papers on Eagle Owl nest characteristics have presented various illustrations of different types of caves, holes, fissures, terraces and nest placements, which mostly reflect local situations and, thus, including them here would have meant dozens of illustrations showing that Eagle Owls may use any kind of cavity in a rock, provided that it offers shelter and visibility of the surroundings. Although Eagle Owl studies have depicted the species as an inland bird, they can also breed successfully along cliffs in coastal areas (e.g. Olsson 1979, Pukinsky 1993, Mitropolskiy & Rustamov 2007, Démarque et al. 2014). Roost sites in rocky habitats may present a typology similar to nests, with Eagle Owls frequently spending the daytime in rock fissures, caves and crevices that are commonly (but not necessarily) protected by bushes or trees. Indeed, several diurnal roosts on cliffs are behind foliage. 99

The Eagle Owl

The type of rocky substratum (e.g. calcareous, basalt, coal, schist and so on) does not seem to have any particular importance (Frey 1973, Blondel & Badan 1976, Görner 1983, Bergerhausen et al. 1989, Balluet & Faure 2004, author’s pers. obs.): the selection of rock types is mostly based on their availability, the ability to dig a hole (presence of a soft soil in which to dig the scrape) and the presence of good cover during the different phases of reproduction. Thus, the presence of a favourable structure (e.g. a cave or fissure, a wellcovered terrace) in which to dig a nest may engender indirect preferences for a given type of substratum, e.g. in some types of rocks (e.g. calcareous and granite) favourable nest locations are more frequent than in others (Cochet 2006). Generally, and when possible, the nest is located in such a way that it is protected from local wind, rainfall and snow, as a dry and warm environment in the vicinity of the nest is obviously preferable (Olsson 1979, Görner 1983, Bergerhausen et al. 1989, Ekimov 2005). Indeed, scrape drainage is very important for keeping chicks dry and warm even during long periods of bad weather. Heavy and prolonged rains may induce unsheltered, incubating females to abandon the eggs, although failures under harsh weather conditions can also be caused by the difficulty males have hunting prey for incubating females (Bergerhausen et al. 1989). In less protected nests, very young chicks can also die because the wet body of the female does not allow her to keep chicks dry and warm. The dry conditions and, thus, warmness in a nest scrape and its immediate vicinity may be quite favourable if compared to the conditions in the cold surroundings in early springtime. For example: (a) nests exposed at east and west may show different temperatures when under direct sunlight, the maximum temperature being 56° (average = 21°) and 34° (average = 17°; Noga 2013), respectively; and (b) the temperatures recorded at one nest in Sweden during the month of May showed that the air temperature in the shade of the cliff was +15°, in the sun +23.5° and at the nest site +33° (Olsson 1979). Olsson also showed that an overhanging rock is useful against snow and rain but the overhang must be limited so as to let in the sun, i.e. there is generally a compromise between two different necessities, shelter and insolation. This is probably especially true for northern countries, where the amount of insolation is more limited than in more southern areas, as well as for those nests at higher altitudes. Nevertheless, we have to bear in mind that nest position may also be the result of individual experience, particularly when new breeders are colonising new areas where they cannot rely on historical sites in which old nest scrapes are still visible. Thus, nest position may be due to the experience of individuals, local weather conditions, as well as the availability of favourable sites, i.e. some nest scrapes may be in relatively uncovered and weather-exposed places because of the lack of better locations. Usually (but not always; Blondel & Badan 1976) the nest is located in such a way that it is possible to easily observe the surroundings (Frey 1973, Blondel & Badan 1976, Olsson 1979, Fremming 1983). In many cases the nest is easily accessible by a flying individual, although in several other locations the owl first needs to perch in the proximity of the nest and then enter it by doing a short jump or walk (e.g. when the entrance of the nest is almost completely covered by a bush). Olsson (1979) observed that a nest may be deserted once trees have grown up and the sunshine and visibility have been lost: in this respect, the Eagle Owl seems to look for the perfect mix of both sun and deep shadow (Simeonov & Milchev 1994). But nest scrapes on a cliff are only one of the most common breeding places for this species. Eagle Owl breeding in stick nests is not a recent phenomenon, although it is possible that the recent expansion of the species in some countries (which sometimes lack rocky habitats) has increased the frequency of this type of nest. Niethammer (1938), Schnurre 100

The nesting site

(1954) and Görner (1977) were among the first to report the use of raptor, heron and stork nests by German Eagle Owls. The use of a carrion crow nest in a pine tree was reported for the French Provence in the 1970s (Blondel & Badan 1976), and Cheylan (1979) reported a case of successful breeding in a Bonelli’s Eagle Hieraaetus fasciatus nest in an ancient quarry in S France. Mikkola (1983) listed the use of Golden Eagle Aquila chrysaetos, Common Buzzard and White-tailed Eagle Haliaeetus albicilla nests; the use of an unidentified stick nest in a quarry was also observed by Doucet (1989) in Belgium. Breeding in nests of White Stork Ciconia ciconia and Black Stork C. nigra, as well as Cormorant Phalacrocorax carbo, inside colonies, has been reported since the 1990s by Profus (1992) and Cherkas (1999) in Poland, where the species was also observed breeding in the stick nests of several forest-dwelling raptors (Pugacewicz 1995). In the 1990s, the breeding of an Eagle Owl in a Common Buzzard nest was reported in the Russian Baranovichi forest (Demyanchik 1990a). At the end of the 1990s, Frikke & Tofft (1997) and Pačenovský et al. (2012) reported Eagle Owls breeding in Goshawk nests in Denmark and Austria, respectively. In the lowland floodplain forests near the lower Morava and Danube rivers, Eagle Owls breed in the stick nests of raptors, Grey Herons Ardea cinerea and White Storks (Zuna-Kratky 2003), similar to the situation in the lowland forests of Belarus where Eagle Owls breed in the nests of raptors and storks (Yurko & Duchits 1993, Gricik 2004). Nests of different raptor species (e.g. Goshawk, Common Buzzard and Black Kite Milvus migrans), and White and Black Storks are also used in Belarus, where almost 30% of the breeding pairs use stick nests to reproduce (Gritschik & Tishechkin 2002). In the Czech Republic, Eagle Owls have been found in the nests of raptors, herons and storks since the beginning of the 21st century (Štastný et al. 2006), and Horal & Škorpíková (2011) reported breeding in the nests of White-tailed Eagles and Goshawks in the floodplain forests of southern Moravia, as well as two instances of breeding in the floodplain forests on the Austrian side, one in an old nest of a Black Stork and one in an unidentified raptor nest. Zemanová (2009) also reported Eagle Owl breeding in raptor and stork nests for Central Moravia. In Hungary one pair nested in a heronry of Grey Herons and another pair has also bred in an artificial nest built in a spruce tree near the town of Sopron for 12 consecutive years (Firmánszky et al. 2006a, Petrovics 2008). In Slovakia, breeding in raptor nests (which was first reported at Plavecké Podhradie in 1979; Danko & Karaska 2002) has been recently confirmed by Šotnár (2007) for a nest of the Lesser-spotted Eagle Aquila pomarina; Eagle Owls breeding in an old nest of a White-tailed Eagle has also been reported for eastern Slovakia (Pačenovský et al. 2012). In Romania, some pairs breed in the nests of Common Buzzards, Goshawks, Golden Eagles, Ravens Corvus corax and Hooded Crows C. cornix; a pair of Eagle Owls has also been found in a Rook C. frugilegus nest within a huge rookery, with the closest active Rook nests located only a few metres away from the owls (Nicola Pârlog, pers. comm.). The case of an Eagle Owl pair breeding in a stick nest in France was reported by Daske (2009) for a heron colony in the Mulhouse Zoo. Several pairs have started to breed in stick nests in the Netherlands (17% of the total nests recorded by Wassink 2011). Breeding in raptor nests has also been recorded in central Spain (Ortego 2003, Ortego & Díaz 2004). Starting in the late 1990s, when the Eagle Owl began breeding in the forests and marshland of Doñana National Park (SW Spain), most pairs have bred in the nests of raptors, storks and herons (Penteriani et al. 2012). Recently, a breeding pair reproduced successfully in a cormorant nest within a colony in Germany (Central Holstein; Peukert 2013), where other cases of breeding in stick nests (e.g. raptors and herons) have also been described (e.g. Robitzky 2007, 2009a, von Lossow 2010, Robitzky & Dethlefs 2012a, 101

The Eagle Owl

Robitzky et al. 2013a). Pairs of herons in a colony occupied by an Eagle Owl pair built their new nests in the immediate vicinity (1.5–2m distance) of the breeding owls (Robitzky et al. 2013b). Five percent of the 58 nests found in S Portugal were in Bonelli’s Eagle and Black Stork nests on cliffs, as well as in tree nests of the white stork (GTAN-SPEA 2015). Except when breeding under the canopy of a forest stand, the selection of a stick nest also means that females and nestlings do not have any cover around them and they are completely exposed and visible (e.g. Cheylan 1979, Peukert 2013). Nests in large tree hollows were described by Mikkola (1983) and Shepel’ (1992). Robitzky & Dethlefs (2011) reported the case of nests in the middle of a huge bifurcation of some branches of old beeches, oaks and poplars (Robitzky 2013). Such nesting sites only occurred in old deciduous trees and in three out of four cases the nesting trees were outside the woodland. Eagle Owls easily breed in artificial nests, including both stick nests and large nest boxes. Frey (1973, 1976) and Görner (1983) were among the first to report the use of artificial nests by Eagle Owls. Görner (1983) described nest boxes with a breeding niche of at least 100cm in width, 70cm in depth and 60cm in height. Horal & Škorpíková (2011) reported two pairs breeding in floodplain forests along the Morava and Dyje rivers (southern Moravia) in wooden nest boxes (75cm width × 50cm depth × 50cm height) placed in trees, which were originally intended for the Saker Falcon Falco cherrug. The case of Eagle Owls breeding in a wooden nest box (80 × 80 × 80cm) installed on a high-voltage pylon for Saker Falcons was described by Mihók & Lipták (2010): the pair successfully bred from 2008–2012 in the open agricultural land of Budkovce (Moravia, Czech Republic). A curious event occurred during one of the successful breeding attempts of the Eagle Owl pair in this nest box: the authors discovered that, together with three Eagle Owl chicks, the box also contained a clutch of four Kestrel Falco tinnunculus eggs in the front right-hand corner! Unfortunately, it was not possible to check the nest box later in the season and, thus, the outcome of the Kestrel breeding is not known. However, it is possible that the Eagle Owl occupied the nest box after the Kestrel pair laid eggs, chasing away the falcons that never came back to the nest. Open nest boxes in forested areas have also been recently occupied by the species in the Netherlands (80 × 80 × 30cm, 3% of the total number of recorded nests; Wassink 2011), Poland (Anderwald 2002, 2006a,b, 2010, Anderwald & Sitkiewicz 2010), where Eagle Owls have also used artificial stick nests (Anderwald 2006a,b), Germany (e.g. Robitzky & Dethlefs 2011, 2012a) and France (an open nest box installed in a quarry, Wilhelm 2010). Likewise, breeding in artificial nests has been reported for Belarus (Kitel 2009) and Latvia (Lipsbergs 2011). To conclude, it would be extremely interesting to understand the effect of nest site characteristics on the ‘search image’ of a juvenile when it is finally time to find and select a place in which to start breeding. That is, how strong is the influence of nest type and features of its surroundings on the learning of juveniles during their growth, especially considering that they spend several months in the natal area before starting natal dispersal? Thus, for example, what kind of breeding site will a dispersing juvenile that has grown up in a stick nest in a stork colony be searching for? Has the imprinting been so strong that it will only search for a similar breeding place or will it be able to start breeding in a cave located on a cliff, for example? And what about a juvenile that grew up in the downtown of a city? Will it be able to start breeding under a bush or a boulder in a forest? In our opinion, this is an intriguing question that could help us to understand why Eagle Owls start breeding in very odd places and if breeding in a peculiar nest site may trigger a sort of ‘lineage’ of individuals preferably searching for what they have experienced during their growth and post-fledging dependence period. 102

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Nest sites Cochet (1991, 2006) approached the study of Eagle Owl nesting sites from an interesting perspective, namely the effect of geomorphology on the emplacement of rocky areas and the distance to the most favourable hunting grounds. Indeed, when possible, Eagle Owls try to nest as close as possible to their prey. Cochet recognises several structures of valleys in which Eagle Owls choose to breed: (1) the peneplain, a low-relief plain representing the final stage of fluvial erosion, where streams show extensive meandering and braiding (the separation of the main stream into a network of small channels). This structure is extremely favourable because it is a mix of (a) rocky areas produced by the advanced erosion of rivers and (b) favourable hunting grounds on the top of the plateau (the uplands on the top of hills eroded by water) and on the frequently large and open valley floors. Depending on the location of the nest (closer to the top or bottom of a peneplain), Eagle Owls may hunt on the top of hills, in the river valleys or both; (2) horst and graben, which refer to regions that lie between normal faults and are always formed together: a horst represents a block pushed upward by faulting, and a graben is a block that has dropped due to faulting. Graben are usually represented by low-lying areas such as rifts and river valleys, whereas horsts represent the ridges standing between/on either side of these valleys. These geomorphological structures are quite advantageous for Eagle Owls: the horsts are generally eroded by water, which creates favourable nesting sites, whereas the graben represent favourable hunting areas close to the nest; (3) the water gap, an opening or cut which flowing water has carved through a mountain range, relatively short in length and very close to open areas. Again, the proximity of nesting and hunting habitats represents a favourable combination for the Eagle Owl; and (4) the edge of faults, a fracture or discontinuity in a volume of rock, frequently crossed by several small rivers that end in a principal one. Again, these areas represent a favourable mix of nesting sites in the valleys eroded by smaller rivers and hunting areas in the main river valley. Thus, different geomorphological structures may create very favourable breeding sites where the erosion of rivers has exposed relatively large cliffs and boulders close to favourable 103

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hunting grounds. However, other elements of the landscape can make these structures more or less favourable to the settlement of a breeding pair, e.g. the amount, structure and age of forested areas in the hunting grounds, as well as the altitude of the structure. Nest sites in forests, as well as in comparatively large patches of trees, where the nest scrape is generally dug at the base of a tree trunk, represent common breeding sites throughout the whole distribution range of the species, but especially in boreal areas. Among the most recent literature, we can find reports of: one pair in Côte d’Or (Burgundy, France; Abel 2007), 11.6% of nesting sites in the flat areas of the Altai-Sayan region (Russia; Karyakin 2007), 24% in the Samara District (Russia; Karyakin & Pazhenkov 2007) as well as in most Russian areas from which we have available information (e.g. Shepel’ 1992, Estafiev & Neifeld 1999, Gritschik & Tishechkin 2002, Bakka 2008), 19% of breeding records in the forests of Satakunta, SW Finland (Helppi & Kalinainen 1984), and 2.6% of recorded nests in the Jeseníky Mountains (Czech Republic; Suchý 2001). Some cases of Eagle Owls breeding in pine, birch and alder forests have also been reported by Pugacewicz (1995) and Cherkas (1999) for Poland. Nests at the base of forest trees have also been found in the Netherlands (Wassink 2011), Czech Republic (Zemanová 2009), Latvia (Lipsbergs 2011), Germany (e.g. von Lossow 2010, Robitzky & Dethlefs 2011, 2012a), as well as in some areas of SW Spain, e.g. pine forests of Doñana National Park and eucalyptus forests in the middle of Andalusian cultivated areas (authors’ unpubl. data), and Central Spain (Ortego 2003, Ortego & Díaz 2004). Only one case, a nest in a chestnut wood (Ravasini 1996), has been reported for northern Italy (Parma Province). Recently, some nests have been found on the ground in relatively open habitats of S France (Tarn department), e.g. in formerly grazed grasslands and open wooded lowlands (Cugnasse et al. 2015). Although in several cases these forest nests are the result of the recent colonisation of some areas that lack cliffs or boulders, in some other regions (e.g. Scandinavia and Russia) nest scrapes dug at the base of a tree are typical breeding places that have always existed. As noted by Gritschik & Tishechkin (2002), nests within forest stands are frequently close (from dozens to a few hundred metres) to stand edges, probably because this location allows Eagle Owls to easily and rapidly reach open areas (e.g. young clear-cuttings and fields) where they find most of their prey. As we have already remarked, quarries (both active and inactive) represent important breeding sites for the Eagle Owl throughout its whole distribution range, as also observed by e.g. Gee (1989) in Belgium; Frikke & Tofft (1997) and Pinstrup (2009) in Denmark; Bassi (2003) and Toffoli & Calvini (2008) in Italy; Lorgé & Conzemius (2007) in Luxembourg; Abel (2007), Dubois et al. (2011), Delgranche et al. (2011a,b), Demarque et al. (2014), Dominique Douay (pers. comm.), Pascal Demarque, Alain Leduc & Bruno Stien (pers. comm.) in France; Hristov et al. (2007) in Bulgaria; Wassink (2011) in the Netherlands; Robitzky (2007), Görner (1983), Dalbeck & Heg (2006) and Robitzky & Dethlefs (2012a) in Germany; and GTAN-SPEA (2015) in Portugal. Quarries are also common breeding sites in Spain. Nests in human buildings (e.g. castles, ruins, cemeteries, houses, water and TV towers, bridges) have also been observed in Central Asia (Mitropolskiy & Rustamov 2007), Russia (e.g. Shepel’ 1992) and in most European countries (see also Chapter 2 and Ortego & Díaz 2004, Dalbeck & Heg 2006, Zemanová 2009, Hrtan 2010, Wassink 2011, Robitzky 2010a, 2012a, Robitzky & Dethlefs 2011 and GTAN-SPEA 2015). Breeding in forest cabins and abandoned barns has been reported for Russia (Shepel’ 1992, Gritschik & Tishechkin 2002) and Finland (author’s pers. obs.), and a nest under a debris crushing machine was found in 104

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an industrial area in the Netherlands (Wassink 2011). One case of breeding on a railway bridge located on a disused railway line was observed in Orava (Karaska 1995) at the end of the 1980s. A breeding pair occupied the hide of a gamekeeper in the Czech Republic (Hudec & Šťastný 2005) and another was found under the wooden cover of a pheasant feeding place (Robitzky 2012b). Similarly, Mikkola (1983) reports some unusual breeding sites like a barn, the attic of a forest cottage, a temporary shelter of poles and branches that foresters erected during the winter to protect them from wind and rain, a hide of a photographer as well as what he called the ‘anthill breeder’. For several years a female bred in different hollows she made in various anthills. As a rare example of the adaptability of the species we can also cite the case of a successful breeding recorded on a French cliff (Puy-de-Dôme department) that is artificially illuminated 136 nights per year between sunset and midnight (Martin 2006). Independent of the increasing number of Eagle Owls breeding in human constructions, which can be the result of both a growing number of such structures in the countryside and a decrease in persecution (these ‘odd’ breeding sites are generally easy to access), the increasing number of observations and breeding events recorded in towns is undoubtedly remarkable (see Chapter 2). Just a few decades ago this species was a relatively shy inhabitant of remote places, avoiding humans wherever possible. Now, Eagle Owls breed on the roofs of modern buildings, hotels and commercial centres, sleep on balconies, patrol shopping streets, urban parks and residential areas, and they are not afraid to attack a leashed dog walking with its owner at night. Indeed, nowadays this species lives in a variety of urban habitats, ranging from suburban areas to highly urbanised city centres. Eagle Owls represent one of the numerous species that have recently colonised our cities, and the reasons behind these new colonisations frequently remain unknown. Colonisations of urban habitats are possibly the result of multiple factors and specific local conditions. In some cases, the Eagle Owl may have been attracted to urbanised areas by abundant, unexploited food resources (e.g. rats, crows or an increasing urban population of Rabbits, frequently the result of pet escapes). The recent appearance of Eagle Owls in urban areas may also be due to the generalised increasing trends of its population in natural areas. After the reduction in persecution, the increasing number of Eagle Owls could have produced a larger dispersal of individuals and/or a greater pressure from rural breeding populations at their capacity level, with some Eagle Owls reaching the periphery of urban areas and, then, starting city colonisations. In addition, behavioural changes in natural populations due to the reduction of direct persecution may also be responsible for the presence of Eagle Owls in cities. Intense and prolonged mortality caused by harvesting imposes selection pressures on target populations (selective removal of certain phenotypes; Allendorf & Hard 2009, Stenseth & Dunlop 2009) and lead to rapid evolutionary changes, even though the harvest is not intended to be selective (Allendorf et al. 2008, Allendorf & Hard 2009). Natural selection maintains a mix of behavioural phenotypes in a single population (Schreiber et al. 2011), the shy–bold behavioural continuum (Smith & Blumstein 2008): bold individuals thrive on risk and novelty, while shy individuals shrink from the same situations (Wilson et al. 1994). During periods of persecution, disproportionate removal of bold individuals may occur because they are more active and incautious, investigate novel stimuli or locations and, thus, are more likely to be shot, which consequently results in an over-representation of shy individuals in the surviving population (Wilson et al. 1994, Biro & Post 2008, Smith & Blumstein 2008, Ciuti et al. 2012, Madden & Whiteside 2014). Additionally, because behavioural changes resulting from harvesting may also be plastic, individuals become more vigilant and avoid contact 105

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with humans during intense harvesting periods (Madden & Whiteside 2014). Increasing population trends after persecution can engender higher variation in temperament within a population (Dall et al. 2004) and the effects of behavioural temperament have the potential to mediate individual responses to human presence (Madden & Whiteside 2014). On the one hand, bold individuals may be more numerous today, and they might be more prone to colonising new habitats like cities, which could explain the increase of these pioneer settlements in urban areas especially after periods of intense persecution. Thus, an increase of bolder individuals in Eagle Owl populations may lead to more individuals taking the risk to occupy new habitats (not only towns but also odd nesting sites, as it is happening nowadays). If this scenario is correct, we will witness an even larger increase in the number of urban Eagle Owls in the near future. An important question is whether, in some cases, Eagle Owls were not ‘pulled’ into urban habitats by good living conditions but rather were ‘pushed’ out of natural areas by unfavourable conditions. We cannot exclude that, locally and more likely in northern countries, the appearance of Eagle Owls in cities could have been, at least at the beginning of these pioneer settlements, the result of the combination of favourable conditions in cities and, for example, severe weather conditions in winter or collapsing food supplies in natural habitats. These urban populations may also play a crucial role in the stability of natural populations, representing an important source of juveniles that, in addition, can connect different populations previously disconnected (this may be especially true for Eagle Owls that may exhibit relatively short dispersal distances; see Chapter 10). That is, large urban areas previously avoided may now acquire a new role in the dynamics of local populations. On the other hand, it is also true that urban Eagle Owls may also suffer increased mortality rates due to collisions with anthropogenic obstacles or novel diseases (Rutz 2008), which in some cases may slow the process of settlement in urban areas or decrease fecundity even if the conditions of cover and food are good.

Nest altitude The altitudinal range at which Eagle Owls have been found breeding is relatively astonishing; or perhaps not, if we consider the adaptability of the species in different environments and with different food resources. As a general rule we can say that, although several nesting sites are located at relatively high altitudes (we consider the threshold for high altitudes around 1,700–1,800m a.s.l.), most nesting sites are at relatively low and medium altitudes, even in those study areas that include mountain ranges, as clearly demonstrated in the following studies: in the Spanish Navarra Mountains, Eagle Owls showed a preference for nesting at lower levels of the mountain range, mostly between 400–800m a.s.l. (Donázar & Ceballos 1984), with a mean nest altitude of 620m in a mountainous area up to 2,400m a.s.l. (Donázar 1988b); in an area of the Sierra Morena (S Spain; Ruíz-Martínez et al. 1996) ranging from 380–1,300m a.s.l., the elevation of nest sites ranged from 450–700m; in the central-eastern Italian Alps, ranging from 180–2800m a.s.l., 58.3% of breeding sites were located at an altitude 1,200m. These examples clearly illustrate that the preference for the species is not directed towards high altitudes, as also reported by Choussy (1971; 500–1,000m), Rigacci (1984; 250–500m), Malafosse (1985; 500–1,000m), Balluet & Faure (2004; 200–850m, with most nests at 400–500m) and Claudio Bearzatto (pers. comm., 2014; 200–400m a.s.l.), even when potential breeding sites are available there. Furthermore, Eagle Owls seem to preferentially select sites at lower elevation, which coincides with the distributional peaks of their main prey species (Sergio et al. 2004b). Diet composition of Alpine and pre-Alpine sites confirms the crucial link between Eagle Owls and low-elevation habitats (Marchesi et al. 2002): Brown Rats are common in wetlands, urban areas and rubbish dumps, which are generally located on valley floors; Hedgehogs are mainly found along the cultivated valley floors; and Edible Dormice Myoxus glis primarily inhabit low-elevation deciduous woodlands. The decrease of food availability at the highest altitudes, together with an increase in distance from the most favourable hunting grounds, may explain the relatively rare breeding of this species in the upper reaches of mountainous areas. Indeed, due to relatively high wingloading (0.71g/cm2; see the section The Eagle Owl in a nutshell within Chapter 1, page 8), Eagle Owls might prefer to breed as close as possible to the most favourable hunting grounds (Donázar 1988b, Martínez & Calvo 2000). Choussy (1971) hypothesised that some pairs breeding at high altitudes are mainly the result of good densities at lower altitudes, and if the number of these breeding sites decreases then the sites at the highest altitudes are the first to disappear. In this sense, and except when very favourable resources are still present at high altitudes, mountain pairs may also be the result of territory saturation at the lowest altitudes. That is, when all the available and best breeding sites are occupied at low altitudes, some pairs may try to reproduce at higher altitudes (especially in good years characterised by high availability of prey). This might also explain the relatively frequent appearance and disappearance patterns of individuals from these high-altitude pairs, which frequently show less site fidelity and/or reproduce more sporadically than pairs at lower altitudes. However, it is not rare to find citations for ‘mountainous’ Eagle Owls. For example, Magnoloux (1977) described Eagle Owls breeding at high altitudes in the French Alps, and Haller (1978) reported Eagle Owls breeding up to 2,020m a.s.l. Three nesting sites at 1,600, 1,700 and 1,800m have been found by Arlettaz (1988) in the Swiss Alps of Central Valais. The highest breeding has been observed by Stefano Bassi (pers. comm.) for the Alta Valtellina (NW Italy) at 2,250m a.s.l. A relatively high-altitude population has been found in the NW Italian Alps (Cuneo province: Bruno Caula, Pier Luigi Beraudo, Massimo Pettavino & Fabrizio Blangetti, pers. comm.; Caula & Beraudo 2014), where most of the pairs breed between 850–1,750m a.s.l. (average altitude = 1,150m). In northern Italy again 107

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(Val d’Ossola), Toffoli & Bionda (1997) reported one pair at 1,950m in an area where the mean altitude of breeding sites is around 500m. In a relatively high-altitude study area in Engadine (E Switzerland), Jenny (2011) reported the case of a breeding pair regularly reproducing at 1,900m a.s.l.

Nest orientation Nest exposure probably depends on local factors, such as temperature, duration of sunlight, prevailing wind and rain directions, and snow fall (e.g., Rockenbauch 1978, Olsson 1979, Mysterud & Dunker 1983, Görner 1983, Bergerhausen et al. 1989). For example, the tendency to predominantly use southern exposed cliffs in Provence (S France; Penteriani et al. 2001) may also be due to protection from local wind: the prevalent direction of the mistral, a high-speed wind up to 100km/h, is N-NW. Similarly, preferred exposure between N and SE seemed to better protect nest sites from prevailing wind and rain in Eifel (W Germany, Bergerhausen et al. 1989). An actual selection for a given orientation might be especially important for nests in relatively open locations, as a nest in a cave is less sensitive to the orientation of the cliff. However, the combined result of 20 studies detailing nest orientations shows that Eagle Owls are able to successfully exploit all available orientations for breeding (Figure 20), although they seem to less frequently use N and NW exposures. This is especially notable in Central and Northern Europe, as well as in mountainous areas (Figure 21), yet N and NW exposures are more prevalent in Mediterranean areas (Figure 22). Orientations between SE and W are the most common, although NE is also quite frequent (Figures 20–22). In a population in N Italy, Enrico Bassi (2001 and pers. comm.) confirmed a tendency to breed on southern slopes, as most nests were located between SE and SW. South and W slopes are also the most commonly chosen by Eagle Owls in Bulgaria (Baumgart et al. 1973) and in the Krasnoyarsk Region (Russia; Ekimov 2005). When thinking about the benefits/constraints of nest orientations we also have to keep in mind that latitude is the primary climatic control of temperature and insolation for a given location. Indeed, the distance north or south of the equator governs the angle at which the sun’s rays strike the earth, the length of the day and, thus, the amount of solar radiation arriving at the surface of the earth. Because average annual temperatures decrease with increasing latitudes, passive heating due to the sun is more important in northern countries than in southern ones: in the northern hemisphere, south-facing slopes are warmer and drier than north-facing slopes, which could explain, at least partially, the lower frequencies of N and NW nest orientations at higher latitudes (i.e. Central and Northern Europe) and in mountainous areas. Indeed, slope angle and orientation with respect to the sun are crucial and may partially compensate for latitude. In spring, when Eagle Owls start breeding, north-facing orientations may still be deep in snow while south-facing slopes are clear. East and W orientations are also affected differently by solar radiation. Soil and vegetation surfaces are frequently moist in the early morning, owing to humidity at night and the formation of dew or frost. On east-facing slopes the sun energy has to evaporate this moisture before the slope can heat appreciably. By the time the sun reaches the west-facing slope, however, the moisture has already evaporated, so the sun energy more effectively heats the slope. Thus, the driest and warmest slopes are those that face toward the SW rather than strictly S or SE. Because topography, characteristics of the nest site and local weather 108

The nesting site N 90 80 70 60 50 40 30 20 10 0

NW

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Figure 20. Exposure of nests from 21 studies in Europe (numbers show percentages).

N

N 100

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Figure 21. Exposure of nests in central and northern Europe (14 studies, including 3 studies in a mountain range of N Italy, 2 in central and NE France, 1 from Austria, 2 from Germany, 3 from the Czech Republic, 1 from Bulgaria, 1 from the Netherlands and 1 from Finland). Numbers show percentages.

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Figure 22. Exposure of nests in Mediterranean Europe (7 studies, including 1 from Southern France (Provence), 3 from central and NW Italy (Liguria), 1 from Spain, 1 from Portugal and 1 from Greece). Numbers show percentages.

Sources for Figures 20–22: Thüringen, Germany (Görner 1983); Satakunta, SW Finland (Helppi & Kalinainen 1984); Central Italy (Rigacci 1984); Greece (Papageorgiou et al. 1993); Strandja Mountain, SE Bulgaria (Simeonov & Milchev 1994); Czech–Moravian Highlands, Czech Republic (Kunstmüller 1996); Styria, Austria (Sackl & Döltlmayer 1996); N Italy (Sascor & Maistri 1996); Provence, S France (Penteriani et al. 2001); Jeseníky Mountains, Czech Republic (Suchý 2001); N Italy (Bionda 2003); Eifel, Germany (Dalbeck 2003); Central France, Loire (Balluet & Faure 2004); E France, Côte d’Or (Abel 2007); Liguria, NW Italy (Toffoli & Calvini 2008); E Czech Republic (Zemanová 2009); the Netherlands (Wassink 2011); S Portugal (GTAN-SPEA 2015); Liguria, NW Italy (Calvini & Valfiorito, pers. comm.); NE Italy (Claudio Bearzatto, pers. comm.); SW Spain (Penteriani & Delgado, unpubl. results)

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conditions are also expected to interact with orientation to determine nest conditions at different exposures, merely taking nest orientation into account might not be sufficient to explain the reasons for the observed nest placement. Intriguingly, it seems that rainfall may also play an important role in determining habitat suitability for Eagle Owls. In a pre-Alpine area in N Italy characterised by high yearly rainfall values, heavy rainfall was associated with the absence of the species from a large sector of the study area (Brambilla et al. 2010). This negative effect of rainfall is in accordance with the findings of another study carried out in eastern Piedmont, along the western boundary of this pre-Alpine area, which is characterised by comparable rainfall values (Bionda 2007). The authors suggest that rainfall effects might also be indirect, e.g. most rainy areas may have wider tracts of unsuitable woodland areas at a large scale that could prevent Eagle Owl settlements.

Nest site habitat Since the beginning of Eagle Owl studies on nesting habitat selection at a landscape level, the preference of this species for breeding sites surrounded by open areas, where Eagle Owls probably find most of their prey and that represent very favourable hunting grounds, has been clearly evident. Open areas in the proximity of nests are still present for those pairs breeding in gorges and canyons, which always have easy access to natural or artificial (e.g. cultivated land) open areas (Cochet 1991, Balluet & Faure 2004). Open landscapes close to nest sites are one of the ‘leitmotivs’ of the surroundings of Eagle Owl nest sites (e.g. Blondel & Badan 1976, Bergerhausen et al. 1989, Defontaines & Ceret 1990, Martínez et al. 1992, Penteriani et al. 1999, 2001, 2002a, Bassi 2001, Bassi et al. 2003a, Dalbeck 2003, Martínez et al. 2003a, Sergio et al. 2004b, Oja et al. 2005, Toffoli & Calvini 2008, Brambilla et al. 2010, Trotti et al. 2013). The need for open areas by the Eagle Owl has frequently been related to the need for profitable hunting areas, where they can easily prey on rats Rattus spp., Hedgehogs and other mammals living in open or semi-open habitats (Martínez et al. 2003a, Brambilla et al. 2010). In Mediterranean regions, where several breeding nuclei strictly depend on Rabbit populations as their more profitable prey, the presence of relatively open areas (e.g. scrublands) in the proximity of nesting sites is mainly determined by the presence of their main prey (Martínez et al. 2003a, Penteriani et al. 2004, Moreno-Mateos et al. 2011, Campioni et al. 2013). Leditznig (1999) showed that those pairs nesting closer to the most favourable hunting areas had the best breeding success. This does not imply that Eagle Owls strictly need the best hunting areas close to the nest, as also demonstrated by Martínez et al. (2003a): in some contexts, the pattern of habitat preference could be the result of long-term changes in prey abundance, which may force individuals to increase hunting ranges by visiting good patches over a larger area, or the lack of suitable nesting sites close to good hunting grounds. We always remember the peculiar case of a radiotagged male Eagle Owl in Doñana National Park that frequently crossed a large area and a marshland to hunt in a Rabbit-rich scrubland up to 5km from his nest. And sometimes, when it was not necessary to return to the nest with prey, he remained roosting during the day on a tree in the hunting area, far from both the female and young. Under certain conditions, the process of habitat selection of Eagle Owls might occur as an initial assessment of the general habitat features at the landscape level (e.g. the presence 110

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of good foraging areas) and, then, by searching for more specific attributes of the habitat, namely an area adequate for breeding and roosting (Martínez et al. 2003a). However, when food is not a limiting factor and it is homogeneously distributed across landscapes, a hierarchical process of habitat selection could be less evident (and necessary) and the location and availability of most favourable nesting sites (all of them having the same amount of prey) may mostly determine pair settlements and distribution. That is, Eagle Owls may show a high adaptability to occupy sites that can be considered as marginal in other situations, a result that may be mostly related to the high abundance of suitable prey (Ortego 2003, Ortego & Díaz 2004). Thus, the difference between the Martínez et al. and Ortego & Díaz studies might be attributed to the very high abundance of Rabbits in central Spain as compared to the eastern part of the country, which would lead to a coincidence in the size of nesting and foraging areas (Ortego 2003, Ortego & Díaz 2004). But open areas are not the only important element of the landscape that is required by a breeding pair: in most of the nesting sites habitat heterogeneity and a mix of open areas and woodlands have been reported (e.g. Görner 1977, Sánchez-Zapata et al. 1996, Sascor & Maistri 1995, 1996, Penteriani et al. 1999, 2001, Brand & Lanz 2005, Toffoli & Calvini 2008), frequently with relatively high percentages of forested landscapes (Frey 1973, Olsson 1979, Donázar 1988b, Martínez et al. 2003a, Brand & Lanz 2005, Nellis 2006). For example, in the Alps and pre-Alps, nesting sites are frequently in the contact zone between forested mountain slopes and cultivated-urbanised valley floors, despite the availability of less human-altered nesting and foraging habitats at higher elevations (Marchesi et al. 2002). Forested areas seem to be especially important when they are associated with the presence of the Edible Dormouse, a prey that can contribute considerably to the diet of the Eagle Owl (Sascor & Maistri 1996, Toffoli & Bionda 1997, Toffoli et al. 1999; Marchesi et al. 1999b, Bassi 2001, Bassi et al. 2003a). Moreover, old-growth forests may represent suitable hunting areas for Eagle Owls because their internal structure (large spaces between trunks and high canopies) does not limit flight manoeuvrability within old stands (Estafiev & Neifeld 1999, authors’ unpubl. data). The presence of forests, however, is not always a positive landscape element for Eagle Owls, especially at low altitudes in Mediterranean habitats. For example, in some areas of the French Provence (Luberon Massif, Penteriani et al. 2002a), when Eagle Owls were not only affected by the loss of a high-value prey (the Rabbit) and the consequent shift to smaller prey leading to loss of hunting efficiency, but also by the closing of the landscape by dense forests, they were probably forced to expend more energy to obtain food. Indeed, the number of fledged young was positively correlated with the amount of open land surrounding the nest site. Thus, the local dependence of the species on open areas and on habitat heterogeneity may be a problem for Eagle Owl conservation in Mediterranean landscapes, where the general trend is toward disruption of the dynamic agropastoral equilibrium maintained by man, which has contributed to the biological diversity and productivity of these seminatural landscapes. The decreasing grazing pressure, especially in inland areas, combined with depopulation and abandonment of agricultural uplands, favours the development of dense Mediterranean forests with a closed structure and a rapid reduction of open areas and landscape diversity. In such a situation, the landscape preferences of Eagle Owls might limit the expansion of the species in the Mediterranean range, and benefit smaller owls living in more forested areas. These results are also consistent with: (i) Leditznig (1996), who showed that the more forested the home range, the lower the reproductive success, primarily as the 111

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result of reduced prey abundance and availability; and (ii) Dalbeck & Heg (2006), who observed that offspring production decreased with the percentage of continuous woodland area around the nest site. It has been hypothesised that the frequently close distances of nesting sites to rivers might not always be the result of water erosion, which creates suitable nesting habitats (Sascor & Maistri 1996, Penteriani et al. 2002a, Balluet & Faure 2004, Toffoli & Calvini 2008). Indeed, the surroundings of streams, creeks and rivers are generally rich with potential prey (fish included) and, thus, Eagle Owls might in fact choose to breed close to them, independent of the presence of rocks and cliffs in proximity to water courses. Similarly, Ortego (2003, 2007) and Ortego & Díaz (2004) hypothesised that, although the preference for nesting close to streams may be due to the fact that rocky outcrops and steep slopes characterise the surroundings of brooks and streams, the surroundings of streams might supply a higher abundance of the main prey in Mediterranean habitats (Rabbits) because along water courses they find good food conditions and softer soils in which to dig permanent refuges. When the landscape is relatively homogeneous, trophic resources are widespread and the density of conspecifics is very high, the distribution of nests may be mostly the result of the interaction between landscape characteristics and the distance to and density of neighbouring pairs (Martínez & Calvo 2000, Martínez et al. 2003a), as well as the availability and distribution of favourable nesting sites (Ortego 2003, 2007, Ortego & Díaz 2004). Indeed, the distribution of individuals can be the result of several interacting factors and, therefore, some caution is needed when establishing causal relationships between breeding pair distribution and habitat features (Martínez et al. 2003a). We have already had the opportunity to see how adaptable Eagle Owls may be to the presence of humans, if this is not directly disturbing the immediate surroundings of the nest. This ability of Eagle Owls is also mirrored by the multiple studies showing the presence of a nesting site close to human presence and/or structures. For example, proximity to roads is common for many nesting sites: (a) nests are relatively close to human settlements and roads in Navarra (N Spain; Donázar 1988b, Donázar et al. 1989a) and N Italy (Sascor & Maistri 1995, Toffoli & Calvini 2008); (b) a few metres to 40), with a weight of 57–58g (Pukinsky 1993). The weight of two fresh eggs obtained from the Berezinsky Nature Reserve was 85 and 86g, which is considered to be a very large egg. However, during incubation, the eggs decrease linearly in weight (Broo & Lindberg 1973): at the 21st day of incubation weight may vary between ca. 52 and 73g, while at the 30th day it is between 48 and 70g, that is to say a decrease in weight of 0.4g per day. Thus, egg sizes seem to vary with the size of individuals in the different areas, being larger for Central and Eastern Europe than in more southern areas. Eggs are laid at 2–4-day intervals and clutch size varies from one to six eggs (Mikkola 1983, Cramp & Simmons 1980). Unhatched eggs or, more frequently, the remains of hatched eggs are generally removed (probably by the female; Choussy 1971) from the nest and left at some distance from it (Cugnasse 1983). Leibundgut (1973) noted that young, captive females lay between 2 and 3 eggs, while older females lay from 3 to 5 eggs. Karyakin (2009) showed that the laying of additional eggs during incubation may take place throughout the whole distribution range of the species, although it seems to be more common in arid regions compared to forested or mountainous regions. After checking 800 nests, he recorded this phenomenon in 21 nests (2.6%): in Russia the presence of nestlings with ‘additional eggs’ represented 2.2% of the observed broods (n = 733), whereas in Kazakhstan this value was 176

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7.5% (n = 67 broods). In most of these cases, Eagle Owls laid one additional egg (57.1%) three or four weeks after starting the incubation of a clutch of 2–3 eggs; more infrequently 2 (19.1%) or even 3 (14.3%) eggs were laid (the latter generally when just one egg had been previously laid). The occurrence of females laying 1–2 eggs in the presence of downy chicks, i.e. 5–6 weeks after the laying of the first egg, was even rarer (9.5%). In the last case, the difference of age between the earliest and latest nestlings of the brood was around 40–50 days. To our knowledge, this is the second time that this phenomenon has been reported in Eagle Owls, as a notable interval among egg-laying was also described by Miltschew (1999) in Bulgaria. Some cases of dwarfism, i.e. when an individual is short in size because of slower-than-usual growth, have also been reported for Eagle Owl chicks (Robitzky 2011, Robitzky & Dethlefs 2012b). Possible multiple broodings have been reported by Martínez et al. (2003b) and Ortego (2004) in SE and central Spain, respectively. As for the former, the story of this peculiar event is as follows: an occupied nest with two young ca. 45 days old was found on 18 April, and when this same nest was subsequently visited on 10 July two nestlings about 30 days of age were discovered. The authors estimated the laying date for the first clutch to be 29 January, and 8 May for the second clutch, but we do not know if eggs were laid by the same female. Rearing second broods is considered unlikely for large birds such as raptors because they generally lay few eggs, have low feeding rates, and nestlings have a slow growth rate and a long post-fledging dependence period. All of this accounts for long and energetically demanding breeding seasons, which is especially true for Eagle Owls. Accordingly, producing more than one clutch may be costly for the parents in terms of both egg production and the rearing of young as they have to take care of the young of both clutches until the start of dispersal. Small and medium-sized owls can produce a second clutch when food availability is high (e.g. Mikkola 1983, Korpimäki 1988, Martínez & López 1999, Zuberogoitia 2000) and, indeed, in the study area Rabbit availability was high. The fact that no second broods have ever been reported for Eagle Owls may indicate that laying more than one clutch entails substantial costs to the parents; however, we cannot discard the possibility that a second brood after a successful one is quite difficult to detect as people generally do not continue nest prospections after nestlings fledge from the nest. Martínez et al. (2003b) suggested two alternative explanations for a double brood by the same pair. First, they hypothesised that the double brood was the result of a successive polygynous mating, i.e. the male had two females nesting sequentially. Second, the authors also suggested the possibility that the female died during the last stages of the breeding period and that another female occupied the nest site. The second female could have started to breed as soon as she entered the new breeding area, using the same nest as the previous owner. In the case reported by Ortego (2004), the multiple breeding in the same reproductive season was discovered on 13 April, when three fledged chicks around a nest were found together with an adult bird incubating two eggs, approximately 50m away from the chicks’ nest (along the same cliff). The author suggested that, in spite of the high density of Eagle Owls in the study area, it seems unlikely that the second clutch belonged to a different pair. According to chick development, the laying date for the three fledglings was estimated to around 5 January, an early date that could have facilitated a second clutch. Indeed, there is a common element in the two reported cases of double brooding that could have enabled their occurrence: the high density of Rabbits, which has the potential to reduce reproductive costs and lead to multiple breeding attempts. A possible case of double brooding has also been reported for the Pharaoh Eagle Owl Bubo 177

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ascalaphus in Egypt (Sándor & Moldován 2010), with a time lag between hatching dates of at least four months. Again, an early laying and the presence of several dumps inhabited by many rats and House Mice Mus musculus, which constitute the main prey of Pharaoh Eagle Owls in that area, might have triggered double brooding.

Replacement clutches Egg production can be a demanding process for birds, and thus parental decisions may be crucial to balance the benefit from the current brood in relation to the cost to the adult in terms of survival, parental care, future fitness and the quality of the offspring. If the cost of attending the nest results in a reduction of parental fitness, e.g. through predation or disturbance, parents may decide to desert the nest and lay a replacement clutch. Several bird species re-nest after a previous failed attempt, and thus replacement clutches provide an important contribution to an individual’s lifetime reproductive success. Nevertheless, the decision to re-lay might be constrained by seasonal factors, timing of breeding losses and female condition. In fact, first clutches are usually larger than the following replacement clutches (e.g. Amat et al. 1999, Gasparini et al. 2006), and females are more prone to re-nest if the loss occurs during brood raising (Antczak et al. 2009). The probability of re-nesting is also higher in females that lay their first clutches earlier in the season (less favourable environmental conditions late in the season have been suggested), which might be correlated with their quality (Hipfner et al. 1999). Moreover, early breeding and high quality of the female may positively influence the reproductive success of a replacement clutch, as well as the quality and the probability of recruitment of the young, at least in comparison with late-season first broods. Relatively little information exists on replacement clutches in Eagle Owls, although these have been observed by König & Haensel (1968), Choussy (1971), who described a replacement clutch 7–11 days after the loss of eggs, Blondel & Badan (1976), Vondráček (1987), Defontaines (2002) and Sokolov (2009). Balluet & Faure (2006) estimated 12 weeks between the initial and replacement clutch and ca. 45 days after egg loss. Olsson (1979) reported a low success rate for replacement clutches; however, the sample size was very small (two out of seven replacement clutches were successful). He found that the distance in metres between the first and second nest scrapes was between 15 and 200m, and he noted that all replacement clutches were in scrapes never used before, even if old and well-used ones were accessible nearby. In one case, two replacement clutches were reported after the first clutch (Kunstmüller 1995). The first replacement clutch was laid 50m away from the first nest, where the previous eggs were broken. After the second clutch was abandoned on 18 April, a third (or second replacement) clutch was laid 200m away. The first and third clutches were destroyed by humans, whereas the second was likely to have been abandoned because of bad weather. As a general rule, if discovered in their nest during incubation, female Eagle Owls prefer to abandon the nest and start a new (replacement) clutch. During a long-term study on this species, Bettega et al. (2011) recorded a number of cases where females, after being disturbed during egg-laying (generally by hunters or people collecting wild asparagus), deserted the first clutch and laid a replacement clutch in another nest. Such information allowed first and replacement clutches to be compared in terms of (a) various morphological and physiological parameters of chicks, (b) timing of breeding, (c) number of eggs, and (d) number of fledglings. When discovered, incubating females are faced with a ‘dilemma’ 178

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on whether to return to the nest (and, consequently, avoid the cost of producing new eggs by incubating the first clutch) or to restart the breeding cycle from the beginning by laying a replacement clutch in a different and potentially safer place. According to lifehistory theory, individuals should maximise their fitness by investing in the most successful clutch: if there is a low risk of first clutch failure, individuals should invest more in the first clutch than in a possible replacement clutch. Conversely, if there is a high risk of first clutch failure, individuals should invest more (or at least equally as much) in a replacement clutch. Besides strategic adjustments, there are also environmental constraints that can directly affect differential investment made in first and replacement clutches. Because the investment made in first vs. replacement clutches is also driven by the risk of clutch loss, the widely-distributed and extremely eclectic Eagle Owl may have evolved an ability to re-nest, to afford the species better chances to breed successfully in each of the extremely different habitats that it occupies. Nest sites with a high risk of disturbance, such as active quarries and towns, and nests on the ground (e.g. most of our study nests in SW Spain, and those in desert, tundra and taiga regions), are common for this species. The vulnerability of nests to predators might mediate the balance of investment between first and replacement clutches, as variation in the number of broods is frequently explained by nest-site features and nest predation (Martin 1995). Thus, one of the proximate factors determining the propensity to abandon nests and start a new clutch might be the female’s capacity to produce chicks that are as good as the ones of the first clutch, i.e. offspring quality between first and replacement clutches should be similar. Accordingly, from 133 nesting attempts, Bettega et al. (2011) recorded 20 replacement clutches following 25 failed clutches. Thus, replacement took place in 80% of the failed clutches. Females generally changed nest sites to lay the replacement clutch, as only a single female in one case used the same nest for re-nesting. A replacement clutch was never laid when the female deserted the nest following a disturbance during or immediately after hatching. Failures in a replacement clutch were never followed by a third attempt to relay. The number of first clutch fledglings (2.4 ± 0.8 chicks, n = 31) was almost the same as that of replacement clutches (2.6 ± 0.8 chicks, n = 7 replacement clutches). First clutches were laid between 28 December and 23 February (average laying date 19 January), whereas replacement ones were laid between 20 January and 15 March (average laying date 10 February). The interval between first and replacement clutches ranged from 6–41 days (mean interval = 20.7 ± 14.0 days). The longest interval (41 days) corresponds to the earliest first clutch that failed (28 December). These authors also found that the characteristics of chicks did not significantly differ between first and replacement clutches with respect to all the variables they considered, i.e. (a) biochemical parameters such as cholesterol, triglycerides and free glycerol, uric acid, urea and total protein concentrations, (b) immune measures of stress and health such as red cells, total leukocyte count, heterophile, lymphocyte, monocyte, eosinophile and basophile ratios, as well as intensity of Leucocytozoon parasites, (c) body mass and morphometric measurements such as forearm, bill, tarsus and wing lengths (p > 0.1 in all cases). Thus, the chicks of first and replacement clutches had similar morphological, physiological and immune characteristics. This result may be explained by several factors. The first breeding attempts followed by a replacement clutch occurred early in the breeding season, allowing the laying of a replacement clutch to be within the normal variation in phenology in the study area. Some studies have indeed found evidence on the importance of breeding early in terms of nestling 179

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quality. Thus, it is possible to hypothesise that, in the case of a failure in the first breeding attempt, high-quality parents breeding early in the season can re-nest and produce a similar number of good-quality nestlings, compared to early first broods (Hipfner et al. 1999), assuming that at least food availability is high. As a consequence, the pairs that produced a replacement clutch may have been of higher quality, because they were early breeders. However, it is important to stress that the breeding season of Eagle Owls in Spain usually starts earlier than at higher latitudes, and trophic resources are more abundant, which might facilitate the laying of replacement clutches among lower-quality pairs as well. Early first clutches influence the probability of re-laying in other large raptors as well, such as the European Bonelli’s Eagle Aquila fasciata (Moleón et al. 2009b) and the Griffon Vulture Gyps fulvus (Martínez et al. 1998). Although breeders showed a high re-laying rate (80%), re-laying never occurred when the female was disturbed within a week from hatching, but the female sometimes delayed her return to the nest and consequently small chicks may have died because they were not yet able to thermoregulate. When a disturbance occurs at the beginning of the nestling period, the investment in the first clutch is probably too high to be compensated for by a replacement one, because of either the physiological condition of the female and/or the lower quality of new chicks. This is a general pattern in large birds of prey, as a failure at an early stage of the first clutch appears to be crucial for initiating a new one (Newton 1979). First and replacement clutches contained a similar number of eggs, whereas for most species of raptors clutch size usually decreases between subsequent nesting attempts (Morrison & Walton 1980). In other species, the size of the replacement clutch can be considerably smaller than the size of the first clutch, although the female can compensate for this by producing bigger eggs (De Neve et al. 2004). Again, prey abundance and early clutches may also be important factors, as females would have sufficient time and food to build up new energy resources. Other studies have actually confirmed the importance of female quality (the quality hypothesis; Verhulst et al. 1995) in affecting clutch size, producing an equal number of eggs in both first and replacement attempts (Christians et al. 2001) or even larger replacement clutches (Wheelwright & Schultz 1994). First and replacement clutches did not significantly differ from each other in terms of the number of fledglings, perhaps because replacement clutches were laid when Rabbits were still abundant, allowing parents to successfully raise an equal number of chicks. Although Bettega et al. (2011) only tested the possibility that chick quality in replacement clutches could be one factor determining the high frequencies of egg desertion and high rates of replacement clutches, it is important to mention that such a trait may generally only represent one of the proximate factors determining the observed pattern of re-laying. In fact, the main evolutionary force influencing the capacity to re-lay is the risk of first clutch loss or abandonment (which explains the occurrence of re-nesting), even though the replacement clutch may be of lower quality or contain fewer eggs than the first one. It is also intriguing that this species seems to adopt a strategy that is ‘halfway’ towards the expectation of the re-nesting hypothesis (Milonoff 1991). Under this hypothesis (i) in species that are unlikely to suffer breeding failure (because they breed in safe sites), individuals maximise their fitness by laying large first clutches early in the season and preserve few resources for re-nesting attempts; whereas (ii) individuals of species breeding in vulnerable nest sites lay smaller first clutches, allowing them to produce more eggs if the first attempt fails. Eagle Owls frequently nest in relatively unsafe places (such as on the ground), but seem to put similar effort into both clutches. This is an unexpected result: under natural conditions only a minority of 180

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pairs lose their eggs during the first breeding attempt (and hence do not have to produce a replacement clutch), so natural selection should favour a strategy that allows individuals to invest most of their resources in the first breeding attempt, retaining only a limited amount of resources for the unlikely event of producing a replacement clutch. In this case study, the temporal stability in the quality of the environment (high and constant food availability) is probably an important factor to be considered when evaluating reproductive output. Due to the fact that reproductive output can be an outcome of constant food availability, parents may provide roughly equal care for all offspring in both first and replacement clutches. Because individuals have to optimally adjust energy use among different life-history traits in order to maximise their fitness, the allocation of resources to one trait is often made at the expense of other traits that usually have less impact on fitness. Under this perspective, the renesting behaviour of Eagle Owls may be seen as a way to solve the trade-off between one trait (survival) favoured over another (reproduction), when one of them has a disproportionate effect on fitness. Thus, the ability to produce a replacement brood of equal quality to the first one may represent a safety strategy for this species that nests on the ground across a large part of its distribution range. Still, several questions on the breeding ecology of the Eagle Owl remain open. For example, does the eclecticism in nesting habits lead to an evolutionary flexibility in the investment in both first and replacement broods or, conversely, could some traits of the life history of this species determine an innate ability to re-nest, consequently allowing this species to breed everywhere?

Incubation period and the different roles of breeders Incubation, undertaken by the female only, should begin immediately after the first egg is laid; however, Mikkola (1983) reported that it in fact starts when the second egg is laid. As reported by Wassink (2010b), at the beginning of incubation the female may leave the eggs alone all day. The length of the incubation period seems to be slightly variable, ca. 32–35 days (34–36 in Cramp & Simmons 1980; up to 37–39 days in Il’ukh et al. 2009). The incubation period (from the laying of the first egg to the hatching of the first young) calculated by the help of webcams was 35–36 days (Wassink 2010b). In captivity, Broo & Lindberg (1973) reported 33 days of incubation. In Romania, the incubation interval seems to vary between 28 and 37 days (Nicola Pârlog unpubl. data), and it has been suggested that this variation is conditioned by weather and temperature, with a warm spring perhaps determining shorter incubating times than a cold spring. During incubation, the female can leave the nest several times (Choussy 1971, Il’ukh et al. 2009, Wassink 2010b), e.g. three times per day on average. Usually, the female returns to the nest after a few minutes: Wassink (2010b) reported that females returned after 11–27 minutes, similarly to that described by Il’ukh et al. (2009); however, the latter observed even longer periods of absence from the nest. Typically, the females left the nest just after sunset (between 18:00 and 22:00) and about an hour before sunrise. Between 22:00 and 04:00 hours the eggs were less often left alone, whereas during the day, and with very few exceptions, the female was almost always on the eggs (Wassink 2010b). The chicks hatch at intervals of several days, generally two, and weigh about 50g when they break out of the egg. To conclude, it is crucial to mention here that the early breeding stages represent the most sensitive and fragile period for this species. Thus, any interaction with incubating females must be avoided. Indeed, if the female is disturbed at the nest when incubating or during 181

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hatching, she often abandons eggs and even chicks only a few days old. This latter case is rarer and largely depends on individuals, but may still occur. Today we have plenty of data on Eagle Owl clutch sizes and chick measurements, and additional nuisances during the early stages of reproduction are not justified. Additionally, females disturbed repeatedly at the nest exhibit a typical behaviour that can easily expose nestlings to predators. Indeed, when a female with young is disturbed for the first time, she tends to fly away from the nest at the last moment and comes back relatively quickly. However, after successive visits to the nest, the female leaves earlier each time and delays her return to the nest; in some cases, a female may decide not to come back to the nest during the day, which leaves the nestlings completely unattended. Thus, repeated visits to the same nest, as well as those early in the season, should be avoided.

The nestling period Sex ratio The knowledge of sex ratios in bird populations is an important aspect of a species life history. Sex allocation is subject to multiple influences, with females being able to adjust the offspring sex ratio according to diverse social, parental and environmental conditions (Hardy 2002). According to the theory of sex-ratio allocation (Fisher 1958), parents should allocate an equal amount of resources to each sex because half of all genes are passed on through the male and half through the female. This means that because exactly half the parental effort (e.g., food) provided to offspring should go to males and the other half to females, the sex with the greatest nutritional needs should be produced in lower relative numbers. Thus, when individuals of one sex are more costly than the other to be produced and/or raised successfully, as in dimorphic species, sex ratios may differ from equality. Information on the sex ratio of owls is scarce and sometimes contradictory. For example: (a) female Common Scops Owls Otus scops seem to control the production of daughters and sons in relation to laying order (Blanco et al. 2002); (b) Hörnfeldt et al. (2000) showed no relationship between hatching order and offspring sex ratio in Tengmalm’s Owl Aegolius funereus; (c) physical condition of parents appears to affect offspring sex ratio in Tawny Owls, female-biased clutches being more frequent in those breeding sites with more abundant prey (Appleby et al. 1997); (d) female Ural Owls produce more males under poor food conditions (Brommer et al. 2003); and (e) Hipkiss et al. (2002) and Hipkiss & Hörnfeldt (2004) observed high interannual variation in the hatching sex ratio of the Tengmalm’s Owl and a significant correlation between favourable food supply and a female-biased sex ratio. Because Eagle Owls exhibit reverse sexual dimorphism and hatching asynchrony, they may possess a mechanism for sex allocation (Olsen & Cockburn 1991), as females may be able to manipulate the sex of their nestlings depending on the environmental conditions. To our knowledge, the only published study on Eagle Owl sex ratios is Mora et al. (2010). They sexed nestlings by molecular procedures using DNA extracted from blood and calculated the average sex ratio of 101 nestlings (n = 45 broods) belonging to 22 distinct nesting sites. Sex ratio was defined as the number of males divided by the total number of individuals (i.e. males/(males + females)). The population they studied exhibited a nestling sex ratio of 0.53, which differed significantly from the binomial distribution (χ2100 = 116.1; p < 0.003), suggesting a possible parental control of sex allocation. Statistical analyses 182

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indicated that the sex ratio of this population was only influenced by the egg-laying date (F = 9.27, p < 0.05; p > 0.1 for all other parameters – year, clutch size, hatching order, quality of the breeding site measured as the amount of Rabbit in the parental diet and Rabbit abundance, the Body Condition Index estimated by a reduced major axis regression using logarithm of both body mass and tarsus length, measures of biochemical parameters, immune responses and parasite loads). That is, when an earlier laying occurred, a greater number of female nestlings were raised. Moreover, when laying occurred later in the year, a greater number of male nestlings were raised. Thus, it seems that female Eagle Owls may adjust the sex ratio of their clutch according to their ability to withstand the costs of reproduction. Under conditions of high food availability, females’ physical conditions are improved and, therefore, they may start breeding earlier and be more able to obtain the necessary prey to raise female nestlings. However, it is not possible to dismiss the possibility that later pairs produce a higher proportion of the less costly sex because food availability will predictably decrease as the breeding season advances, this being particularly critical for late breeders (i.e. Eagle Owls may adjust the sex ratio according to food availability during the nestling rearing period rather than in relation to their own condition). Finally, because information on offspring mortality prior to DNA sampling was not collected, the increased number of males in late broods might also be due to differential mortality at the beginning of the nestling phase (i.e. relatively more females dying in late broods).

Hatching asynchrony Hatching asynchrony in altricial birds is the serial production of offspring that occurs when the incubation of asynchronously laid eggs starts before clutch completion. Hatching asynchrony has several, mutually non-exclusive consequences, e.g. (a) within-brood differential growth, particularly manifested in large broods when the time interval in egglaying is more than one day, (b) a competitive hierarchy among nestling broodmates and (c) asymmetric sibling competition, frequently resulting in post-hatching mortality of the last hatched offspring. Siblicidal brood reduction can occur directly (usually through wounding) and indirectly, e.g. through forced starvation or expulsion from the nest. The phenomenon occurs mainly in species that have weaponry such as claws and sharp beaks, including birds of prey (e.g., Bortolotti 1986). However, siblicide is rare in owls even though they generally produce large broods with a time interval between the laying of eggs exceeding one day (Mikkola 1983, Voous 1988). Thus, species able to raise large broods should accordingly have evolved mechanisms to reduce within-brood mortality caused by sibling aggression, and one such way to reduce offspring mortality caused by siblicide is to favour the production of the larger sex among late-laid chicks. Penteriani et al. (2010a) tested two hypotheses possibly explaining sex allocation in Eagle Owl broods as a function of clutch order. The first hypothesis (the energetic cost hypothesis) takes into account the fact that the costs of feeding and caring for young are assumed to vary among reproductive phases and, as parental expenditure on progeny of both sexes is equal, chicks of the energetically less expensive sex should more frequently be produced later in large clutches. Males are smaller than females and thus presumably represent the energetically less expensive sex, as observed in some birds of prey and owls, and hence require fewer resources (Anderson et al. 1993). Thus, following this hypothesis it is possible to predict that male chicks would be more common than female chicks as the 183

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third and fourth offspring, as opposed to first and second, to reduce the energetic costs of brood-rearing. The second hypothesis (the reduced sibling aggression hypothesis) says that if body size is a poor indicator of potential parental costs and/or the larger sex does not significantly increase the biological costs to parents, then sex allocation within a brood could partly result from a reduction of conflicts among nestlings. In such circumstances physical strength could be important in shaping the competitive relationship among siblings. In this case, two scenarios can be envisioned: (1) due to the species’ sexual dimorphism, sibling aggression or competition for food is likely to be extremely strong between the first- and the last-hatched chicks. Consequently, more females would be expected among third- and fourth-hatched chicks if the first-hatched is a female. In fact, a female that hatches first in a brood will rapidly become substantially larger and heavier than a male that hatches fourth, consequently increasing the risk of mortality of the smaller young; and (2) because the Eagle Owl is slightly size-dimorphic, the sex (and consequently the size) of the first-hatched chick may not be important in determining sex allocation and, due to the competition among the first- and last-hatched chicks, more females should still be expected among the third- and fourth-hatched chicks, irrespective of the sex of the first-hatched chick. The above-mentioned study was conducted during five breeding seasons (2003–2007) in an area consisting of several geographical subunits in southern Spain: (1) the Sierra Norte of Seville (the Sierra Morena massif, SW Spain), and (2) three neighbouring hilly areas in the regions of Murcia and Alicante (SE Spain). Some traits of the Eagle Owl might facilitate the evolution of aggressive competition among siblings, including: (a) direct feeding prevalent throughout the nestling period (i.e. food passes directly from the adult’s beak to that of the chick), as selection favours sibling aggression in species in which dominant chicks can monopolise the food that parents bring to the nest; (b) effective weaponry (larger nestlings are able to injure smaller brood mates); (c) nest-site topography, which reduces the chances of escaping, such as cliffs, caves and large trees; (d) differences in age and size of brood mates due to hatching asynchrony; (e) food items are often large and infrequent, clustered in bouts or meals, and as aggression is more costly than begging or fighting for food, larger food items are a significant reward for the aggressor; and (f ) a long nestling period (at least 30–35 days), which favours aggressive interactions among brood mates and increases the possibility that cohabitation will coincide with periods of food shortage. As the first step for testing the two study hypotheses, the authors estimated the size frequency and the proportion of males (hereafter sex ratio) for broods from SW and SE Spain, and for both areas combined. Actually, sex ratio was calculated as the number of male chicks relative to the total number of chicks. The sex of 349 Eagle Owl chicks from 137 broods (SW Spain = 34 and SE Spain = 103 broods) was examined. Determination of the hatching order of nestlings by monitoring egg-laying and hatching sequence is impossible in this species because of the extremely high risk of nest abandonment due to disturbance. Moreover, if nests are observed when owlets are 30–40 days old, determination of the hatching order may be complicated as a result of sexual size dimorphism. To overcome these limitations, the studied broods were examined when their offspring were less than a month old, at which age the effect of hatching asynchrony predominates over that of sexual dimorphism (Penteriani et al. 2005b). Chicks were sexed through molecular procedures (DNA extraction from owlet blood). The sex ratio of owlets in both regions was close to 0.5 (SW Spain = 0.46; SE Spain = 0.53), corresponding to the general sex ratio of 0.48. However, SW Spain (χ287 = 121.6, p < 0.0025), SE Spain (χ2260 = 369.4; p < 0.001) and both samples combined (χ2348 184

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= 483.3, p < 0.001) differed significantly from a binomial distribution, suggesting parental control of sex allocation. The proportions of different brood sizes were, respectively for SW and SE Spain, 26.5% and 7.8% (1 chick), 38.2% and 22.3% (2 chicks), 26.5% and 49.5% (3 chicks), and 8.8% and 20.4% (4 chicks). These percentages were not significantly different from the modal clutch size of 2 for SW and 3 for SE Spain (SW Spain: χ23 = 0.5, p < 0.8; SE Spain: χ23 = 0.0, p = 1; both samples combined: χ23 = 0.0, p = 1). For clutch sizes of four eggs: (a) there was a high probability (79%) that the third chick was a male (partially supporting the energetic cost hypothesis), and (b) the fourth chick was slightly more frequently a female (57.9%). The latter probability was not apparently affected by the hatching of a female as the first chick, as the probability that the fourth chick was a female was only 33.3% (reduction of sibling aggression hypothesis, scenario 1). Further statistical analyses supported the results obtained by the Bayesian estimation of probabilities for the energetic cost hypothesis for the sex of the third chick (Penteriani et al. 2010a). Although the probability that the fourth chick was a female was approximately 60% in Eagle Owl broods, no significant relationships were detected between the sex of the fourth chick and the sex of the other chicks. Thus, to summarise, the main pattern highlighted in sex allocation may be considered a tradeoff between the energetic cost hypothesis and scenario 2 within the framework of the reduced sibling aggression hypothesis. In broods of four chicks, Eagle Owls seem to invest in the less energetically costly sex as the third hatchling (favouring the hatching of males). This could favour the production of female chicks (the larger sex) as the last (and smallest among the chicks in the nest) of the brood. This strategy has the potential to reduce sibling aggression, as maternal strategies for adjustment of brood-mate sex are the result of selection to minimise the detrimental effects of sex-specific sibling interactions and, thus, is expected to maximise the fecundity of breeders. The observed patterns of sex allocation might be a way of resolving the apparent paradox represented by the contrasting effects of hatching asynchrony vs. sibling rivalry in those species that can raise large broods. If hatching asynchrony is adaptive, but has the collateral effect of increasing brood-mate aggression and killing, a possible solution could be to commence incubation prior to clutch completion as a way of increasing egg viability and to adjust sex allocation within broods to favour the larger sex as the last hatchers (to eliminate or reduce sibling aggression). As the risk of siblicide mainly depends on the sex composition of the brood, disparity in hatchling size may be an important factor involved in siblicide. However, even if the adjustment of sex ratio within broods appears important, the success of clutches may be determined by the relative position of each sex within the brood. In the Eagle Owl, the sex composition that provides the highest number of surviving offspring may be the one that produces a female as the last chick in broods of four eggs in cases when the energetically less expensive male is the third chick. Evidently, because Penteriani et al. (2010a) only focused on hypotheses related to a specific aspect of within-brood sex allocation, it is not possible to exclude the role of other factors contributing to sex allocation. However, by studying the same populations at multiple areas over several years, the authors probably reduced potentially confounding effects of variability in environmental conditions affecting the study populations. There is a relationship between hatching order and sex allocation, with females being able to control the sex of chicks hatching from individual eggs in the laying sequence. Because posthatching control is likely to be energetically more expensive and directly affects the lifetime reproductive success of individuals, pre-ovulation control mechanisms should be favoured in species able to produce large broods, in particular through manipulation of the sex of offspring in high-ranking positions within the brood. 185

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General development of chicks, from nestlings to fledglings Penteriani et al. (2005b) studied the morphological and morphometric development of 19 chicks from eight nests, from the first day after hatching (i.e. 1 day old) until they were 60 days old, after which it became difficult to find, approach or catch the young. Moreover, from this stage onwards, the visible differences in plumage become very subtle, making exact age estimation quite difficult. Nests were visited every five days, taking both photographs of the young and morphometric measurements of the body parts most useful in describing patterns of growth in this species (Delgado & Penteriani 2004): lengths of the forearm, bill, tarsus and wing, as well as body weight. The growth rate (K) and curve were calculated following Ricklefs (1967, 1973), the latter being fitted by the von Bertalanffy equation. Moreover, to provide a better comparison of growth patterns with those of other owl species, we calculated (Ricklefs 1968): (1) the ratio (R) between the average weight of young and the asymptote of the growth curve, and (2) an inverse measure of growth rate (t10−90), which represents the time required for growth between 10 and 90% of the asymptote. This time interval represents a practical index because it varies directly with temporal features related to growth (such as the duration of incubation and nestling periods), allowing comparisons between species (Ricklefs 1967). The most remarkable aspect of young development is the rapid increase in weight and size gain during the first 30 and 40–45 days, respectively (Table 10 and Figure 47). After these two time thresholds, there is a notable reduction in the rate of mass gain. Consequently, morphometric differences among young in subsequent 5-day periods are Table 10. Average (mean ± SD) weight gain and morphometric measurements (range in brackets) for forearm, bill, tarsus and wing lengths of Eagle Owl young from 5 to 60 days old. Age

Forearm (mm)

Bill (mm)

Tarsus (mm)

Wing (cm)

Weight (g)

5

35.1±3.2 (31.8–39.4)

13.0±0.4 (12.6–13.5)

32.5±5.2 (26.1–38.4)

74.3±13.1 (62.0–90.0)

131.3±45.9 (80.0–170.0)

10

48.2±3.5 (45.8–50.7)

17.3±3.1 (15.1–18.5)

40.9±2.2 (39.4–42.5)

100.0±14.1 (90.0–110.0)

285.0±21.2 (270.0–300.0)

15

67.1±4.0 (62.6–70.2)

18.4±0.5 (18.1–19.00)

54.1±2.7 (52.4–57.2)

163.3±5.8 (160.0–170.0)

530.0±75.5 (450.0–600.0)

20

90.9±1.9 (88.8–92.3)

21.4±0.2 (21.2–21.6)

68.8±8.2 (60.4–72.0)

226.7±20.8 (210.0–250.0)

810.0±36.1 (780.0–850.0)

25

112.0±8.5 (106.4–121.7)

22.8±0.4 (22.5–23.4)

69.3±3.1 (66.4–72.5)

275.0±21.8 (260.0–300.0)

1003.3±205.5 (870.0–1240.0)

30

126.0±6.6 (121.6–133.6)

24.7±0.2 (24.5–24.9)

80.6±4.2 (75.8–83.6)

355.0±8.7 (350.0–365.0)

1166.7±125.8 (1050.0–1300.0)

35

164.0±10.4 (152.0–170.0)

24.9±1.9 (22.7–25.0)

81.5±5.2 (77.8–87.5)

396.3±35.6 (359.0–430.0)

1323.3±144.7 (1230.0–1490.0)

40

177.0±5.3 (170.0–182.1)

27.6±1.7 (26.2–30.0)

94.4±0.1 94.3–94.4)

482.6±26.3 (460.0–520.0)

1375.0±64.6 (1300.0–1450.0)

45

177.5±3.5 (175.1–180.0)

27.4±0.3 (27.2–27.7)

94.8±1.6 (93.1–96.7)

565.0±21.2 (550.0–580.0)

1400.0±424.3 (1100.0–1600.0)

50

185.0±7.1 (180.0–191.2)

27.7±1.2 (26.7–28.5)

99.5±0.5 (97.5–100.0)

573.3±23.1 (560.0–600.0)

1533.3±152.8 (1400.0–1700.0)

55

188.7±9.3 (185.2–192.1)

29.1±1.7 (27.3–30.7)

100.6±7.1 (98.3–108.2)

575.0±7.1 (570.0–580.0)

1590.0±141.4 (1500.0–1800.0)

60

200.0±10.0 (190.0–207.6)

31.1±0.9 (30.4–31.7)

105.3±5.4 (101.5–109.1)

585.0±21.2 (575.0–600.0)

1775.0±176.8 (1600.0–1900.0)

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Table 11. Growth and life history parameters of owl species (for which growth rate is available in the literature). Age of 1st flight (days)

Clutch size

Adult weight (g)

Ratio (R)

63–70

2–3

1175

approx 50

7–9

approx 30

Tyto alba (10) Bubo bubo (19)

Species (n) Bubo virginianus (2) Bubo scandiacus (unknown) Otus asio (4)

Growth rate

Locality

Source

Kansas (USA) Baffin Is. (Canada) California (USA)

Hoffmeister & Setzer 1947

(K)

(t10–90)

1.02

0.094a

32.9

1922

0.88

0.101a

30.7

3–5

150

0.80

0.264

16.7

67

2–7

408

1.40

0.152

29.0

California (USA)

approx >50

2–5

1900

1.42

0.044b

48.8

Andalusia (Spain)

Watson 1957 Sumner 1928 Sumner 1929 Pickwell 1948 Howell 1964 Penteriani et al. (2005b)

Notes: Equation used to fit the growth curve: a = Gompertz; b = von Bertalanffy; none = logistic.

A

B

700

80

500

% of asymptote

Length (mm)

600

400 300 200

60 40 20

100 0

100

0 10 20 30 40 50 60

Age (days)

0

0 10 20 30 40 50 60

Age (days)

Figure 47. (A) Starting from ca. 40 days old, Eagle Owl young (n = 19) show a general decline in the rate of morphometric development in mean length of (from top to bottom): wing (grey line), forearm (solid black line), tarsus (broken black line) and bill (dotted black line). That is, after the first 40 days of life, morphometry may yield inaccurate estimates of age. (B) A similar trend is observed for mean weight increase, as illustrated by the growth curve (calculated from the von Bertalanffy equation), expressed as a percentage of the asymptote. This type of growth curve is typical of species whose nestling weight levels off below adult weight and where growth is almost completed after the young leave the nest (from Penteriani et al. 2005b).

not as evident as in the initial phase. Calculated growth parameters, as well as the growth parameters of other owl species for which such information was available in the literature, are given in Table 11. The K value was 0.044, corresponding to a weight increase of 0.025kg/ day. The time interval for growth from 10–90% of the asymptote was 48.76 days, a relatively large value compared with other Strigiformes (Table 11). Broo & Lindberg (1973) observed that males weighed significantly more than females from the third to the 16th day; after the 16th day females increased more in weight than males. In Belarus (Demyanchik 1990b), two chicks at the day of hatching weighed 37 and 46g; on the 24th day the rate of weight gain was 60g/day and on the 40th day was 30g/day. At the age of 45 days the average weight of chicks was 1,660g. Young Eagle Owls fit the general growth pattern in nestling raptors, 187

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which is characterised by an early short period of slow weight gain and morphological development, followed by a period of rapid weight gain and general growth, and then a second stage of slower development. The first phase lasts about 10 days, during which growth and body development are slower than in the next 20–30 days (i.e. the second phase). A third ‘plateau’ phase in growth can be observed from about the 30th (for weight gain) and 40th (for morphometry) day of life. Relying solely on morphometric data during this third period could produce critical errors in ageing young. More evident and useful for ageing young are the differences in morphology (see Figures 48–58, in which all the most important details for correctly ageing young are described). Nestlings remain totally white until 10–12 days, when they start to show a barred appearance and a greyish-beige uniform pattern. Starting from ca. 30 days, rapid and noticeable feathering (i.e. remiges) occurs, as well as the development of ear-tufts and a well-defined facial mask. Scapulars and coverts become evident at 45 days. The previously reported growth slowdown also corresponds to slow changes in morphological development. However, plumage characteristics become more useful and reliable for ageing at this stage. In fact, several plumage features (e.g. feathers appearing on the upper part of the throat and at the base of the neck, emergence of wing coverts, better definition of white and black patches on facial masks, three horizontal bands of feathers on the wing) may allow young to be aged reliably until they are 60 days old. Modifications of the plumage are consistent among young, i.e. the pattern of feather growth is the same among all young of the same age. A similar description of plumage development has been reported by Scherzinger (1974) and Lossow (2010), the latter for young from 2 to 58 days old, showing the reliability of morphology for ageing; additionally, morphology does not seem to depend on sex and size variations of individuals across their distribution range. The growth rate of Eagle Owls, as in the other owl species represented in Table 11, is typical of altricial species characterised by slow growth and a long nestling period. In fact, all of these species nest in secure and well-protected sites, allowing young to remain in the nest longer than those that are exposed to predators and inclement weather. It is interesting to note that the Eagle Owl growth rate is the lowest among those owl species for which growth data are available, although it is similar to that of the Great Horned Owl Bubo virginianus. Finally, although clutch sizes of raptors and large passerines generally show a positive correlation with growth rate, Eagle Owls (as in Barn Owls Tyto alba and Snowy Owls Bubo scandiacus) have large clutches, and these are not associated with high growth rates.

Young behaviour at the nest Between feeds, as well as during the day, young intersperse periods of sleep with exploration and other activities, e.g. they catch objects with their claws, focus on nearby objects with staring and wide head-circling movements, exercise their wings, call, jump, scrape and bite at their feet and feathers. Thus, their diurnal activity is relatively intense (e.g. Choussy 1971, Blondel & Badan 1976, Penteriani et al. 2000). Young generally sleep leaning against one another, frequently on or against prey, and sometimes bill one another gently. The young increase their awake time slowly when they are between 30 and 40 days old (Wassink 2010b). At that time, most of their daily activity takes place between 16:00 and 00:00 hrs, as recorded by Wassink (2010b); after midnight activity starts to decline and young engage in much less activity between 07:00 and 16:00 hrs, when they are mostly sitting in the nest cavity. 188

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Figure 48. Chicks at 1–3 days post-hatching. Nestlings are covered in a whitish first down and their eyes are still closed. Some parts of the body are still naked (e.g. shoulders, belly), resulting in some light pink patches. The eyes start to open at 4 days and are completely open by the age of 6–7 days. At this time, the eyes have a grey-blue pupil and a dark yellow iris. Nestlings lie prostrate, the body in contact with the substrate, and they have pink toes and tarsus with light grey claws. They may give an acute and plaintive call (see Chapter 12).

Figure 49. Chick at 10 days post-hatching. Nestlings are still covered in white down. Sheathings are clearly visible along the shoulders, in correspondence to the future rectrices (close to the rump), scapulars and remiges, as well as on the upper part of the tarsus. Two parallel shaft lines mark the thorax. The belly starts to be covered by down.

Figure 50. Chicks at 15 days post-hatching. The white colour of down tends to be increasingly grey-beige, and the appearance of a secondary down gives the birds a ‘streaked’ aspect (especially on the wings, shoulders and rump). The down is now abundant around the body and has a woolly appearance. The development of pin feathers starts to be apparent, and primaries and coverts are ca. 7 and 13mm, respectively, outside the pin feather sheaths. Eyes are more protruding and a yellow-orange iris appears around the grey-blue pupil. Covering of the belly by down is still incomplete. The still present egg tooth and initial white patch around the bill are also evident in the picture. Nestlings are now able to assume an aggressive posture, opening the wings and snapping their bills, even if they are not yet very stable on their feet. If not really necessary, it is strongly recommended that nests are not checked before this age.

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Figure 51. Chick at 20 days post-hatching. The streaked aspect of nestlings is more evident, with the body entirely covered by dense down except on the belly, still incompletely feathered in the middle. The eruption of remiges from pin sheaths becomes more evident, especially for secondaries. Pin feather development between the nape and back starts. Bill colour is darker and vibrissae appear surrounding the beak. The white patch around the bill now markedly contrasts with the down around it, even if its development is limited to a small patch on the lower extremity of the bill. At this stage the nestlings start to emit their typical chwcitsch or chwätch call (see Chapter 12). The yellow-orange iris is larger and the pupil more blue (the grey texture is disappearing). Feet and claws start to resemble those of an adult.

Figure 52. Chick at 25 days post-hatching. The white patch around the bill is even more evident and the vibrissae are more abundant around the bill. On the crown, future ear-tufts start to develop, although at this stage they are only made of down. The egg tooth disappears at this stage of nestling development. The general appearance of the young is darker, due to the more visible streaks. Down development on the belly is now complete. The black mask around the eyes appears. Pin feather development of the remiges, especially primaries and coverts, is now clear. Rectrice sheathings also appear.

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Figure 53. Chick at 30 days post-hatching. The general appearance is quite similar to the previous stage, but with all the above-cited morphological characteristics more developed. That is, more defined black and white patches on the facial mask, more prominent ear-tufts and longer pin feathers (largest pin feathers are ca. 8–9cm, sheathing included). Dark brown patches on the feet have disappeared and they are now a homogeneous cream-beige colour.

Figure 54. Chick at 35 days post-hatching. Changes between 30 and 35 days are pronounced, especially in the facial mask and wings. White contours around the bill and eyes are well marked, as well as the black spot on the upper part of the eyes. Ear tufts are longer and stick out clearly from the crown. Remiges (now also secondaries) and rectrices continue their eruption from pin sheaths and begin to form a quite visible horizontal bar along the wings. At this stage nestlings can walk out of the nest and roost several tens of metres from it. If disturbed, they can throw themselves from the nest and glide a considerable distance away.

Figure 55. Young at 40 days post-hatching. Primaries are about two-thirds of full length and the emergence of tail feathers from sheaths is now pronounced. Feathers are starting to emerge on the nape and head, especially on the facial mask near the eyes.

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Figure 56. Young at 45 days post-hatching. Primary remiges and rectrices (ca. 8–9cm of feather is visible out of the sheath) are ca. 80% and 40% of their ultimate length, respectively. Secondaries are still encased in ca. 7–8cm sheathings. More and more nape, scapular and dorsal feathers are erupting from pin sheaths, which begin to contrast markedly with the body down. Several feathers also appear on the upper part of the throat, at the base of the neck. Wing coverts begin to emerge. White and black patches on the facial mask are better defined.

Figure 57. Young at 50 days posthatching. Feather sheaths of secondaries are now reduced to ca. 4–6cm. Wing coverts continue their development and contrast more with the remaining downy areas. This represents a useful ageing element: at this stage, wings appear separated into three clearly defined horizontal bands, i.e. a downy pale band between dark primaries and upper coverts. Rectrices have reached about half their final length. A well-defined black line now separates the auricular area from the head.

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Figure 58. Young at 60 days post-hatching. Morphological changes begin to be less evident at each stage and, consequently, ageing from the general feather pattern is increasingly difficult. The most evident trait is the appearance of several well-developed feathers on the neck and back.

As an anti-predator response (e.g. Schnurre 1936, Desfayes & Géroudet 1949, Guichard 1956, Scherzinger 1974), starting at ca. 15 days of age, Eagle Owl young bill-snap and hiss in response to any disturbance. When approached, the young within a nest may also lean against one another, slowly turn their heads from left to right, blink their wide eyes, and bill-snap. In addition, young may raise wings in an incipient threat posture, similar to that of adults; and when threatened, young may, although rarely, lie on their back and raise their claws. A similar behaviour may also be exhibited after fledging, when young are out of the nest. Cannibalism, i.e. the consumption of debilitated or already dead nestlings, has been reported by several authors (Frey 1973, 1992, Haller 1978, Berezovikov & Vorob’yov 1986, Suchý 1990, Grüll & Frey 1992), even when food resources do not seem to be scarce (Miltschev 2008). Predation on young by adults of neighbouring nests is also possible, and not necessarily linked to food scarcity, as we recorded in our study area in SW Spain. The remains of at least one of the two ringed chicks of a nest were found in the nest of the neighbouring pair breeding only 250m from the first one. 193

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Parental behaviour during the nestling period Eagle Owl young are brooded almost constantly for two weeks, less so thereafter (Frey 1973). Nevertheless, the time females spend at the nest is variable and may depend on factors not strictly related to the development of young, e.g. characteristics of the nest, individual variability in brood investment and local weather. In their first week of life the chicks are barely left alone (Wassink 2010b). Thereafter, females may start to leave the young alone for longer periods of time. At three weeks of age, young Eagle Owls are suddenly left unattended for larger intervals (e.g. from 600 to more than 1,200 minutes per day in Wassink 2010b). However, this pattern may change depending on local conditions and individual variation in behaviour. A possible explanation may be that the young have a higher food demand and, thus, the female is forced to participate in hunting. However, in some cases, the nest is too small to accommodate the female and all of the nestlings, which are thus left alone in the nest. At one month, if the young are still in the nest, they are frequently left alone during the daytime and much of the night; however, the female is always perched very close to the nest and guards it. Frequently, males also roost close to the nest during the day, although we have observed the diurnal roost of a male breeding in a quarry at a distance of 1.5km from the nest, and Buhot (2009) reported the occurrence of males roosting ca. 1km from the nest. Both males and females may vigorously defend young, although females seem to be the more aggressive sex in this context, at least towards human intruders. Cramp & Simmons (1980) reported that males may be passive, even if young are in danger; however, as also observed by Choussy (1971), Defontaines (1999) and Gritschik & Tishechkin (2002), we have observed on some occasions males vigorously defending their young. Nevertheless, the intensity and efficiency in young defence vary considerably between individuals, which may depend on many non-exclusive factors, for example, individual personality and specific brood investment. Young defence is not limited to the nestling stage, being also very strong when fledglings are scattered around the nest during the post-fledging dependent phase. Indeed, although it seems that parents are more aggressive from sunset to sunrise, daily attacks on humans may occur. Some males and females may perform certain disablement types of distraction-lure displays (as also reported by Frey 1973, Cramp & Simmons 1980 and Defontaines 1999). For example, when humans are close to young, an individual may fly to a nearby and very visible place and then start to stare, droop its wings, simulate a loss of balance and fly around the intruder, increasingly farther from the nest. The intensity of these distraction-lure displays is extremely variable depending on the individual, time of day and period of the disturbing event, as well as the frequency with which the nest has been disturbed during the breeding season. A similar behaviour has also been reported by Ramanujam (2014) for young Indian Eagle Owls Bubo bengalensis. März (1953), Cramp & Simmons (1980), Choussy (1971), Demyanchik (1990b), Beuchat (2008) and Karl-Otto Jacobsen (Lurøy, Nordland, pers. comm.) reported that after disturbance some females may move young from the nest to a safer place. In addition, many people generally accept that females remove nestlings when disturbed, although there is still no firm confirmation of this behaviour for most of the cases that have been described. In many cases we have been able to demonstrate that the supposed nestling removal was in reality due to the failure of a first breeding at the small chick stage and the appearance of new young in another nest because of a replacement clutch. Nevertheless, available information 194

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does not allow the complete discard of this type of behaviour, which is considered relatively frequent by Demyanchik (1990b). Demyanchik (1990b) reported that young Eagle Owls can be repeatedly transported out of and into the nest several times during the nestling phase, with this event having been observed up to four times for a single nest. Indeed, the transporting of young seems to have been proven for the Great Horned Owl, as reported by Brady (1976) who described the displacement of two nestlings on two consecutive days to a distance of about 70m from the original nest.

Prey delivery by the parents Prey delivery largely occurs during the night, the first one being generally delivered by the male a little after sunset. However, as also reported by Choussy (1971), Eagle Owls (prevalently males) may also carry prey to the nest during the day, largely in the afternoon or shortly after sunrise. Mysterud & Dunker (1983) reported that the duration of adult visits when feeding was quite variable, and in some cases the prey was just flung into the scrape and the adult disappeared immediately, after only 2–20 sec. They never observed males actively feeding young and, thus, their visits were shorter (48 ± 58 sec) than those of females (294 ± 339 sec). When the female visited the nest, the prey was torn into pieces and the young were fed. However, Defontaines & Defontaines (1998) reported a peculiar case in which a male was able to successfully feed both the young and its female, the latter having lost one wing (probably during the breeding period). The number of feedings ranged from 0–11 per day in Wassink (2010b). Most feedings took place from 15:00 to 23:00 hrs, with less feeding between midnight and 5:00 am, followed by an increase around sunrise. Evidently, rates of feeds may be quite variable (e.g. Kranz 1971, Blondel & Badan 1976), mainly depending on the number of young, length of the night, weather conditions and type of prey the adults are able to find and hunt in the vicinity of the nest: the larger the prey, the fewer the prey deliveries.

The fledging and post-fledging dependence period Fledging and duration of the nestling period As also remarked by Frey (1973) and Blondel & Badan (1976), when the nest is on the ground or close to it, nestlings may leave the nest when they are 4–5 weeks old (even if some chicks may start wandering around the nest one week before). To leave a nest on a cliff (and, in some cases, a tall tree), nestlings should be older. It is difficult to say exactly at what age nestlings on a cliff will fledge or, at least, leave the nest, because this mainly depends on the surrounding area. Clearly, a completely vertical cliff will require a real first flight, which may happen when the young is ca. 2 months old (Mebs 1972 reported an age of ca. 10 weeks) or, at least, when the largest feathers of the wings are well developed, to slow down the fall like a ‘parachute’. This form of leaving the nest may also occur as a response to the approach of a potential predator to the nest. But, even on a vertical cliff, if the young can jump from the nest to a neighbouring trunk or rock, or it can climb easily in the surroundings of the nest, it may also fledge earlier. Indeed, young Eagle Owls have a surprising ability to climb even nearly vertical cliffs with the help of their claws. At ca. 2 months of age young can fly and 195

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leave nests on vertical cliffs or high points like buildings or stick nests in trees. Thus, fledging is directly correlated with nest site morphology: the more elevated the nest on a high place or vertical cliff (where fledging needs to be a first real flight), the later fledging takes place.

Movements during the post-fledging period In birds, the period from fledging to independence is known as the post-fledging dependence period. During this stage, juveniles gradually have to: (1) enhance muscular development, (2) experiment with the external world and conspecifics without the protection of the nest, and (3) learn essential skills to survive as an adult. This period is a crucial phase in the life of an Eagle Owl because it represents the time during which individuals reach the necessary body condition for dispersal. Reproductive efforts invested by parents during the previous stages may be lost if post-fledging occurs in hostile and uncertain environments or conditions. This stage of a bird’s life is also an extremely dynamic phase, in which juveniles increase their mobility and parental protection decreases. If the increased travel occurs in hostile or unknown habitats, mortality could increase due to, for example, predation (Sunde 2005), electrocution (Sergio et al. 2004a), as well as starvation and disease (Aebischer et al. 2005). This increase in juvenile mortality has the potential to affect breeding populations because of the reduction of the number of juveniles that will start dispersion. Thus, the knowledge of movement patterns during the post-fledging period may be one of the most useful tools to allow us to better understand this final phase of the breeding cycle, as well as to be aware of the possible risks that the fledglings may be confronted with. This is particularly important in the case of endangered species that during their first exploratory movements may be faced with various stochastic events. In a preliminary study on post-fledging movements, Penteriani et al. (2005b) followed eight offspring that were radiotagged (n = 4 nests) every 3–5 days, thus collecting 168 location fixes from 45 (when fledglings start to move in the vicinity of the nest) to 150 days of age (when the first dispersal event was recorded). Because fewer than 20 high-quality fixes were obtained for the majority of juveniles, it was not possible to estimate home range size for fledglings (Kennedy et al. 1994). During post-fledging, the movements of the radiotagged owls showed that: (1) the mean distance from the nest was 504 ± 266m; (2) the mean distance from the nest increased significantly with age (t = −3.68, p = 0.0001): 492 ± 307m for juveniles < 100 days old (n = 43) and 1040 ± 88m for juveniles > 100 days old (n = 125). However, starting from 85 days old, the absolute maximum distance between a juvenile and its nest could rise to 1,500m; (3) the mean distance between siblings was 280 ± 13m, with maximum distances of 698 and 1,318m for juveniles of < 100 and > 100 days, respectively. The mean distance between siblings increased significantly with age (t = −2.43, p = 0.03), being 168 ± 15m for juveniles < 100 days old (n = 43) and 489 ± 81m for those > 100 days old (n = 125). To our knowledge, young Eagle Owls show the longest post-fledging period and the greatest distances from the nest ever recorded for an owl species. In comparison, the young of some other owl species spend relatively little time at the natal site before dispersing. In general, little or no information exists on the post-fledging areas of young. This is peculiar given: (1) the recognised importance of such areas for animals and (2) the frequently reported high mortality rates of young prior to dispersal. Such characteristics are likely to be more extreme in human-altered landscapes, where, for example, current evidence seems to indicate that predispersal mortality could markedly affect Eagle Owl offspring, reducing the 196

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actual breeding success of nesting sites (Sergio et al. 2004a). Such a mortality risk is probably increased by the large amount of time (an average of six months at least) that young spend in the post-fledging area. Such long exposure to potential mortality factors exaggerates the importance, for conservation management, of identifying and taking into account potential mortality factors acting within post-fledging areas, especially for those species that move largely around their nest before dispersal. Finally, occasional exploratory movements during post-fledging should be taken into account when censusing occupied nests or evaluating breeding success by passive auditory surveys of young. In fact, the best stages for listening to begging calls also coincide with the time of the most distant movements from the nest, increasing the possibility of bias in nest checking and productivity evaluation. Because during post-fledging the young may perch far from their nest for more than one day, several listening sessions should be planned on different days before considering a breeding place as unoccupied or unsuccessful. Moreover, because at this stage siblings usually move together and stay in close proximity throughout the night, it may be possible to not hear any calls at all for a full night even in close proximity to the original nest. Subsequent to this first, preliminary study of the post-fledging dependence period of young, a more detailed study on post-fledging movements was carried out by Delgado et al. (2009a). In this study, 41 radiotagged fledglings (24 males and 17 females) from 13 different nests were followed in SW Spain (Sierra Norte of Seville) from the beginning of their displacements around the nest to the start of dispersal. Natal dispersal started when the distance between successive moves became larger than the average distance travelled by each animal (Delgado & Penteriani 2008, see Chapter 10). During the post-fledging dependence period radiotracking was performed two times per week during the whole night, from one hour before sunset to one hour after sunrise. In these radiotracking sessions all nests were visited and owlets from all family units radiolocated simultaneously, with a 1-hour time interval between successive individual locations (no more fixes were necessary because fledglings are not very mobile and they alternate movements with long resting periods). Following Penteriani et al. (2005b) and Delgado & Penteriani (2007), the post-fledging dependence period was divided into six periods of 20 days. Owlets were sexed by molecular procedures using DNA extracted from blood, and to determine individual physical condition several morphological, biometrical and biochemical parameters were recorded: morphological and biometrical measurements were summarised into a body condition index estimated by a reduced major axis regression, using the log of both body mass and wing-length. From plasma samples, cholesterol, urea and total protein concentrations were determined. From the 41 tagged owlets 1,962 locations were obtained, showing that average step length (i.e. mean distance between successive owlet locations), distance from the nest, between-sibling distances and size of post-fledging areas increased with time throughout the post-fledging dependence period (Figure 59). Step lengths were short (mean = 344 ± 32m) from the time juveniles left the nest until 20 days after fledging. During the next 60 days, the distance travelled between successive moves significantly increased, Figure 59A), reaching its highest value close to dispersal (734 ± 161m). Also, juveniles were closer to the nest during the first 20-day period (334.8 ± 25.5m), travelling farther from the nest during the rest of the post-fledging dependence period, reaching a maximum distance just before dispersal (Figure 59B). Sex of the owlet and time spent in the post-fledging period showed a significant effect on between-sibling distances. Until 20 days after owlets left the nest, family 197

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Figure 59. Movement characteristics of owlets (n = 41) by 20-day periods during the post-fledging dependence period. Means and 95% confidence intervals are provided. (A) Mean distance between successive nightly locations, (B) distance from the nest, (C) distances between siblings, and (D) size of post-fledging areas (Delgado et al. 2009a).

units were closer together, independent of the sex of juveniles (197 ± 30m). Between-sibling distances increased with time as individuals became increasingly mobile (Figure 59C). The closest proximity was between siblings of different sexes (238 ± 31m), the next closest was between males (290 ± 19m) and the farthest was between females (310 ± 25m). Just before dispersal, family units seemed to dissolve, with a mean spacing between individuals of 613 ± 149m. Post-fledging areas varied with time since fledging: owlets increased the areas prospected from 0.2 ± 0.1km2 when they left the nest to a maximum of 0.9 ± 0.3km2 just before dispersal (Figure 59D), whereas sex appeared not to affect the size of these postfledging areas. In addition, the tortuosity of paths followed during displacements significantly decreased with time since fledging. Paths were more tortuous when owlets left the nest than at the end of the dependence period: at the time that owlets travelled significantly farther distances, their movements described straighter paths. Generally, it seemed that these paths during the dependence period were significantly unoriented. Finally, body condition index, blood cholesterol, urea and total blood protein concentrations did not show any significant effects on owlet movements. That is, fledglings seemed to change their movement behaviour independent of their physical condition. 198

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The analysis of fledgling movements allowed the detection of several notable behaviours during post-fledging. After leaving the nest, owlets moved with short steps, focusing their activities at, or very close to, the nest. Consequently, at this time, the post-fledging areas prospected by young were limited to areas around the nest. After a few weeks, the movements of fledglings showed a marked change, with their movement trajectories longer than initially observed. Moreover, they were frequently located farther from their nests. This change in post-fledging behaviour is probably due to the increased flying ability of owlets. In fact, when Eagle Owl young leave the nest (approximately 35–45 days in the study area) their: (a) remiges and rectrices are only 80% and 40% of their final length, respectively; (b) secondaries are still encased in 7–8cm sheathings; and (c) wing coverts are only starting to emerge (Penteriani et al. 2005b). Because at this stage owlets cannot fly but rather only walk and jump among rocks and bushes, the movement patterns we observed mainly reflect this form of displacement, i.e. that of a ‘terrestrial bird’ that cannot use its wings. Although fledglings continued to expand their post-fledging area size indefinitely until departing from the natal area, most post-fledging areas included the nest within their activity area throughout the entire post-fledging dependence period. Thus, the nest still represents a focal point all through the post-fledging period. In the early post-fledging period, siblings tend to move together, with a consistent distance between them during the first weeks. But, as independence neared, the distance increased progressively and family units seemed to partially dissolve. Increased sibling distance can be due, in a non-mutually exclusive way, to conflict behaviours between juveniles or improved flight abilities. Although inter-sibling distances increased during the entire dependence period, juveniles of different sexes tended to stay close together during the whole post-fledging dependence period. Such a link among siblings does not necessarily end after the start of dispersal: in contrast to other raptor species, some Eagle Owls move together during the first steps of dispersal. The spatial and temporal movement patterns were not related to individual physical condition, i.e. all fledglings have a similar pattern in movement behaviour. This suggests that post-fledging movement behaviour may be a general ability acquired during this phase as the result of the combination of several traits, as follows. Fledglings showed unoriented movement, which is not surprising for the following three reasons: (1) they are fed and protected by their parents, and consequently they do not need to direct their movements towards specific shelters or foraging areas; (2) after leaving the nest, young are embedded in new surroundings of which they have to learn, i.e. they move randomly during landscape explorations; and (3) their flight abilities and perceptual range are not yet completely developed, so directionless walks represent the best strategy to cover larger areas at small scales. Moreover, during the early post-fledging period, movement paths were more tortuous and became progressively straighter as independence approached. Since the paths were unoriented, we suggest that this increase in straightness may indicate an increase in their perceptual range (i.e. the maximum distance from which an individual can perceive the presence of a particular landscape element as such). That is, when animals are moving with an unoriented search strategy, the behavioural mechanisms governing movement are working at small spatial scales and those small scales are determined by the perceptual range. Increasing the perceptual range increases the natural step length of their movement path, resulting in straighter paths. From the earliest stages of post-fledging individuals are developing, day to day, their perceptual range (i.e. the ability to perceive habitat at a distance). Perceptual range has 199

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important ecological implications since it may determine the appropriate search strategy and, consequently, influence both fledgling survival and the distribution patterns of individuals. Finally, tortuous paths may have resulted from both incomplete growth and cognitive abilities (as perception and imperfect knowledge of the parental breeding place). With age, individuals further develop both their flight and cognitive abilities, and perceptual range increases as fledglings become more familiar with their surroundings. This combination of traits enables them to search more rapidly and over larger areas, resulting in straighter movement paths. In fact, when their wings are fully developed, their displacements are realised by flying over bushes, rocks and trees and not by walking and jumping among them anymore. Thus, the post-fledging dependence period represents an intensive phase of experience and learning, and its influence may well shape fledglings’ behavioural strategies during their successive, crucial phase, i.e. natal dispersal. Adults continue to feed their young until their dispersal departure. For example, Il’ukh et al. (2009) observed adult feeding young until late autumn (end of November in their Russian study area).

Brood-switching and adoption Nest-switching, the (permanent or temporary) incursion of one or several foreign young into another brood (which generally occurs when fledglings are capable of flight but are not yet completely independent of parents) and their subsequent adoption by a foster family (parents + native young), has been recorded in many bird species (Riedman 1982), birds of prey and owls included (Penteriani & Delgado 2008). One of the most intriguing aspects of this form of alloparental care is the investment of resources by birds into non-genetic offspring, instead of allocating breeding effort exclusively to their own genetic contribution to future generations. Such behaviour seems incompatible with classic evolutionary theory, because it apparently violates the Darwinian principle by which selection does not act to benefit competing genotypes. To explain adoption in birds, several hypotheses have been proposed and tested. The potential explanations for nest-switching and alloparental care can be sorted into two main groups (Redondo et al. 1995, Bize et al. 2003). First, there may be adaptive explanations such as (1) benefit to fledglings (e.g. better care than in the natal nest, acquisition of a dominant rank within a younger brood, reduction in ectoparasite load), and (2) kin-selected benefits from a parental perspective when young switch to nests of related adults, diluting predation risk, or increasing breeding experience of related individuals. Second, there may be non-adaptive explanations such as reproductive errors or adoptions with negligible costs that do not affect reproductive success or survival of the foster parent. The different intensities of selection pressure for intruder chicks (surviving vs. dying) and foster parents (cost of investment in unrelated fledglings) can also be conceived of as an arms race (Pierotti & Murphy 1987). Under such a scenario, chicks are likely to ‘win’ (and consequently gain adoption) because the selection pressure on chicks to survive is stronger than on foster parents to discriminate and reject intruders. Detailed observations of two cases of nest-switching in radiotagged fledglings have been reported in Penteriani & Delgado (2008). Because nest-switching occurred during the post-fledging dependence period (i.e. owlets were no longer in the nest), hereafter we consider it more appropriate to replace the term nest-switching with brood-switching. In the first case, due to the difficulty of simultaneously recording all the movements of a total number of seven owlets (three 200

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switchers + four resident fledglings), owlet position was located once per hour. In the second case of brood-switching, because both the foster female and the switcher were radiotagged, it was possible to follow the two individuals simultaneously and continuously. The broodswitching in 2004 took place after two thirds of the post-fledging dependence period. Three fledgling males left the parental nest site and joined the nearest occupied nest. This other nesting site was already occupied by four resident young (two males and two females), resulting in a total brood of seven fledgling owlets. Two of them were the same age as the intruders, and the other two were older. The switching birds never returned to the parental breeding site, as revealed by the home range that they explored until dispersal, which never overlapped with the home range of the male of the parental nest. From the moment the brood-switching occurred to the start of dispersal, the switching birds moved closer to the foster nest than the home nest and occupied a smaller range than the resident fledglings. The mean distances of the three switchers and the four resident fledglings to the foster nest were 357 ± 207m (range: 33–1,197m) and 415 ± 246m (range: 60–2,318m), respectively. The mean distance between the seven owlets was 394 ± 267m (range: 29–1,661m). The mean distance among the three switchers was 320 ± 243m (range: 35–915m), whereas mean distance among the four resident fledglings was 390 ± 337m (range: 17–2,263m). Distances among switchers, resident fledglings and switcher vs. resident fledglings were not significantly different (χ2 = 5.68, df = 2, p = 0.06). The second example of brood-switching occurred at the beginning of early dispersal attempts. In fact, when the fledgling male left the parental home range, he did not come directly to the foster area. When he established himself in the breeding area of the foster parents, this area was occupied by two resident fledglings (one male and one female), which were both older. This switching individual also did not return to the parental area. The area that he explored when in the foster area largely overlapped (in both extension and spatial location) with the home range of the foster female. A mean distance of 598 ± 307m (range: 266–1,191m) was recorded between the adopted fledgling and the foster female; and the mean distance between the switcher and the foster nest was 788 ± 274m (range: 269–1,191m). In both cases of brood-switching, the switchers started dispersal before the resident fledglings (n = 10, t = 3.99, p = 0.007), even if they were younger than the resident owlets. Finally, adoption did not affect the fate of either switching or resident owlets. In fact, when excluding the four young that were lost immediately after the start of dispersal, one owlet was shot (i.e. mortality due to a stochastic event not related with brood care), another survived at least one year after fledging (when the battery of the receiver failed) and four individuals were still alive at the end of the year (one of them, a female, reproduced successfully in her first year of life). The age of dispersal of the young within the broods in which broodswitching occurred (n = 10, ͞x ± SD = 161.1 ± 14.7 days old, range = 126–175) did not differ (t = −0.047, df = 67, p = 0.96) from the mean age of natal dispersal of the population at that time (n = 59, ͞x = 170 ± 20.5, range = 131–232 days old). Based on these two observations, brood-switching by Eagle Owls has unusual characteristics when compared with brood-switching in other raptor species. Specifically, only male switchers were recorded, and in both cases the switchers joined broods with older fledglings. Adoptions were permanent (switching fledglings never returned to the parental area and stayed with the foster family until dispersal). The integration of alien fledglings with resident young appeared to be a fluent, non-aggressive event, as indirectly revealed by the small distances between the fledglings of different broods and the switcher and 201

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foster parents; as well as the fact that relatively young switching birds integrated with older resident fledglings, a risky strategy for switching birds if interactions with resident young are aggressive. Although we did not directly observe foster parents feeding alien owlets, the fact that switching birds stayed for so long with the resident fledglings (not overlapping with the home range of their parents at the time) implies that they were getting alloparental care from foster parents. Brood-switching seems to be an infrequent behaviour in this species. In fact, in a sample of 74 radiotagged young during a period of four years, we only recorded two cases of brood-switching involving four individuals (5.4% of all marked owlets, n = 20 nests). Moreover, because brood-switching generally occurs when population density is high, this Spanish population should be characterised by high rates of brood-switching as it exhibits one of the highest breeding densities recorded in Europe. However, as also pointed out by Kenward et al. (1993) and Roulin (1999a), in those species whose fledglings beg loudly for long periods (as in Eagle Owls), it should be easy for a switcher to detect potential foster families even from large distances. The two cases of brood-switching reflect the two non-mutually exclusive models advanced to explain facultative brood-switching in semi-altricial species (Gilson & Marzluff 2000), i.e. brood-switching may arise from: (1) a nonrandom, deterministic behaviour governed by the conditions experienced by the fledgling in the natal area (as we can suppose for the first case of brood-switching); or (2) random predispersal movements of fledglings (i.e. the last reported brood-switching). Foster adults did not appear to benefit from adoptions. No adoptions took place in singlechick broods (where parents could gain breeding experience) and brood-switching does not lower the risk of predation of young because old owlets (those > 100 days) are the same size as adults and predation is virtually non-existent in the study area. High frequencies of wandering fledglings might select for non-aggressive adults. For example, frequent chases of intruder young may not be energetically efficient for adults. This could be especially true in high-density populations when adjacent pairs are so close that they are within the range of movement of young during the post-fledging dependence period. The mean net straight line distance between the whole set of owlet locations and the nest (mean ± SD = 504 ± 266m, ranging up to 1,500m; Penteriani et al. 2005b) can explain frequent encounters between strange young and foster parents in the high-density conditions of the study area. Generally, brood-switching has been characterised by younger (and presumably subordinate) fledglings of large broods moving to a nesting site with younger chicks (Poole 1982, Bustamante & Hiraldo 1990), in which the former become the dominant fledglings and can improve their food intake (consequently increasing their probability of survival). Moreover, they may benefit from a longer period of parental care if they establish themselves in younger broods (Pierotti & Murphy 1987). In Eagle Owls, however, brood-switching did not extend the period of parental care for switchers because they were adopted within older broods and did not delay the start of dispersal. Intruding fledglings may simply have taken advantage of a food surplus in a situation of high prey availability, reducing or avoiding aggression by foster parents, which simply fed all the young in the nest area. As suggested by Kenward et al. (1993), brood-switching in raptors may be more similar to a process of brood parasitism than adoption.

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CHAPTER 8

Breeding performances Breeding is an extremely complex event in the life cycle of the Eagle Owl, which may be the result of several non-mutually exclusive factors whose combination may vary depending on local conditions. Internal (e.g. individual quality and experience) and external (e.g. landscape structure and elevation, climate, intra- and interspecific interactions, prey availability) factors can act at different moments of the breeding period and with different intensities depending on individual state and local conditions. Thus, the rules determining breeding output in an area are not necessarily the same as in another one. Yet, on the basis of the available information, we aim here to identify general patterns and some specific effects at both global and local scales.

Breeding performance across the distribution range Timing of egg-laying The decision of when to start breeding is crucial for animals, as this starting point will determine the temporal sequence of all the other, sequential events that compose the breeding season, i.e. clutch size and its viability, successful chick rearing, fledging and start of dispersal, which also depend on the climate, prey availability and hunting success under which these breeding events will develop. On the one hand, Helppi & Kalinainen (1984) reported that egg-laying in SW Finland starts around March–April, when the average daily temperature exceeds 0°C, but that the amount of snow or food does not seem to affect the start of laying. Similarly, Beneyto & Borau (1996) noted that there was no relationship between local weather or nest site 203

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characteristics and the timing of egg-laying. They observed that each breeding site, year after year, seemed to show a consistency in the time at which eggs were laid. We indeed observed the same phenomenon in our 15-year study of the Sierra Norte (SW Spain) Eagle Owl population: every nesting site was characterised by a given timing of egg-laying, which remained more or less stable throughout the years. And, in several cases, the timing of egglaying was relatively different between nest sites that were located less than 1km from each other (Figure 60). We hypothesise that, due to the strict dependence of each owl pair on the resources in the immediate vicinity of the nest and the generally limited extension of the home range (especially core areas, which usually include hunting areas; Campioni et al. 2013), the consistency in the time at which egg-laying starts might depend on the trophic resources available in each home range. This dependency on local resources in our study area is so great that breeding dates were also observed to remain constant in different nesting sites even when a breeder was replaced. Thus, in the absence of changes that can modify food availability, in the same breeding site the pair may start incubation at a similar time year after year. Evidently, other factors may affect the timing of egg-laying (e.g. harsh weather, individual quality and age, stochastic events), but food has the potential to be the main driver (as also pointed out by Serrano 2001). In particular, when the home range size of Eagle Owls is small, their breeding cycle is more directly linked to the characteristics of the immediate vicinity of their nests, which may also explain why neighbouring nest sites may show an important difference (e.g. sometimes even weeks) in the timing of egg-laying. In south European Rabbit-dominated landscapes, Rabbit availability may represent the main determinant of the egg-laying date, as also suggested by Serrano (2001). Similarly, Cochet (1999) found a disparity of approximately one month in the dates of egg-laying between two neighbouring populations of the French department of Ardèche.

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Figure 61. Relationship between the average egg-laying date and (A) the proportion of breeding failures and (B) average reproductive success. The regression lines are indicated in the figures (from Dalbeck & Heg 2006).

On the other hand, Marchesi et al. (2002) and Balluet & Faure (2006) showed that, as a general rule, the lower the nest-site altitude, the earlier the egg-laying: in particular, the latter authors recorded that there was a delay in egg-laying of ca. 10 days for every 100m of altitude. Dalbeck (2003) and Dalbeck & Heg (2006) reported that there is a positive relationship between elevation of a nest site and the timing of breeding: mean laying dates at elevations below 300m were on average five days earlier than above 300m and climatic conditions in both January and February were also associated with the timing of breeding. In addition, independent of laying dates, reproductive success and elevation were also inversely related, suggesting that there might be additional negative effects of breeding at high altitudes. Leditznig et al. (2001), Penteriani et al. (2002a), Marchesi et al. (2002), Dalbeck & Heg (2006) and Pérez-García et al. (2011) found a clear negative relationship between laying date and productivity (i.e. higher productivity is achieved by those pairs that lay eggs earlier in the breeding season) in pre-alpine Austria, Mediterranean France, the north Italian preAlps, W Germany and SE Spain. As also suggested by Dalbeck (2003) and Dalbeck & Heg (2006), it can be expected that the severity of winter conditions and pre-breeding food availability will affect the accumulation of body reserves and, consequently, the timing of laying and clutch size. Eagle Owls need ca. 7–8 months for breeding and raising young, thus an important benefit of early laying might be the prolonged time available for raising owlets to independence before the following winter. Indeed, in years when clutches are produced early, the proportion of breeding failures is low and reproductive success is high (Dalbeck & Heg 2006, Pérez-García et al. 2011; Figure 61). Hence, early breeding may improve offspring survival. However, Donázar (1989) did not find any significant relationship between the number of young and the timing of egg-laying, which may be explained by the local availability of food throughout the whole breeding season. Thus, local conditions may represent the key to understanding the rules of egg-laying timing across the Eagle Owl distribution. As a general pattern, the egg-laying date significantly increases, or is delayed, with both latitude (n = 32 studies, r = 0.84, p = 0.0001; Figure 62A) and longitude (n = 32, r = 0.58, p = 0.001; Figure 62B), but it is independent of altitude (n = 26, r = –0.09, p = 0.65; Figure 62C). These trends seem to indicate that variations in the date of egg-laying may primarily represent variations in local prey availability (which changes along latitudinal and altitudinal gradients, see Chapter 5). That is, if feeding resources are abundant, Eagle Owls 205

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may start breeding early, independent of climatic conditions. Evidently, harsh conditions at the highest altitudes prevent the presence of abundant prey early in the year that, at a large scale, may explain late egg-laying in mountain habitats (Table 12 and Figure 62C). At a distribution range level (n = 32 studies reporting egg-laying dates), the mean egg-laying date is 25 February (range of mean egg-laying dates = 20 January – 9 April); the mean egglaying date of Scandinavian and Baltic countries (n = 4 studies) is 27 March (range of mean egg-laying dates = 17 March – 9 April), whereas the mean egg-laying date of Mediterranean countries (n = 9 studies) is 5 February (range of mean egg-laying dates = 20 January – 24 February; Table 12 and Figure 63). The earliest egg-laying dates start around the second half 100 of December (Table 12 and Figure 63).

B

Figure 62. Patterns of egg-laying variation expressed on an ordinal scale (Julian date, 0 = 1 January) by (A) latitude, (B) longitude and (C) altitude. Egg-laying date significantly increases following latitude and longitude, but is independent of altitude.

Egg-laying Egg-laying Egg-laying date datedate

90 100 100 80 90 90 70 80 80 60 70 70 50 60 60 40 50 50 30 40 40 20 30 30 20 20

100 90 100 100 80 90 90 70 80 80 60 70 70 50 60 60 40 50 50 30 40 40 20 30 30 20 20

35

40

35 35

40 40

45

50

55

(º) 45Latitude 50 55 45Latitude 50 55 (º) Latitude (º)

60

65

70

60 60

65 65

70 70

Egg-laying Egg-laying Egg-laying date datedate

A

100 90 100 100 80 90 90 70 80 80 60 70 70 50 60 60 40 50 50 30 40 40 20 30 30 20 20

5

0 0

5 5

10 15 20 25 30 35 40 45 50 10 15Longitude 20 25 30(º) 35 40 45 50 10 15Longitude 20 25 30(º) 35 40 45 50

Longitude (º)

Egg-laying Egg-laying Egg-laying date datedate

C

0

206

0 0 0

100 200 300 400 500 600 700 800

Altitude (m above sea 600 level)700 800 100 200 300 400 500 100 200 300 400 500 Altitude (m above sea 600 level)700 800 Altitude (m above sea level)

Breeding performances

Table 12. Parameters of fecundity of Eagle Owl populations across their distributional range. When available, mean, range (in square brackets) and sample size (in round brackets) are shown. Egg-laying datea

N eggs

N youngb per pair

3.3 (17)

2.7 (31)

2.3

1.3

1.5

0.7

3.0 (14)

N youngb per successful pair

Breeding pairsc

Nestling successd

57% 47% 2.5 (19) 2.6

0.3 (10)

17 Mar (51)

2.7 (33) 2.4 (29)

28 Mar (276) 2.5 (49) [27 Mar–28 May]

SE Russia

Ryabtsev & Rezin 2009 Karyakin et al. 2009 Lõhmus et al. 1997

Estonia

Nellis 2006

1.2 (15)

54% (28)

Estonia

Nellis 2013

2.1 (40)

70% (48)

Latvia

Lipsbergs 2011

Belarus

Gritschik & Tishechkin 2002

65% (382)

C Finland

Mäkinen 1990

CW Finland

Korpimäki 1980

SW Finland

Lagerström 1978, 1983–1991

2.0 (92) 2.1 (497)

70% (1115)

1.5 (335)

2.1 (242)

Finland

Haapala et al. 1993

2.1 (3923)

Finland

Valkama & Saurola 2005

SW Finland

Penteriani & Delgado unpubl. data

0.7 (408)

1.6 (250)

SE Sweden

Olsson 1979, 1986, 1997

44% (597)

2.5 (38)

1.3 (117)

2.4 (115)

2.4 (75)

2.0 (73)

42% (231)

2.3 (177) 0.6 (549)

1.0 (304)

58% (549) NE Czech Rep.

1.2

1.6 (2808) 3.1 (18)

16 Mar (428)

Czech Rep.

Vořišek 1995a,b

S Czech Rep.

Kunstmüller 1996

Hungary 1.8

68%

2.0 (22)

Poland

Profus 1992

Slovakia

Danko et al. 1995a,b

0.6 (25)

Bulgaria

Baumgart et al. 1973

Germany

Mammen & Stubbe 2005 Robitzky 2007

1.2 (26)

2.1

59% (17)

N Germany

1.1 (46)

1.3 (46)

63% (46)

CE Germany

0.8 (537)

1.7 (208)

37% (537)

CE Germany

1.4 (138)

2.1 (138)

67% (138)

W Germany

2.4 (31)

2.7 (28) 77% (43)

W Germany W GermanyNL E Germany

2.7 (30)

Suchý 2001 Petrovics 2006

2.1 (2176)

2.3 (33) 0.4 (113)

88.8% (138)

54% (56)

0.7 (360)

23 Mar (8)

Shepel’ et al. 2005 Karyakin & Pazhenkov 2007

Estonia

2.0 (37)

2.6 (14)

25 Feb

W Russia SW Russia

20% (10)

2.3 (46)

5 Mar

Shepel’ 1992

1.6 (5159)

9 Apr (45) 25 Mar (272)

Mitropolskiy & Rustamov 2007

W Russia

1.5 (2)

1.2 (45) 1.3 (1115)

Central Asia

70% (122) W Kazakhstan

2.0 (17) 1.3 (140)

Source

3.1 (55) 1.8 (71) 1.1 (28)

Country

Förstel 1977 Lanz & Mammen 2005, Lanz & Pille 2005 Dalbeck 2003, Dalbeck & Heg 2006 Wassink 2010a van Lierop & Wassink 2012 Knobloch 1979

E Germany

Knobloch 1981

1.7

1.9 (24)

SW Germany

Rockenbauch 1978 Rockenbauch 2005

1.5

1.9 (465)

80% (465) SW Germany

1.1 (51)

2.3 (24)

47% (51)

SE Germany

Lossow 2010

1.2 (155)

1.6 (118)

71% (301)

SE Germany

Geidel 2012

The Netherlands

Wassink 2010a

2.4 (33)

2.7 (44)

207

The Eagle Owl

Egg-laying datea

N eggs

N youngb per pair

1.8 (40)

0.9 (66) 1.4 (40) 1.3 (703)

N youngb per successful pair 1.6 (29) 1.8 (24) 2.0 (461)

0.9 (248)

2.0 (184)

10 Mar (248)

Breeding pairsc

Nestling successd

81% (66) E Switzerland 60% (40) S Austria 65% (703) S Austria 74% (248)

1.4 (13) 1.5 (38)

16 Mar (5) 23 Feb (2)

SE Austria

1.7 [82]

12 Mar (11)

E Austria

Grüll & Frey 1992

Swiss Alps

Burnier & Hainard 1948

2.0 (2)

S Switzerland

Desfayes & Géroudet 1949

2.4 (18)

Belgium & Lux

Gee & Weiss 1987

NE France

Guichard 1956

NE France

Cochard 2011

CE France

Balluet & Faure 2006

2.0 (31)

CE France

Pialoux 1994

1.5 (21)

CS France

Cochet 1985, 1989

2.4

CS France

Cochet 1989, 1999

1.9 (13)

CS France

Cochet 1994

S France

Choussy 1971 Defontaines & Ceret 1990

2.2 [66]

80% [82]

18 Feb (7) 1.7 (107)

2.0 (93)

87% (107)

S France

70% (27)

S France

Martin 2008

2.0 (252)

89% (252)

S France

Defontaines 2002 Brugière et al. 1989

1.7 (27) 15 Feb

1.8 (225) 2.3 (16)

2.0 (70)

S France

2.7 (9)

2.0 (43)

S France

Cugnasse 1983

2.1 (13)

S France

Malafosse 1985

1.9 (9)

S France

Cochet 1985

2.3 (28)

S France

Thiollay 1969

7 Mar (13) 2.7 (17) 27 Jan (14)

3.1 (15)

20 Feb

2.6 (42)

[27Dec–11Apr]

2.4 (87)

16 Feb (106)

S France

Blondel & Badan 1976

1.4 (50)

2.7 (13) 2.0 (37)

74% (50)

S France

Bergier & Badan 1979

1.5 (70)

1.7 (206)

89% [245]

S France

Bergier & Badan 1991

1.6 (347)

1.8 (306)

92% (347)

S France

Penteriani et al. 2002a, 2003, 2004

S France

Buzzi & Tavernier 2002

4 Feb (16)

2.1 (16) 2.3 (10) 1.4 (24)

[12Feb–25Mar]

2.7 (10)

1.5 (24)

98% (24)

0.7 (63)

1.6

44%

1.1 (7)

2.1 (10)

208

Bassi et al. 2003a

N Italy

Trotti et al. 2013

CN Italy

Leo & Bertoli 2005 Sascor & Maistri 1996

1.8 (47)

74% (59)

NE Italy

1.8 (79)

51% (160)

NE Italy

Marchesi et al. 2002

1.4 (14)

2.0 (10)

71% (14)

NE Italy

Toffoli & Calvini 2008

1.9 (103)

50% (85)

1.3 (18) 29 Jan (75)

Sériot 1985

N Italy

1.3 (59)

0.9 (103) 2.6 (121)

S France

0.9 (160) 2.3 (6)

28 Mar (5)

[19Jan–23Mar]

Jenny 2011 Frey 1973 Frey 1992 Leditznig & Leditznig 2010, Leditznig et al. 2001 Sackl & Döltlmayer 1996

2.0 (2)

3.0 (2) 3.1 (7)

SW Austria

Source

2.3 (9)

1.7 (18) [19Feb–9Mar]

Country

1.4 (159)

1.8 (132) 2.1 (38)

70% (105)

NW Italy

Casanova & Galli 1998

NW Italy

Bionda & Brambilla 2011

NW Italy

Caula & Beraudo 2014

Portugal

GTAN-SPEA 2015

N Spain

Donázar 1989

Breeding performances

Egg-laying datea

N eggs

N youngb per pair

N youngb per successful pair

Breeding pairsc

Nestling successd

1.5 (12) 2.6 (15) [20Jan–29Mar]

[14Jan–28Jan]

2.0 (62)

2.3 (54)

1.4 (29)

2.3 (36)

1.9 (88)

2.2 (82)

2.9 (42) 1.5 (49)

[8Jan–2Apr] [19Dec–20Mar]

3.7 (41)

[13Dec–31Mar] [24Dec–8Apr]

72.0% (29)

2.9 (97)

2.4 (45)

2.2 (435)

2.6 (534)

2.8 (162)

3.1 (148)

1.6 (39)

2.0 (30)

1.8 (247)

2.5 (174)

88.0% (706)

79.9% (134)

Country

Source

NE Spain

Real et al. 1985

NE Spain

Beneyto & Borau 1996

69% (29)

NE Spain

Serrano 2001

86% (44)

SE Spain

J.A. Martínez unpubl. data

64% (55)

SE Spain

Martínez et al. 1992

95% (49)

SE Spain

Martínez & Calvo 2001

70% (602)

SE Spain

León Ortega 2015

93% (177)

SE Spain

Pérez-García et al. 2011

74% (34)

SW Spain

Penteriani & Delgado unpubl. data

SW Spain 71% (243) Penteriani & Delgado unpubl. data (Sierra Morena)

Median when available; if not, mean is shown; b under the term young we included what different authors have reported as young, fledged young, nestling and fledgling, because most of the information is related to young still in the nest and/ or almost ready to fledge, when mortality is in general very low; c percentage of pairs that started breeding; d percentage of pairs that successfully raised chicks to fledging. a

Egg-laying date

10 Apr

20 Mar 28 Feb 7 Feb

Belarus SW Finland SW Finland SE Sweden SE Bulgaria W Germany - The Netherlands Eifel (D) Bayern (D) Baden-Wurtemberg (D) SW Austria Belgium & Lux Bourgogne (F) Loire (F) Puy de Dôme (F) Hérault (F) Lozère (F) Massif Central (F) Alpilles (F) Alpilles (F) Alpilles (F) Luberon (F) Ariège (F) NW Alps (I) CE Alps and pre-Alps (I) Navarra (E) Zaragoza (E) Murcia (E) Murcia (E) Alicante (E) Sierra Norte (E) Doñana (E) Portugal

18 Jan

Figure 63. Egg-laying dates by country (n = 32 studies), from Scandinavia to the Mediterranean. Multiple measurements for the same country occur where different egg-laying dates have been recorded in different areas. D = Germany; F = France; I = Italy; E = Spain.

209

The Eagle Owl

Clutch size The appearance of sterile eggs is a relatively rare event, as reported by Donázar (1989), Estafiev & Neifeld (1999) and Karyakin et al. (2009), and also verified in our Spanish study area in the Sierra Morena. However, 35% of the eggs did not hatch in Shepel’s study (1992). Data from 26 studies that reported information on clutch size show that the number of laid eggs is negatively related to latitude (r = –0.47, p = 0.02), i.e. the lower the latitude, the bigger the clutch size (Figure 64A), but does not correlate with longitude (r = –0.22, p = 0.28; Figure 64B) or altitude (r = –0.26, p = 0.31; Figure 64C). When taking into account all available information on clutch size, the mean number of laid eggs is 2.7 (mean clutch size range = 1.5 – 3.7 eggs; Table 12 and Figure 65). Mean frequencies of clutch sizes (n = 16 studies; Figure 66) are 8% for one egg, 40% for two eggs, 38% for three eggs, 12% for four eggs and 2% for five eggs. In central and north Europe, clutch sizes of four and five eggs are rarer than in the south. For example, Jenny & Strimer (2011) stated that when they found a breeding pair at 1,370m a.s.l. in the Lower Engadine with five small chicks (and four of them fledged in June), this represented the first record of a clutch with five hatched chicks and the fifth record of a successful brood with four fledglings in Switzerland. Karyakin et al. (2009) in W Kazakhstan only found five eggs in 7% of clutches. Clutches of six eggs were reported by Mitropolskiy & Rustamov (2007) for Central Asia.

4.0

4.0

A

3.0

3.0

2.5

2.5

2.0

2.0

1.5 0

1.5 0

4.0

35

40

45

50

Latitude (º)

55

60

65

C

Clutch size

3.0 2.5 2.0 1.5 0

210

0

0

10

20

30

40

Longitude (º)

50

60

70

Figure 64. Patterns of mean clutch size variation by (A) latitude, (B) longitude and (C) altitude. Clutch size significantly increases when latitude decreases, but is independent of longitude and altitude.

3.5

0

B

Clutch size

3.5

Clutch size

3.5

100 200 300 400 500 600 700 800

Altitude (m above sea level)

Sierra Norte (E)

Navarra (E)

S France

Alpilles (F)

Loire (F)

S Austria

Baden-Wurtemberg (D)

Bavarian Vogtland (D)

Poland

Romania

Czech-Moravian Highlands (CZ)

Czech Republic

Slovakia

SW Finland

50% Portugal

Sierra Norte (E)

Alicante (E)

Catalonia (E)

CE Alps and pre-Alps (I)

Alpilles (F)

Alpilles (F)

Alpilles (F)

S France

S France

Puy de Dôme (F)

Loire (F)

Bourgogne (F)

S Austria

Bavarian Vogtland (D)

Belarus

W Germany - The Netherlands

Czech-Moravian Highlands (CZ)

NE Czech Republic

Czech Republic

Finland

CW Finland

W Kazakhstan

Perm Oblast (W RU)

Perm Oblast (W RU)

Central Asia

0

Belarus

Perm Oblast (W RU)

Frequencies of egg numbers Mean clutch size

Breeding performances

4

3

2

1

Figure 65. Clutch size by country (n = 26 studies), from Central Asia, Russia and Scandinavia to the Mediterranean. W RU = western Russia; CZ = Czech Republic; D = Germany; F = France; I = Italy; E = Spain. 100%

75%

5 4 3 2 1

25%

0%

Figure 66. Clutch size frequencies (different grey tones represent different numbers of eggs) by country (n = 16 studies), from Russia and Scandinavia to the Mediterranean. W RU = western Russia; CZ = Czech Republic; D = Germany; F = France; I = Italy; E = Spain.

211

The Eagle Owl

Fecundity When looking at the population level, some statistics such as mean or median productivity do not really tell the entire story. In fact, not all breeding places have similar fecundity because of population heterogeneity, which is generally explained by site-dependency of territorial individuals. That is, individuals settled in a given breeding site strictly depend on the resources available in their surroundings. For this reason, for example, not all the nest sites of a population contribute in the same way to the fecundity of the whole population. In a 16year study in the central-western Alps in Italy, Brambilla & Bionda (2013) showed that the most productive 50% of nest sites produced 85% of all fledglings, with the most productive 30% of nest sites fledging 61% of all young. This scenario stresses the significant role of the quality of individual breeding places for general population dynamics and, thus, their importance for the stability and viability of a given population. Such results are even more extreme than the heterogeneous patterns of fecundity reported for the species in southern France (Luberon Massif ), where almost all pairs contributed to the annual production of juveniles during good years, but only a few pairs did so in poor years (Penteriani et al. 2004). Indeed, in the Italian study population, some nesting sites apparently have never had any fledged young. These peculiar patterns of variation in fecundity may have important conservation implications. The disappearance (e.g. because of disturbance or mortality due to anthropogenic causes and also local changes in habitat quality) of those pairs with the highest productivity might result in a change in the overall population dynamics from stability to decline (Brambilla & Bionda 2013). As a consequence, the maintenance of high reproductive nesting sites is crucial for the conservation of the entire population, as highquality breeding sites may contribute with disproportionately large numbers of offspring to the overall population. The probability of clutch loss may also be correlated with topography (Ortego 2007), suggesting reduced predation in situations of markedly irregular landscapes. By breeding in the most inaccessible areas, Eagle Owls may minimise the chance of nest detection and subsequent clutch predation. This pattern could be primarily relevant in situations where the combination of cliff scarcity and high density of breeding pairs forces some pairs to occupy places that may be considered marginal in other populations (Ortego & Díaz 2004). Because clutch loss is the greatest cause of reproductive failure in Ortego’s population, occupying the best topography probably bestows the most important fitness benefit in relation to habitat choice (Ortego 2007). One population characterised by high stability and fecundity is the one located in the Sierra Norte of Seville (e.g. Delgado et al. 2013), where the occupancy rate was very high over many years: breeding pairs always reproduced successfully (Figure 67), and the only context in which we did not find any evidence of reproduction was when a pair disappeared (e.g. one or both members of a breeding pair died). After controlling for the effect of year, no significant differences among nest sites were detected for productivity (t = 20.14, p = 0.89). Finally, there was no effect of landscape structure, diet and resource abundance on mean reproductive output. All these results point to a relatively homogeneous population, which is characterised by breeding sites of similar quality showing rather similar annual variance in productivity. Remarkably, the absence of the effect of diet (i.e. mainly Rabbits) in this study area is probably due to a homogeneous high density of this prey across the entire study area. 212

Breeding performances

Number of fledglings

6 5 4 3 2 1 0

1 2 3 4 5 6 7

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Nest number

Figure 67. Pattern of mean productivity of Eagle Owl breeding sites during a 10-year period. Differences in the fecundity distribution between nest sites were very small, indicating a relatively homogeneous population, which is characterised by breeding sites of similar quality (from Delgado et al. 2013).

Several studies have shown that nestling mortality is relatively low (e.g. Frey 1973, Blondel & Badan 1976, Olsson 1979), e.g. 4.6% in Frey (1992), 5.7% in Donázar (1989), 7.3% in Geidel (2012) and 4.4% in the Sierra Morena over a period of ten years (Penteriani & Delgado unpublished data), even if Shepel’ (1992) reported a chick mortality of around 13%. Among the possible causes we can suggest are: (a) relative stability of Eagle Owl prey in many areas of its distribution, (b) eclecticism in diet and, thus, the ability to shift between very different prey species if the availability of the main prey decreases, as well as (c) strong nest defence by both mates. However, extreme climatic conditions, such as heavy rain (Penteriani & Delgado unpublished data) or long periods of frost (Estafiev & Neifeld 1999), may increase nestling mortality. The effect of heavy rains at the beginning of the nestling period may be particularly strong in those areas where nests are not well protected, e.g. on the ground, as intense rain conditions have the ability to kill almost all young chicks (Penteriani & Delgado unpublished data; see also specific information on rainfall effects in Short- and long-term effects of weather and climate, page 222). When considering the entire Eagle Owl distribution: (a) the mean number of young per breeding pair (n = 65 studies) is 1.5, and ranges from 0.3 to 2.9 (Figure 68); and (b) the mean number of young per successful pair (n = 67 studies) is 2.0, and ranges from 0.6 to 3.1 (Figure 69). The comparison between fledged young per breeding and successful pairs highlights that the latter shows a more homogeneous pattern than young per breeding pair. This essentially means that, when Eagle Owl pairs are able to reproduce successfully, the end result is almost the same across the whole species distribution. On the other hand, differences among areas emerge when we consider the entire population, i.e. successful pairs + pairs that fail to reproduce. 213

Samara (RU) W Kazakhstan Latvia Estonia SW Finland SE Sweden Finland NE Czech Republic (CZ) Schleswig-Holstein (D) Bavaria (D) Eifel (D) Baden-Wurtemberg (D) Germany E Austria S Austria W Balkan Swiss Alps Belgium & Lux Puy de Dôme (F) Hérault (F) Massif Central (F) S France S France Alpilles (F) Luberon (F) Bergamo (I) N Italy (I) NW Alps (I) CE Alps and pre-Alps (I) Catalonia (E) Alicante (E) Murcia (E) Sierra Norte (E) Portugal

Mean number of young/successful pair

214 Central Asia

Figure 69. Number of young per successful pair, from Central Asia, Russia and Scandinavia to the Mediterranean. RU = Russia; CZ = Czech Republic; D = Germany; F = France; I = Italy; E = Spain. Portugal

Sierra Norte (E)

Murcia (E)

Murcia (E)

Zaragoza (E)

Catalonia (E)

CE Alps and pre-Alps (I)

NW Alps (I)

NW Italy (I)

CN Alps (I)

Luberon (F)

Alpilles (F)

Ardenne (F)

Ardèche (F)

Puy de Dôme (F)

Loire (F)

Baden-Wurtemberg (D)

Bayern (D)

Bavaria (D)

Bavarian Vogtland (D)

Zittau Mountains (D)

SE Austria

S Austria

E Austria

Poland

NE Czech Republic

Czech Republic

CW Finland

SW Finland

Finland

Estonia

Perm Oblast (W RU)

Mean number of young/breeding pair

The Eagle Owl 3.0

2.3

1.5

0.8

0.0

Figure 68. Number of young per breeding pair, from Central Asia, Russia and Scandinavia to the Mediterranean. W RU = western Russia; D = Germany; F = France; I = Italy; E = Spain.

4

3

2

1

0

3.5

A

3.0 2.5 2.0 1.5 1.0 0.5 0

0

35

C

40

45 50 Latitude (º)

55

60

65

3.0 2.5 2.0 1.5 1.0

Mean number of young/successful pair

Mean number of young/successful pair

3.5

Mean number of young/successful pair

Breeding performances 3.5

B

3.0 2.5 2.0 1.5 1.0 0.5 0

0 10 20 30 40 50 60 70 80 90 100 110 120 Longitude (º)

Figure 70. Patterns of mean number of fledged young per successful pair by (A) latitude, (B) longitude and (C) altitude. Mean number of fledged young per successful pair shows a slight significant increase when altitude decreases.

0

0 100 200 300 400 500 600 700 800 900 1000 Altitude (m above sea level)

100

75

50

25

0 Perm Oblast (W RU) Perm Oblast (W RU) W Kazakhstan Estonia Latvia C Finland SW Finland Poland NE Czech Republic W Germany - The Netherlands Schleswig-Holstein (D) Bavarian Vogtland (D) Bavaria (D) Bavaria (D) Zittau Mountains (D) Eifel (D) Bayern (D) Baden-Wurtemberg (D) Grisons (CH) S Austria SW Austria Bergamo (I) N Italy (I) NW Italy (I) NW Alps (I) NE Alps (I) CE Alps and pre-Alps (I) Loire (F) Puy de Dôme (F) Hérault (F) Alpilles (F) Luberon (F) Zaragoza (E) Alicante (E) Murcia (E) Murcia (E) Murcia (E) Alicante (E) Sierra Norte (E) Doñana (E)

% of pairs that successfully raised chicks to fledging

0.5

Figure 71. Percentage of pairs that successfully raised their chicks to fledging along the distribution of the species, from Russia and Scandinavia to the Mediterranean. W RU = western Russia; D = Germany; CH = Switzerland; I = Italy; F = France; E = Spain.

215

The Eagle Owl

75 5 4 3 2 1

50

25

0

Samara (RU) Perm Oblast (W RU) W Kazakhstan Pirkanmaa (SW Fin) SW Finland SE Sweden Slovakia Hungary Hungary Czech Republic Schleswig-Holstein (D) Bavarian Vogtland (D) Zittau Mountains (D) E Germany Baden-Wurtemberg (D) Grisons (CH) S Austria Loire (F) Puy de Dôme (F) Zaragoza (E) SW Spain SE Spain SW Spain (Sierra Norte) SW Spain (Doñana)

Brood size (% frequency)

100

Figure 72. Brood size frequencies (different grey tones represent different numbers of chicks) by country (n = 24 studies), from Russia and Scandinavia to the Mediterranean. RU = Russia; W RU = western Russia; SW Fin = southwestern Finland; D = Germany; CH = Switzerland; F = France; E = Spain.

Thus, the factor that most differentiates Eagle Owl fecundity among populations is the number of pairs that do not reproduce successfully, with mountainous areas and some boreal regions showing the lowest rates of fecundity. Such global trends are confirmed by the non-significant correlation between the mean number of young per successful pair and both latitude (r = –0.19, p = 0.13; Figure 70A) and longitude (r = 0.12, p = 0.32; Figure 70B), as well as the negative relationship between the mean number of young per successful pair and altitude (r = –0.31, p = 0.03; Figure 70C). Donázar (1990) found that latitude was the most important factor affecting the brood size of Eagle Owls, but the sample size used for this analysis was smaller (eight study cases), which may explain the disparate results. Nevertheless, the above-mentioned author considered that the loss of chicks would be more frequent at northern latitudes because of the harsher climate and, consequently, more limited food availability. The overall percentage of pairs that successfully raise their chicks to fledging is generally high, yet averages 69% (n = 41 studies) and ranges between 37 and 98%. There is no clear trend in the variation of this parameter along the distribution of the species (Figure 71), although most of the higher percentages of successful pairs are in southern and central Europe. Mean frequencies of brood sizes are (n = 24 studies; Figure 72) 24% for one chick, 43% for two chicks, 26% for three chicks, 6.9% for four chicks and 0.1% for five chicks. The number of pairs that breed with success (i.e. pairs that fledge at least one young) may also vary considerably across years, as described in the Loire Department (east-central France), where Balluet & Faure (2006) observed that the number of successful pairs ranged from 86 and 100% (mean = 80%; Table 12), although in one year it decreased to 45%, probably due to bad weather and/or low availability of the main prey. 216

Breeding performances

Table 13. Description and values of the dietary variables used to explain Eagle Owl reproductive performance (n = 26 Eagle Owl breeding sites). Variable

Description

Mean ± SD (range)

Rabbit biomass

Percentage of biomass in the diet composed of Rabbits

Rabbit mean weight

Mean weight of all Rabbits consumed (g). The weight of each Rabbit individual was estimated using humerus, femur or tibia length

63.3±15.8 (24.8–93.7) 468.3±91.7 (262.0–670.6)

Juvenile Rabbit proportion

Number of juvenile Rabbits divided by the total number of Rabbits consumed. Age was estimated considering bone size and ossification of the distal and proximal epiphyses

0.43±0.15 (0.24–0.75)

Rat biomass

Percentage of biomass in the diet composed of rats (Rattus spp.)

Mean weight of alternative prey

Mean weight of all prey species other than Rabbits (g). Most frequent species were rats, Hares, partridges and pigeons

Diet diversity

Shannon diversity index applied using the numeric percentage of each prey taxonomic order in the diet

0.51±0.11 (0.28–0.73)

Superpredation

Numeric percentage of mesopredators (owls, diurnal raptors, mammalian carnivores) from the total prey

1.5±1.3 (0.0–3.8)

9.5±11.0 (1.8–53.5) 271.7±72.3 (161.8–436.0)

Source: from Lourenço et al. 2015.

Influence of food and diet-related variables on breeding performance The relationship between diet and fecundity has frequently been studied in the framework of optimal foraging theory, and it is generally accepted that reproductive performance is largely explained by food intake and the effort associated with obtaining it (Perry & Pianka 1997). Several Eagle Owl studies have specifically shown that: (1) pairs with a diet based largely on high-value foods (e.g. Rabbits and rats) bred earlier and produced large broods (Cochet 1999, Penteriani et al. 2002a, 2003, Campioni et al. 2013); (2) the abundance and availability of prey can be limited by the structure, composition and altitude of the landscape surrounding nesting sites, with direct consequences on fecundity (e.g. Penteriani et al. 2002a, 2003, Leditznig 2005a,b); (3) higher productivity was associated with a higher proportion of a main, profitable prey (e.g. rat; Marchesi et al. 2002) in the diet of individual pairs, and when this prey decreases in availability the diet becomes more diverse, which, in turn, may result in low productivity; (4) when a new viral disease, namely the Rabbit haemorrhagic disease, started affecting Rabbit populations in the Mediterranean area, brood size, number of fledged young per breeding pair and nesting site occupancy decreased significantly in those populations highly dependent on Rabbits (Martínez & Calvo 2001, Martínez & Zuberogoitia 2001). But the relationship between prey abundance and fecundity is not always direct and easy to interpret. In some cases, other diet-related variables (e.g., diet diversity, mean prey weight, proportion of juvenile individuals) may explain reproductive performance better than just prey abundance. For example, Dalbeck (2005) did not find a clear and strong relationship between prey choice and (a) young condition (mass and size) and (b) reproductive success; and only Rabbits and small voles correlated positively with the number of young, but only to a small extent. 217

4.0

A

3.5 3.0 2.5 2.0 1.5 0

Mean egg-laying date

80

1 2 3 Superpredation (%)

4

Coefficient of variation of breeding success

Mean breeding success (number of chicks)

The Eagle Owl

B 0.8 0.6 0.4 0.2 0.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Rat biomass (arcsine transformed)

C Figure 73. Relationships between: (A) mean breeding success and superpredation (i.e. percentage of mesopredators in the diet of Eagle Owls) (R2 = 0.13); (B) the coefficient of variation of breeding success and rat biomass in the diet (R2 = 0.12); and (C) mean egg-laying date and Rabbit mean weight in the diet (R2 = 0.14). Plots show linear regression trend lines with lower and upper 95% confidence intervals (broken lines) (from Lourenço et al. 2015).

60

40

20

0 300

400

500

600

Rabbit mean weight (g)

The effects of Eagle Owl dietary features (Table 13) on breeding performance have been studied by Lourenço et al. (2015) in SW Spain (Sierra Morena). Superpredation proved to be the most important variable affecting mean breeding success, with lower breeding success associated with a greater consumption of mesopredators (Figure 73A). Additionally, greater mean breeding success was associated with less diverse diets, greater Rabbit biomass and lower rat biomass in the diet. Rat biomass percentage was the variable with the highest relative importance on the coefficient of variation of breeding success: high variation in breeding performance was associated with a diet containing a high percentage of rats (Figure 73B). Rabbit mean weight showed the highest relative importance when analysing the relationship with mean laying date, i.e. earlier laying dates were associated with a higher consumption, on average, of small Rabbits (Figure 73C). Moreover, earlier laying dates may be associated with less diverse diets, containing a greater proportion of rats and fewer mesopredators. In summary, other variables besides Rabbit biomass percentage may also show relative importance in explaining breeding parameters. These results suggest that, in landscapes characterised by the high availability of a given prey (Rabbit), less diverse diets, with a high Rabbit biomass percentage, may afford greater reproductive success, whereas a high rat biomass percentage seems to be associated with a greater variation of breeding success, and earlier laying dates with the consumption of smaller Rabbits. 218

Breeding performances

Despite the relatively low explanatory power of the models related to the variables that better explained the three reproductive parameters, it is interesting to highlight the importance exhibited by diet-related variables other than the biomass of the main prey (Rabbits in this study), which is generally the most frequently studied factor reported in the literature. This result suggests that these factors can contribute to a better understanding of the variations in breeding performance within populations living in apparently homogeneous environments. In our case, breeding performance appeared to be negatively affected when the proportion of the main prey in the diet was reduced (which is probably a function of prey availability). Following the prediction of optimal foraging theory, this may force individuals to search for alternative prey, such as rats, thereby increasing dietary diversity. It is known that high availability of the main prey species results in dietary specialisation. Further, as specialised diets allow greater foraging efficiency, it is expected to be associated with higher reproductive success. However, under some conditions dietary diversity has been found to be positively correlated with reproductive performance. The Eagle Owl is a generalist predator that specializes in Rabbits when they are abundant, with obvious benefits to its breeding success (Serrano 2000b, Martínez & Calvo 2001, Martínez & Zuberogoitia 2001, Penteriani et al. 2002a; Pérez-Garcia et al. 2012). In this Spanish study area, the spatio-temporal availability of a profitable prey (Rabbits) can be considered predictable and stable, and thus a positive association between diet specialisation and reproductive performance is expected. The apparent negative relationship between dietary diversity and breeding success of Eagle Owls would also corroborate the well-established idea that Rabbits are one of the most profitable prey species for large predators in Mediterranean ecosystems (Delibes-Mateos et al. 2007, Moleón et al. 2009a, Lourenço et al. 2011a). Finally, earlier mean laying dates seemed to be associated with the consumption, on average, of smaller Rabbits, which suggests that Eagle Owls may take advantage of the breeding season of Rabbits. Consequently, those individuals that breed earlier should include in their diet a greater proportion of juvenile Rabbits, which should be easier to capture and more numerous than adults. However, there is still an unanswered question: why did the parameters describing prey size variations show such a weak effect on reproductive performance, contrary to what would be expected under the optimal foraging theory? Wild Rabbits show great size variation (70–1,400g; Soriguer 1981a) and, although they may be generally considered easy prey for Eagle Owls when they reach high densities (Penteriani et al. 2008), different age classes should be associated with different capturing efforts, as a consequence of their abundance and behaviour. Juvenile Rabbits are easier to capture due to their greater exposure and inexperience (Soriguer 1981a,b). However, the availability of young Rabbits is more limited across seasons, and their relatively small size (70–300g) makes them less profitable than adults (assuming theoretically that searching and capturing effort are similar for the two age classes). On the other hand, subadult and adult Rabbits (500–1,400g) may be more profitable due to their larger size, although their lower abundance and more alert behaviour may require greater searching effort in terms of time and distance travelled. Still, adult Rabbits are a more constant and predictable food source year-round. In areas where Rabbits are very abundant, adults seem to be the most preyed upon age class (Hiraldo et al. 1975, Pérez Mellado 1978). However, in this study area, most Rabbits eaten by Eagle Owls were juveniles, and indeed the breeding season of this predator in Mediterranean ecosystems seems to be synchronised with the breeding season of Rabbits in order to take advantage of this seasonally abundant food. Here, the prevalence of juveniles may be due to the fact that diet sampling primarily 219

The Eagle Owl

included material accumulated during the breeding period (January–June) of Eagle Owls, and only a part of the sample might correspond to the non-breeding period, when juvenile Rabbits are less available and capture effort must focus on adults and subadults. The pattern of the weight of the Rabbits captured by Eagle Owls in this study is very similar to the age structure of Rabbit populations studied in nearby areas during the period January–June (Soriguer 1981a), which may indicate that, in fact, Eagle Owls are not very selective in terms of Rabbit size, but instead capture different age and size classes mostly as a function of their availability. In addition, we consider two other reasons that may explain the generally low relative importance of the variables expressing the role of Rabbits in the diet of Eagle Owls, despite this species being the main prey. First, Rabbit abundance is generally high in our study area, and consequently, the availability of the main prey species may not be a limiting factor sufficient to cause strong variations in reproductive performance (Campioni et al. 2013). Second, typical Rabbit abundance estimates (based on either counting faeces or individuals) may not directly reflect prey availability, since the latter can result mostly from the interaction between prey abundance and prey detectability (Ontiveros et al. 2005). In summary, the study illustrates what appear to be two non-exclusive directions taken by Eagle Owls to maximise breeding success in a scenario of relatively high prey abundance: (a) maximise the size of the Rabbits captured, and (b) take advantage of the period when juvenile Rabbits (easier age class to hunt) are most abundant. Thus, there may be complex interactions between prey size variation, prey abundance and seasonal variations of both (as also observed by Dalbeck 2005), which represent a particular challenge when explaining the breeding performance of vertebrate predators. Since diet-related variables other than the most commonly used (i.e., the proportion of the main prey in the diet) may help understand variations in breeding parameters, it may be crucial to include information on both prey abundance and dietary data, and in particular variables describing dietary diversity and the mean size of prey. Finally, it is important to keep in mind here that, depending on local factors, breeding performance may be the result of different factors and their combinations. That is, climatic events, landscape and nesting site characteristics, as well as prey availability and distribution, all play a role in determining the fecundity of a population, and their effective contribution will strongly depend on local conditions and may vary considerably from one area to another.

Influence of landscape Habitat preferences, that is the disproportionate occupancy of habitats (higher or lower use than average availability), might be related to changes in individual fitness (e.g. clutch size, frequency of clutch loss, number of fledglings and chick quality), and if habitat preferences are adaptive, individuals settled in the most preferred habitats should show higher fitness values. When analysing whether different components of fitness vary along habitat preference gradients, Ortego (2007) showed that there was a clear effect of given landscape components (irregular topography, high cover of scrubland and great length of and close proximity to watercourses) on the probability of clutch loss and clutch size, as well as on offspring quality (i.e. Eagle Owls settling in preferred habitats raised higher-quality young). Thus, the probability of clutch loss seemed to be subjected to directional selection along the habitat gradient analysed by Ortego. He suggested that, by settling in inaccessible areas, Eagle Owls could minimise the chance of nest detection and subsequent clutch predation, 220

Breeding performances

strongly indicating that natural and/or human predators represent an important selective agent shaping habitat preferences. Moreover, habitat quality in terms of favourable hunting grounds and prey availability (i.e. better foraging performance) may also contribute to the production of higher-quality young. Habitat characteristics of nesting sites can affect reproductive performances. In Dalbeck & Heg's 2006 study, high breeding performances were associated with: (1) nest sites located on volcanic rocks, followed by schist, sandstone, limestone, sand/gravel and clay; and (2) basins, followed by cliffs and then rock slopes. These authors suggested that volcanic rocks warm up more quickly in the sun, and basins and cliffs provided warmer habitats in winter than slopes due to temperature inversions; indeed, slope formations were situated near the bottom of valleys, where during temperature inversions cold air accumulates at night. This may be particularly important during incubation in late winter, when steep drops in temperature and snow gales may occur. In addition, slope formations are generally smaller than other rock types and may thus be more accessible to predators. When investigating the fitness consequences of habitat selection, Brambilla et al. (2010) found that Eagle Owl productivity was positively associated with cliff length, which might have relevance in the specific context in which the study was carried out (e.g. intra- and interspecific competition, cliff structure and exposure, location of longer cliffs and so on). Open landscapes around the nest site seem to have a positive influence on breeding performance. For example, the number of fledged young was positively affected by the percentage of open land within a radius of 1,000m around the nest site in Penteriani et al. (2002a, 2003, 2004): in nesting sites surrounded by a larger amount of open habitat (i.e. the main hunting ground), the number of fledged young was higher than in nesting sites within the more forested landscape. Again, the frequency of breeding attempts that produced three young was positively correlated with the percentage of open land within a radius of 1,000m around the nest, whereas breeding attempts that produced one young were negatively correlated with the percentage of open land. Eagle Owl breeding performance has also been related to the amount of open areas in the surroundings of the nest in the Loire Department (east-central part of France), where it seems that more than 20% of open habitat is needed to breed successfully and to produce larger brood sizes (Balluet & Faure 2006). Similarly, Defontaines & Ceret (1990) found a significant effect of open areas on reproduction, and Dalbeck & Heg (2006) observed a negative correlation between the proportion of dense forested areas managed for wood production in the vicinity of nest sites and reproductive success. Indeed, dense patches of forest may be avoided as hunting ground, even if mature forests are among the preferred hunting grounds (Leditznig et al. 2001). This possibility is also supported by the evidence that in this study area, the diet of Eagle Owls is dominated by Hedgehogs, Hares, Rabbits, rats and pigeons which prefer mixed and open landscapes, as well as riverbanks (Dalbeck 1996, Dalbeck 2003). In the same way, Bionda & Brambilla (2011) showed an effect of the wetland interspersion index (factor potentially affecting prey abundance and hunting opportunities) and the extent of urban land cover on both the number of fledged juveniles and the probability of breeding success rather than failure. The wetland interspersion index has been reported as important for Eagle Owl breeding success (Sergio et al. 2004b) because it may represent an indirect estimate of Brown Rat density, which is the most important owl prey in some Alpine populations (Marchesi et al. 2002) and is particularly abundant in areas rich in water habitats (Sergio et al. 2004b). On the other hand, 221

The Eagle Owl

a higher cover of urbanised areas negatively affects breeding output, as also reported by Bassi et al. (2003a). Indeed, urban areas may be locally unsuitable for hunting, and their occurrence in the vicinity of nesting sites may reduce the availability of suitable foraging areas. Finally, in homogeneous populations characterised by nesting sites of similar quality showing similar breeding performances, it is also possible that there is no effect of landscape structure, nor of diet and resource abundance, on reproductive output (Delgado et al. 2013), or that more subtle and difficult-to-detect effects are acting on breeding individuals.

Short- and long-term effects of weather and climate We have already alluded to the negative effects of heavy rainfall during chick rearing, as observed for other raptors (e.g. Mearns & Newton 1988, Kostrzewa & Kostrzewa 1990, Penteriani 1997, Selås 2001, Rodríguez & Bustamante 2003, Krüger 2004). A particular study by Bionda & Brambilla (2011) showed that rainfall may increase chick mortality due to the increased risk of hypothermia in young and the reduced hunting performance of adults, resulting in food shortage for chicks. The latter might be the most important effect in those areas in which Eagle Owls nest almost exclusively in cliff caves, which protect chicks from direct rain. The timing of breeding is an important determinant of reproductive success in birds and, for Eagle Owls, early breeding seems to be beneficial. For example, Leditznig et al. (2001), Penteriani et al. (2002a) and Marchesi et al. (2002) found a clear relationship between egglaying date and productivity. The weather in January and February, which is just before and during clutch initiation in the Eifel region (W Germany), seemed to be a crucial factor in determining differences in average laying date and subsequent reproductive success (Dalbeck 2003, Dalbeck & Heg 2006): onset of laying was delayed during periods of bad weather and low temperatures in late winter. Additionally, elevation was significantly associated with the timing of breeding: pairs breeding at higher altitudes showed a lower reproductive success via delayed breeding. Bad weather might directly lead to the loss of broods due to cooling of the eggs or hatchlings, and also act indirectly by affecting the timing of breeding and reproductive success via prey abundance and prey accessibility (Dalbeck & Heg 2006). The relationship between the start of the breeding season and weather conditions, especially temperature, has also been detected in Lower Austria (Leditznig & Leditznig 2010). That study showed that breeding pairs started breeding increasingly early in the year: in the study area as a whole, the breeding season started, on average, on 5 March instead of 15 March, i.e. 10 days earlier than at the start of the study period (1986–2008). Between 1987 and 1989, the breeding season in the Alpine foothills started on average on 14 March, while between 2005 and 2008 it started on 28 February. Thus, it seems that there is a relationship between the start of the breeding season and the average monthly temperature, especially the average temperatures in February and March. Higher temperatures lead to earlier breeding, which may be one of the consequences of climate change at higher latitudes, where the effects of global warming are expected to be more intense (Solonen 2010, Penteriani et al. 2014a).

222

CHAPTER 9

Home range behaviour On the one hand, home range refers to the area over which an animal travels during its dayto-day activities and that contains the most essential elements for its survival. On the other, territory refers to an exclusive portion of the entire home range that is defended to exclude other conspecifics and, consequently, does not overlap (or overlaps less) with the home range of neighbouring residents. Therefore, as with many other territorial species, Eagle Owls show territorial behaviour only in a restricted portion of the home range (Delgado & Penteriani 2007). Breeders maintain their territory year-round and over several years, having well-defined home ranges with internal core areas (e.g. the active nest and hunting areas) of more intense use (Delgado & Penteriani 2007). Within their home ranges, breeders may select precise plucking and defecation sites within their nesting sites (Penteriani & Delgado 2009a), repeatedly use the same call-posts during displays (Delgado & Penteriani 2007, Penteriani et al. 2007a) as well as the same foraging areas (Delgado et al. 2009b), and tend to be faithful to the same diurnal roosts (Delgado et al. 2009b). Home range behaviour comprises complex and dynamic patterns of space use resulting from routine activities associated with basic aspects of species’ life-histories (Börger et al. 2006). The intrinsic complexity of home range behaviour results from the simultaneous influences of both internal and external factors. The structure and composition of the home range environment and the availability of main trophic resources have been shown to represent some of the key factors determining differences in home range behaviours (Saïd et al. 2009, Rivrud et al. 2010). Moreover, home range behaviours are dynamic during an individual’s lifetime (Börger et al. 2008). As individuals change their needs and tasks during different periods of the year, and seasonal changes alter the spatial distribution of resources, home range behaviour is expected to vary over time. As different home range behaviours ultimately reflect the ability of individuals to react to their experiences as they move (Dall 223

The Eagle Owl

et al. 2005), it is important to understand when and under which circumstances different behaviours are displayed. Further, variations in home range behaviours are not only based on external factors but also on the intrinsic characteristics of Eagle Owls. For example, changes in the internal state of individuals may determine the time allocated to specific behaviours (e.g. food acquisition, territoriality and reproduction), thus affecting the properties of the resultant home range patterns. It is also expected that behaviours and home range patterns are influenced by individual heterogeneity. The time allocated to different behaviours may have relevant consequences at both the individual and population levels through modulating survival, reproduction and, therefore, population dynamics (Morales et al. 2010). Here, we will first summarise the relatively scarce literature published thus far on Eagle Owl home range behaviour, and then we will turn our attention to home range behaviour during dispersal. In particular, we will discuss differences in home range behaviour in Eagle Owls depending on their social status (breeder vs. disperser).

Home range behaviour in breeders Eagle Owls show high variability in home range behaviour among different geographical areas (Table 14, Figure 74A) and temporal scales (i.e. between seasons and years; Table 14, Figure 74B). For example, Leditznig (1996) and Leditznig et al. (2001) observed that the size of the home range of a female in 1991 was 75km2, whereas in 1993 the same female explored an area of only 24km2. Christoph Leditznig was, to our knowledge, among the first to tag adult Eagle Owls with radio transmitters. By recording the behaviour of one male and one female (Leditznig 1992), he observed that during the pre-laying period the male explored an area of 100km2, travelling a maximum distance of 7.5km. Home range size decreased during the incubation period (size = 35km2, maximum distance travelled = 4.4km) and even more during the nestling period (size = 25km2, maximum distance travelled = 3.3km), but increased again during the post-fledging dependence period (size = 55km2, maximum distance travelled = 4.9km). This increase in home range size is likely the result of an increased demand for food by older young and the fact that parents (especially females) generally follow young displacements after fledging. During 80 days of monitoring, the female spent 69% of the time at a maximum distance of 500m from the nest, 19% of the time at a maximum distance of 2,000m, and 12% at a maximum distance of more than 2,000m. Later on, by presenting the results of eight additional breeders, Leditznig (1996) and Leditznig et al. (2001) confirmed that home range size clearly varied between different time periods (Table 14), and that it may also depend on breeding success. For example, when the female is in the nest, males hunt in a relatively small area of their entire home ranges. However, during the post-fledging dependence period, male home ranges were larger, sometimes up to the maximum size observed (Leditznig 1996). Females were always in the vicinity of the nest during the nestling period, but the area they prospected increased significantly after 3–4 weeks and quickly reached the maximum size (Leditznig et al. 2001, Dalbeck 2003). When analysing home range size at a global temporal and spatial scale, Leditznig (1996) observed that male Eagle Owls exhibited a home range that varied between 30–100km2, whereas female home ranges ranged from 26–128km2. He thus did not observe significant differences between males and females, either for home range size or for habitat used within home ranges. Both males and females showed a preference for 224

Home range behaviour

Table 14. Home range sizes (HRS; km2) of the Eagle Owl across its distributional range (from north to south) at both seasonal (breeding and non-breeding seasons) and overall scales. HRS can be estimated using two methods: Minimum Convex Polygons (MCP) or fixed Kernels (K). These methods are based on different percentages of observations, e.g. 50%, 75%, 95% and 100%. HRS (seasonal scale)

HRS Non- (overall Method Breeding breeding scale)

Area

Country

Source

41.4

51.0

MCP(100)

5m+6f

High-Jaeren & Dalane

Norway

Oddane et al. (2012)

32.0

42.3

MCP(95)

5m+6f

High-Jaeren & Dalane

Norway

Oddane et al. (2012)

49.1

106.2

K(90)

5m+6f

High-Jaeren & Dalane

Norway

Oddane et al. (2012)

9.6

9.7

K(50)

5m+6f

High-Jaeren & Dalane

Norway

Oddane et al. (2012)

K(90)

1m

Sleneset

Norway

Bevanger et al. (2011)

3.2 1.1

K(50)

1m

Sleneset

Norway

Bevanger et al. (2011)

8.4

11.4

K(100)

3f

Jutland

Denmark

Jacobsen et al. (submitted)

6.1

9.9

K(90)

3f

Jutland

Denmark

Jacobsen et al. (submitted)

0.9

5.5

K(75)

3f

Jutland

Denmark

Jacobsen et al. (submitted)

0.03

1.0

K(50)

3f

Jutland

Denmark

Jacobsen et al. (submitted)

-

1f

Northrine-Westfalia

Germany

Dalbeck (2003)

-

4m+4f

Northrine-Westfalia

Germany

Dalbeck et al. (1998)

20.5

MCP(95)

1m

WeißenburgGunzenhausen/Bavaria

Germany

Sitkewitz (2005)

14.0

K(95)

1m

WeißenburgGunzenhausen/Bavaria

Germany

Sitkewitz (2005)

-

1m+1f

Mostviertel

Austria

Leditznig (1992)

1m+1f

Mostviertel

Austria

Leditznig (1992)

3m

Mostviertel

Austria

Leditznig (1999)

3m+3f

Mostviertel

Austria

Leditznig (1996)

4.7 10–100

35 ; 25 ; 55 a

b

c

100 23–26 138 12.6

12.7

21.7

MCP(95)

1f

Valais

Switzerland

Nyffeler (2004)

4.5

5.0

29.2

MCP(95)

1f

Valais

Switzerland

Nyffeler (2004)

8.6

K(95)

11 m + 11 f

Murcia

Spain

León (2015)

2.5

K(90)

17 m + 9 f

Andalusia

Spain

Lourenҫo et al. (2015)

1.9

K(90)

24 m + 10

Andalusia

Spain

Campioni et al. (2013)

3.1

K(90)

10 f

Andalusia

Spain

Campioni et al. (2013)

2.2

K(90)

24 m + 10 f

Andalusia

Spain

Campioni et al. (2013)

0.3

K(50)

24 m

Andalusia

Spain

Campioni et al. (2013)

0.5

K(50)

10 f

Andalusia

Spain

Campioni et al. (2013)

0.4

K(50)

24 m + 10 f

Andalusia

Spain

Campioni et al. (2013)

1.5 ; 1.4 ; 2.1c

1.5

K(50)

24 m + 10 f

Andalusia

Spain

Campioni et al. (2013)

0.3 ; 0.2 ; 0.3c

0.2

K(50)

24 m + 10 f

Andalusia

Spain

Campioni et al. (2013)

0.9

K(95)

2m

Andalusia

Spain

Penteriani et al. (2012)

0.1

K(95)

2f

Andalusia

Spain

Penteriani et al. (2012)

0.6a

K(95)

2m

Andalusia

Spain

Penteriani et al. (2012)

1.2c

K(95)

2m

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non-breeding males and females breeding males and females non-breeding females breeding females breeding males 0 10 20 30 40 Home range size (km2)

Figure 74. Home range sizes of male (black) and female (grey) Eagle Owls across their distributional range (A = Austria, D = Denmark, ROK = Republic of Korea, CH = Switzerland, E = Spain, N = Norway) at both (A) overall and (B) seasonal (breeding and non-breeding seasons) scales. White columns represent values from those studies reporting home range sizes of males and females together. Panel B graphically exemplifies the home ranges of male and female Eagle Owls in Spain, as well as their spatial overlap (from Campioni et al. 2013).

open areas and places near water, probably because in these areas there is a high availability of prey (Leditznig 1996). Interestingly, Leditznig (1996) also noticed that some portions of the home ranges of neighbouring pairs, mainly hunting areas, overlapped. He anticipated that this situation could be even more common in the case of Eagle Owl populations characterised by a high density of breeding pairs. Dalbeck et al. (1998) and Dalbeck (2003) also observed the existence of considerable interactions between three neighbouring pairs in the "Eifel Hills" (Northrine-Westfalia, Germany), and suggested for the first time the possibility that the Eagle Owl may not be a monogamous species throughout its entire life (Dalbeck et al. 1998, Dalbeck 2003). 226

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However, more recently, other studies on a larger sample of individuals have found differences in home range behaviours between males and females during the breeding cycle (Penteriani et al. 2012, Campioni et al. 2013). Sex-dependent tasks have the potential to affect movement decisions and thus contribute to differentiating the patterns of space use of males and females. Figure 75 depicts the home range behaviour of Eagle Owl breeders in Doñana National Park (Penteriani et al. 2012). Breeding Eagle Owls moved the shortest distances during the pre-laying (mean ± SD = 340.6 ± 214.8 m; Figure 76A) and incubation periods (240.7 ± 172.7 m; Figure 76A), with their activities mainly performed in the vicinity of the nest (Figure 77). During the nestling and post-fledging dependence periods, however, breeders moved the greatest distances (533.4 ± 428.3m and 483.1 ± 252.3m, respectively; Figure 77). Penteriani et al. (2012) observed that males in general performed longer displacements especially during the breeding period (Figure 75A) than females (Figure 75A), probably because they are responsible for female feeding and breeding territory defence (Penteriani & Delgado 2009a). These two activities may require males to continuously move back and forth to and from the nest site (Figure 77) in order to (1) prevent intruders from approaching their breeding areas, (2) perform territorial displays and (3) search for food. It is well known that reproduction is energetically expensive for both mates, but, from a movement perspective, males have to sustain more continuous activities, travel over longer distances and undergo higher rates of movement (Figure 77). In addition, breeders have two peaks of high activity during the night (Figure 76B), corresponding to sunset (1,464.0 ± 1,027.0m) and sunrise (351.3 ± 231.2m). At sunset, breeder activities are performed close to the nest. At that time individuals usually spend more time in activities like social communication rather than foraging, for example. After sunset displays, breeders usually fly to their foraging areas, and these movements increase movement distances (Figure 76B). Movements within foraging areas are typically short, which is to be expected from a sit-and-wait predator. Before sunrise, breeders return to the roosting place, which in general is close to the nest (Figure 76B). Interestingly, Figure 75A and Figure 75B show differences in home range behaviours of two Eagle Owl males in Doñana. The male in Figure 75A did not successfully breed, and thus, these patterns are in line with the notion that home range size may also depend on breeding success (as previously observed by Leditznig 1996 & Leditznig et al. 2001). The smallest home range size in Doñana National Park was observed during the pre-laying (males 94.8 ± 64.2 ha; females 18.9 ha) and incubation periods (males 61.8 ± 39.2 ha; Figure 78), subsequently increasing during the post-fledging dependence period (males: 122.8 ± 125.0 ha; females: 67.1 ± 65.7 ha). Female home range sizes were smaller than male home range sizes (Figure 78), as the range of the former was more restricted to the vicinity of the nest (Figure 77). However, it is important to highlight here that the size of the home range strongly depends on local conditions and may also reflect social constraints, such that in a population characterised by a high density of breeding pairs, and as a result of their extremely territorial behaviour, males may exhibit smaller home ranges than females (Campioni et al. 2013), as females are allowed to intrude into the territories of neighbouring pairs with less conflict (Penteriani et al. 2007a). These results are consistent with those reported by Barraquand & Murrell (2012), who found that greater territoriality will in general select for small home ranges. The relatively high frequency of contests among neighbouring males (Delgado & Penteriani 2007) might be determining the sometimes high rate of home range overlap between males, as well as the higher percentage of home range overlap for breeders than for dispersing individuals, whose males do not confront each other during vocal displays. 227

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Figure 75. Home range sizes (Kernels 95, 75 and 50; unit: ha) and nightly distances between consecutive steps (m) of two males (panels A and B) and two females (C and D) in Doñana National Park (SW Spain) during (1) incubation, (2) nestling, (3) fledging and (4) pre-laying periods. The Kernal home-range estimator is an intuitive interpretation of home range as a probability, showing the likelihood that any point in space will be occupied by a given animal at any given time.

In order to understand the mechanisms involved in resource exploitation, clear knowledge of habitat selection is required (Beyer et al. 2010). Habitat selection involves different aspects of the individual’s life history and has strong implications for individual fitness (e.g. survival, fecundity, and mating success; Millon et al. 2010, Morosinotto et al. 2010). For example, Nyffeler (2004) followed two radiotagged adult females in the Valais region (Switzerland). One female was marked in September 2002, which did not breed in 2003, whereas the other female was marked as a juvenile in 2002 and then bred in 2003. The radiotagged females were tracked during one complete night per week over 15 weeks using the 95% Minimum Convex Polygon (MCP; Harris et al. 1990, White & Garrott 1990). These two females showed similar home range sizes for summer and winter (Nyffeler 2004), even though one female showed a much larger home range than 229

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Figure 76. (A) Seasonal movement patterns of breeders. (B) Nightly breeder distances between successive steps from one hour before sunset (hourly block = 0) to one hour after sunrise (hourly block = 16) The different symbols represent different individuals. Dotted lines indicate activity peaks around sunset and sunrise. From Penteriani et al. (2012). 2 Distance (km)

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Figure 77. Distances of males (left) and females (right) from their nests during the life cycle, with confidence intervals. 400

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Figure 78. Home range sizes of adult Eagle Owls during the life cycle. Black dots represent the home range size of males, and grey dots denote the home range size of females.

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the other at both seasonal (summer: 12.6km2, winter: 12.7km2 vs. summer: 5km2, winter: 4.5km2) and global scales (29.2km2 vs. 21.7km2). Short temporary trips to more remote sites explain the larger home range at the global scale than at the seasonal scale. Edges of woods (which separates shelter from open areas), wooden power poles and single trees (preferred perch site for both displays and sit-and-wait hunting) had higher values of usage by these two Swiss females. The preference for open habitat exhibited by the two radiotagged Eagle Owls is also supported by other studies, although in several cases the number of radiotagged individuals was very low. For example, Oddane et al. (2012) analysed the home range size and habitat used by 11 breeders in Norway. They used two types of transmitters, one collecting a single location every two hours over 75 days (n = 906 locations), and the other collecting one location per individual every night (n = 1,100 locations). For home range estimation, the authors used both the Minimum Convex Polygon (100% and 95%) and Kernel methods (Worton 1989; 90% and 50%). The observations were split into spring/summer (i.e. the breeding period; 3,831 locations) and autumn/winter (7,570 locations) periods. Home range size was smaller in the breeding period (median values: 41.4km2 for the 100% MCP, 32km2 for the 95% MCP, and 49.9km2 for the 90% kernel) than in the autumn/winter (median values: 51km2 for the 100% MCP, 42.3km2 for the 95% MCP, and 106.2km2 for the 90% kernel). However, the centres of activity (i.e. nesting and foraging areas), represented by the 50% kernel, were similar in both periods (breeding: 9.6km2, nonbreeding: 9.7km2). During all of the seasons, Norwegian Eagle Owls always preferred to roost in mountainous areas during the day, whereas outside of the breeding period they frequented open rocky outcroppings and forests. On the other hand, Eagle Owls preferred relatively open lowlands during the breeding season, which probably represent main hunting grounds. As another example, one male radiotagged in Weißenburg-Gunzenhausen (Bavaria, Germany; Sitkewitz 2005, 2007), exhibited a home range (MCP) of 20.5km2 during winter and 9.3km2 during spring/summer. The Core Convex Polygon (95% of the locations) was much smaller: 14km2 during winter and 6.0km2 during spring/summer. Again, this male displayed a general preference for open habitat types such as areas with low vegetation cover, grassland and agricultural areas. The use of these open habitats increased if single trees or poles were available. A preference for open habitats was also recorded in a telemetry study in Poland (Dylawerski 2006). Additional information on the factors that may influence home range features and breeder movements comes from three females radiotagged in 2010 in Jutland, Denmark (Jacobsen et al. submitted). The GPS-tags were set to take a position every hour, recording a total number of 7,620 positions, out of which 2,701 were between sunset and sunrise. Jacobsen and colleagues estimated the net distance (i.e. a straight line) between each position and the nest in three time periods: (1) nestlings younger than 14 days old; (2) young from 14 days to 45 days; and (3) fledglings older than 45 days. They observed that females moved further from the nest as young increased in age, both during the day and at night. For example, when young were less than two weeks old, females remained at a mean distance of 13m from the nest during the day and 19m during the night. However, when nestlings were older than two weeks, mean distances from the nest increased up to 816m during the day and 1,324m at night. The maximum distance observed between female positions and the nest was 5,067m, and it occurred when young were older. Indeed, when young fledge, the distances between successive moves increases during the next 60 days, such that the nest 231

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site is not always a focal location (Delgado et al. 2009b). Dalbeck et al. (1998) suggested that adult Eagle Owls prefer roosting places up to 4.7km away from the nest in order to be close to hunting areas at dusk, which might increase hunting success. However, at least during the breeding period, most diurnal roosts are close to the occupied nest (author’s pers. obs.). Jacobsen et al. (submitted) also did not observe that adult Eagle Owls roost closer to hunting areas and they suggested that roosting places may mainly be selected as a function of their safety. To estimate home range size, Jacobsen et al. (submitted) used both MCP and Kernel (50%, 75%, 95% and 100%) methods. They observed that 24-hour home ranges were on average 14.7km², even if they hunt in a more restricted area. One of the females flew up to 5km from the nest to hunt in a harbour area, where she hunted gulls (Jacobsen et al. submitted). Finally, these authors also observed that females prefer to hunt in open landscapes such as pastures, arable lands, as well as human settlements like cities and harbours, probably because in their region prey abundance and availability decrease in more forested areas. Lourenço et al. (2015) presented the results of a long-term study (from 2003–2010, Sierra Norte of Seville, SW Spain) using multi-model inference procedures to elucidate the influence of diet-related variables on home range behaviour. The diet of Eagle Owls, determined at 26 breeding sites, was described by using seven variables: (1) Rabbit biomass percentage; (2) mean weight of Rabbits; (3) proportion of juvenile Rabbits; (4) rat biomass percentage; (5) mean weight of alternative prey; (6) diet diversity; and (7) superpredation. Although the relative importance of diet-related variables explaining Eagle Owl home range behaviour was generally low, the mean weight of prey species other than Rabbits, diet diversity and Rabbit biomass somewhat helped to explain home range parameters (Lourenço et al. 2015). These results suggest that when Rabbits are less available, Eagle Owls may increase home range size in order to obtain alternative prey, increasing their dietary diversity at the same time. We expect we will find more remarkable differences in home range behaviours from the 10 breeders (six marked as juveniles and four as adults) marked up to date in Finland (Valkama, Saurola, Penteriani & Delgado unpubl. data), a country where Eagle Owls mostly depend on voles and their intrinsic cyclic dynamic. This is an ongoing project, which is still in the phase of collecting data. By looking at the maps we are obtaining, however, we have already observed that two males have spent most of their time far away from their breeding sites (10–12km), because they were regularly travelling to a dumping site. This suggests that, in some cases, Eagle Owls may prefer to pay the costs of travelling long distances in order to include a constant source of food within their home ranges rather than staying in a smaller home range close to their nests where they will depend on lower prey availability. Finally, another interesting external factor that has been observed to affect home range behaviours is moon phases (Penteriani et al. 2011a), such that individuals show higher activity levels during moonlit nights. The potential for owls to detect prey might increase with increasing light, as does the effort involved in encountering active prey under these conditions. Yet, the general pattern of high activity of breeding Eagle Owls during moonlit nights may represent a cost/benefit trade-off between preying on less active/more concealed prey and taking advantage of the easier visual location of prey (Penteriani et al. 2011a). The above studies show that there is a high variability in Eagle Owl home range behaviour within and among individuals. In general, it is well known that individual heterogeneity is an essential ingredient for understanding ecological systems. Kang and 232

Home range behaviour

collaborators (2013) provided an example of individual heterogeneity in Eagle Owl home range behaviour. They analysed the home range behaviour of three females and one male in Korea. This study used both MCP (100%) and Kernel (95%, 90% and 50%) methods. Although the average home range based on MCP was 39.1km2, the individual who showed the broadest home range explored an area of 80.5km2, whereas the owl exploring the smallest home range only moved within an area of 8.5km2. When using the Kernel method, the authors found that the area for the 95% Kernel was 27.8km2, and the area for the 50% Kernel was 1.1km2. The largest home range with this method was 91.9km2, whereas the smallest home range was 5.6km2. Actually, each individual is the result of a series of complex, reciprocal interactions between factors that can occur throughout its lifetime and are responsible for the emergence of different personalities (Sasha et al. 2004, Stamps & Groothuis 2010). Yet, the above studies have not explicitly modelled this important source of variation, probably owing to the extremely small sample size. For this reason, when Campioni and colleagues (2013) studied Eagle Owl home range behaviour, they specifically based their study on a conceptual framework that recognises home range behaviour as the result of the simultaneous influences of temporal, spatial and individual-level processes (Börger et al. 2006, 2008, Horne et al. 2008, Indermaur et al. 2009, van Beest et al. 2011). This integrative approach recognises that different home range patterns may emerge at multiple spatio-temporal scales (e.g. McLoughlin & Ferguson 2000, Anderson et al. 2005, van Beest et al. 2011). To characterise Eagle Owl home range behaviour, Campioni et al. (2013) analysed space use, rhythms of activity and movement patterns during the different phases of the Eagle Owl biological cycle and at different spatial scales. From 2004–2010, 34 breeders (24 males and 10 females) from 24 nests were individually followed throughout the night (from 1 h before sunset to 1 h after sunrise; total time duration = 3,333 h) during 296 continuous radiotracking sessions. A new location (total number of locations = 5,298) was recorded each time there was a change in the position of the focal individual. The continuous radiotracking sessions were performed year-round in an attempt to obtain a homologous dataset across the different phases of the owl’s biological cycle. Space use was studied at two different temporal scales. First, the seasonal scale relies on the biological cycle of the species, i.e. the pre-laying, incubation, nestling and fledgling/post-fledging dependence periods. Second, to describe general patterns (i.e. global home range and core area sizes) an overall scale encompassed the entire period during which it was possible to follow an individual. They first estimated home range size through the Kernel method. They calculated the 50% (representing the internal core area) and 90% (to establish home range boundaries) kernels using the least squares cross-validation procedure (Silverman 1986). Movement behaviour was characterised by five variables: (1) total distance, as the sum of the distance between successive steps of nightly displacements; (2) step length, as the distance between successive locations; (3) speed, as the step length divided by the time interval between successive locations; (4) turning angle between successive movements; and (5) time step, as the time elapsed between successive moves. The movement variables were analysed at two different spatial scales, home range and core area, and two temporal scales, overall and seasonal. The nocturnal activity of Eagle Owls was estimated using two indices: (1) the time spent inside the core area(s); and (2) movement frequencies (a) per night and (b) within the core areas. Core area activity is a measure of the time devoted to main activities, such as hunting, feeding and territorial defence. This study demonstrated the important role of the individual in shaping home range characteristics. Actually, Campioni 233

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et al. (2013) detected both inter-individual variations in home range behaviour and intraindividual consistency in the way Eagle Owls behaved. Inter- and intra-individual effects largely contributed to shaping (1) home range and core area sizes and (2) movement patterns during different periods of the biological cycle. At the overall timescale, female home range size was larger and showed higher inter-individual variation than male home range size. The size of core areas for females was also larger than for males, although the variation in core area size was consistently similar between sexes (Campioni et al. 2013). Because territoriality is stronger between males than females, the latter may intrude more easily in the home ranges of neighbouring pairs and, consequently, the size of their home range can be larger than that of males. However, home range behaviour did not vary across the biological cycle, suggesting stable home ranges. The extremely high density of breeders in this study area could have strongly limited conspicuous home range expansions/ contractions. Each Eagle Owl home range seems to have a well-defined location and size throughout the year. The size variations of home range and core areas at the overall timescale were partially explained by edge density (i.e. habitat heterogeneity) and sex, which is in line with previous studies (e.g. Kie et al. 2002; Saïd & Servanty 2005). An increase in the amount of edge density resulted in a decrease of home range and core area sizes. Higher densities of edges have the potential to aggregate different patch types in a reduced space (Tufto et al. 1996, Revilla et al. 2004), consequently producing a more clustered distribution of basic resources. As an end result, such crowded resources can reduce individual rates of movement and, thus, home range sizes. Because of the dependence of Rabbits on this combination of edges, shrubs and open patches (Lombardi et al. 2003, 2007), it is not surprising that there is a correlation between certain components of Eagle Owl movement patterns and rhythms of activity with landscape structure and composition. Finally, home range internal structure was related to differences in the state of individuals, with those in better condition having a simpler internal home range structure. In addition, males exhibited a slightly greater number of core areas than females; the core areas of males were located at greater distances from the nest than those of females. Some of these areas of frequent use, but relatively far from the nest, may represent specific locations where males perform territorial displays at the edge of their home range towards neighbouring males. This relationship could suggest, for example, that the existence of fewer core areas and smaller distances between breeding and foraging sites may reduce movements and, consequently, minimise daily energetic expenditures allocated to unprofitable and costly activities. The authors observed that the effect of the different phases of the biological cycle became evident at the level of movement patterns, with individuals travelling longer distances during incubation and nestling periods than during pre-laying and fledging/postfledging periods. These differences in movements may be explained by the different tasks that a breeder needs to perform during the different periods of the year, such as territorial and sexual displays, feeding of the incubating female, feeding of both the female still in the nest and growing nestlings, as well as the feeding of older and bigger young during the post-fledging dependence period. Path tortuosity and movement rate (i.e. the time elapsed between consecutive movements) were affected by habitat composition and the age of owls. In particular, older breeders showed more tortuous and faster movements in habitats with dense vegetation, which might be the best strategy for optimal hunting. Similarly, the distance between successive locations and the time elapsed between the successive movements of an owl were slightly sex-dependent, with females travelling shorter distances 234

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at longer time intervals than males. The activity patterns of Eagle Owls were quite constant year-round and did not show any clear differences between periods. However, the analyses of movement patterns highlighted a slight difference between sexes, with males moving at higher rates than females. Within core areas, movement and activity rates seemed to be somewhat influenced by edge density, with individuals showing higher movement and activity rates when the density of edges decreased (Campioni et al. 2013). These higher rates might be associated with lower prey (Rabbit) availability in those portions of the hunting grounds where edges decrease. Yet, a heterogeneous landscape composed of a mix of shelter and open habitats (where edge density increases) may allow for the presence of more abundant and evenly distributed groups of Rabbits, which consequently reduce the need of longer displacements and higher rhythms of activity during hunting.

Home range behaviour during natal dispersal During natal dispersal, Eagle Owls may establish relatively stable home ranges, i.e. temporary settlement areas where juveniles benefit by having knowledge of the abiotic and biotic environment (Delgado el al. 2009b, Penteriani et al. 2011b). For example, when studying nine juveniles that were marked in Schleswig-Holstein (Germany), Frölich (1986) observed that the average extension of the home range prospected by juveniles was 34km2, varying between 15km2 and 44km2. Frölich (1986) suggested that the observed variation in individual behaviours might be the consequence of differences in prey availability in the area occupied by the tagged owls. He also observed a high degree of overlap between the home ranges of individuals, even though some central regions always remained exclusive, possibly reflecting the first indication of territorial behaviour. In additional studies, Penteriani et al. (2011b) and Penteriani & Delgado (2012) also observed that juveniles explored relatively stable areas (Figure 79) during dispersal, and that the permanence period in those areas ranged between several days and approximately one year. Both males and females, and indiscriminately during the wandering and stop phases of the dispersal period (see Chapter 10), showed similar permanence times in their home ranges (Penteriani et al. 2011b). Throughout the entire dispersal period, juveniles occupied a mean home range of 8.5 ± 4.5km2, even though the average size per night of these home ranges was somewhat smaller (~2km2) and slightly larger for males than for females. However, the size of home ranges can also exhibit temporal variation, being larger at the beginning of the dispersal process (Delgado et al. 2009b). In line with the results presented by Frölich (1986), Penteriani et al. (2011b) also found that dispersing Eagle Owls were surrounded by several conspecifics, whose numbers are greater during the wandering than the stop phase (Figure 80). Moreover, overlapping home ranges occurred more frequently between individuals belonging to different dispersal phases (Figure 80) and during the wandering than the stop phase (Penteriani et al. 2011b), when home range overlaps were rarer: wandering vs. wandering individuals = 39.8% overlap; stop vs. stop = 23.2%; wandering vs. stop = 37%. When spatially overlapping, the distances between males were slightly less than between females. In particular, both male−male distances (1.0 ± 0.6km) and female−female distances (1.1 ± 0.2km) were less than male−female distances (1.4 ± 0.7km). Only minor differences were recorded in the distances among sibling owls (1.0 ± 0.6km) compared to the distances among non-siblings (1.2 ± 0.8km). 235

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Figure 79. Examples of home ranges for two dispersing males (black, A and B) and two dispersing females (grey, C and D). Each group of polygons (calculated using 95% Minimum Convex Polygons) represents the nightly area occupied by a given individual (from Penteriani & Delgado 2012).

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Figure 80. An example of the home range overlap of floaters (black = males, grey = females) during the wandering and stop phases of natal dispersal (from Penteriani & Delgado 2012). Different line thicknesses represent different individuals.

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From early dispersal to breeding Individuals of different status (e.g., dispersers vs. breeders) may show differences in their home range behaviours (Campioni 2012). One of the reasons for these disparities is the amount (and quality) of information individuals have about the surroundings they inhabit. Information, recently defined as a ‘fitness enhancing resource’ (McNamara & Dall 2010), is vital to an organism’s adaptive behaviour, and affects choices related to habitat selection, mate choice, foraging, social interactions and space use (Dall et al. 2005). The better informed an individual, the better it can develop and adjust its behaviour to meet the demands of a variable world (Dall et al. 2005). Evidence is accumulating that individuals make adaptive use of information during dispersal, from departure to settlement (Bowler & Benton 2005, Clobert et al. 2009). Individuals need to update their perception of the biotic and abiotic environments to be able to compare different alternatives and thus benefit by increasing their likelihood of choosing the best-matching behaviour. Thus, individuals need time to acquire knowledge about the surroundings in which they move and, consequently, adopt some site-specific mechanisms or rules which allow them to exploit habitat patches optimally (Dall et al. 2005). Moving during natal dispersal and learning are therefore intertwined processes (Delgado et al. 2009b). Natal dispersal involves considerable time spent alone travelling across unknown areas, and therefore the costs of dispersal are usually high due to both mortality risks and missed reproductive opportunities (see Chapter 10). But the costs of uncertainty during dispersal may be reduced by becoming familiar with the environment. Thus, the comparison of home range behaviours of dispersing Eagle Owls vs. breeders represents a unique scenario to understand the effects of local familiarity on individual decisions (Delgado et al. 2009b). In addition, characterising differences of home range behaviours at the lifetime scale has other important applications, such as providing information about the ability of individuals to track environmental variation. Furthermore, it may help to identify the underlying behavioural mechanisms by which profitable areas are detected and exploited (Barraquand & Benhamou 2008); such information may provide concrete and useful support for management strategies of crucial areas for species conservation. Thus, there is a pressing need to study the different home range behaviours individuals may exhibit throughout their lifetime. Some investigations explicitly attempted to identify changes in the home range behaviours of Eagle Owls at the lifetime scale, i.e. from disperser to breeder status (Delgado et al. 2009b, Campioni 2012, Penteriani et al. 2015). These studies explored the possibility that dispersers facing novel environments should exhibit more dynamic and complex space use and activity rhythms than breeders, which are assumed to have a perfect knowledge of the landscape they inhabit as well as the need to deal with specific and repetitive tasks associated with reproduction. By addressing this question, the authors attempted to identify the key life history traits, behaviours and external factors potentially determining home range behaviours, keeping in mind that social status may crucially impact the way in which individuals use their surroundings. Interestingly, they found that, for example, appreciable differences in home range behaviour between the sexes began only with the acquisition of a breeding place (Penteriani et al. 2015). During their dispersal period males and females showed similar home range overlap (Penteriani & Delgado 2012), probably because individuals do not have any territorial and/or reproductive objectives when in their dispersal grounds (Delgado et al. 2009b), whereas the percentage of breeder home range overlaps 237

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A

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Figure 81. Examples of home range overlap (based on Kernel 95, 90, 75 and 50) of breeding males (n=12; A) and females (n=9; B). Different shades of grey are used to differentiate individuals when they overlap each other (from Penteriani et al. 2015).

(Figure 81) was higher for male–male (mean ± SD = 22.7 ± 22.9%) than for male–female (mean ± SD = 13.3 ± 17.9%) and female–female (mean ± SD = 14.7 ± 14.8%) pairs. The higher rates of home range overlaps between males principally occur at the borders of their home ranges and may be due to territorial displays, when males approach each other. Breeders also exhibited larger home ranges than dispersing individuals, with higher nightly variation of home range size. Again, such variations in home range behaviour may reflect status-dependent needs and constraints, which might be related to the breeding cycle. On the other hand, the characteristics of the dispersers’ home range may mirror the solitary 238

Home range behaviour

lifestyle of these dispersing individuals (Penteriani & Delgado 2012), which only need to provide food for themselves and for which the absence of territorial behaviour allows them to move with fewer restrictions than breeders. Dispersing individuals also have higher activity levels within their core areas than breeders (Penteriani et al. 2015). As we have seen, Eagle Owls switch their home range behaviours during their lifetime, alternating between more explorative movements when inhabiting new environments and more exploitative movements within a more stable home range (Delgado et al. 2009b). Dispersing individuals are expected to pay the cost associated with high exploration rates during the wandering phase, which will be replaced by the benefits of information during the stop phase (Delgado et al. 2014a), e.g. when the disperser settles within a more stable home range (Delgado et al. 2009b). Therefore, dispersing individuals may be more active than breeders because they still have an imperfect knowledge of the surroundings in which they move. The breeder’s familiarity with the surroundings of their nest site could contribute to the reduction in activity in comparison to dispersers. Yet, breeders spend large amounts of time roosting close to the nest or performing vocal displays (Penteriani 2002, Delgado & Penteriani 2007, Campioni et al. 2010), whereas juveniles essentially roost, hunt and survey new areas (Delgado et al. 2009b, Penteriani & Delgado 2012). Because of the different home range behaviours displayed by breeders and dispersers, it is now evident that to quantify individual differences in space use it is important to take into account the potential influence of social status. This was indeed the aim of two studies carried out by Campioni et al. (2010, 2012). In the first study, Campioni et al. (2010) looked at the selection of breeder and juvenile post sites in order to understand whether social status determines their spatial location. Indeed, both breeders and dispersers might use distinct post sites to perform different routine activities such as hunting, vocal and visual displays (see Chapters 12 and 13), landscape exploration and so on. That is, depending on their different social status, the diverse trade-off between costs and benefits of breeders vs. dispersers can influence their behavioural decisions. While the strategy of breeders is primarily aimed at maintaining their resources and mates, the dispersers’ strategy is principally directed at searching for an empty breeding site while reducing the risks associated with conspecific aggression due to visible intrusions. The analysis of 679 post sites (225 for breeders and 454 for dispersing individuals) showed that posts differed significantly between the two social statuses, i.e. breeders preferred visible locations and dominant posts, whereas juveniles selected hidden posts, especially when crossing or sharing areas with breeders. Breeders may take advantage of visible locations to declare their status as territory holders, whereas dispersers may benefit from a more secretive life by wandering unnoticed within occupied territories. This secretive life could help juveniles to reduce the risks associated with conspecific aggression. Moreover, in species characterised by aggressive territorial behaviours and weapons, several benefits can be gained by a territory holder selecting dominant and visually connected posts. Breeders might avoid being involved in dangerous aggressive encounters with occasional intruders crossing their territorial boundaries as the latter are aware of their presence from afar. From the top of their dominant posts, territory holders might be acting as continuous signallers during the entire time spent perching. For dispersing Eagle Owls that do not need to defend a territory, and whose principal need is the avoidance of aggressive encounters with breeders, it may be advantageous for them to go unnoticed when gathering social and spatial information, while avoiding risky 239

The Eagle Owl

circumstances. The use of less visible post sites by dispersing individuals can be explained, at least partially, by the complex array of behavioural patterns that territorial Eagle Owls can exhibit. Moreover, voluntary selection of less dominant posts may also represent a way to communicate no intention of intrusion if discovered by a territorial individual. The secretive behaviour of individuals during dispersal may therefore allow them to overlap broadly with defended territories. Notably, Campioni et al. (2010) also observed a higher frequency of occurrence of post sites of dispersing female Eagle Owls within breeders’ home ranges than by dispersing male owls. This might be the consequence of the generally lower aggressive response towards females than towards males. In addition, because polygamy can occur in Eagle Owls (Dalbeck et al. 1998), a dispersing female entering a holder’s territory might also represent to a male the possibility of occasionally reproducing with two females. In the second study, Campioni et al. (2012) compared habitat characteristics near 15 nesting sites of breeders with those of 75 diurnal roosts chosen by dispersing owls in Doñana National Park. Again, Campioni et al. (2012) expected that the use of those locations would reflect the different cost-benefit trade-offs related to the status of breeders and dispersers. Indeed, the structure of forest stands may mainly reflect: (a) the need of breeders to fly easily within the breeding stand during the reproductive period; and (b) the necessity of dispersers to remain unnoticed. As expected, breeders and dispersers were shown to select different forest stands with different vertical structures. Compared with dispersers, breeders preferred more mature and open stands characterised by higher trees, suggesting that individuals of the same population but differing in social status can display different habitat use. The different patterns of habitat use of breeders versus dispersers may be explained by the tasks and constraints associated with the differences between these two social groups. For reproduction occurring within forested stands, the different activities that breeders perform in the area surrounding the nest specifically require easy access to the nest. This access is provided by the more open structure offered by the oldest stands. In contrast, non-territorial juveniles are free from these temporal and spatial constraints: they depend primarily on foraging and on conspecific avoidance. We may expect the strategy of the territorial breeder to be directed so as to maximise individual benefits by selecting suitable breeding conditions that provide long-term individual benefits and increase fitness. In contrast, the strategy of the non-territorial dispersers appears to minimise the short-term negative effects of natal dispersal through behavioural mechanisms, such as specific cover use. Finally, the possibility also exists that the observed choices of dispersers may be determined by habitat-mediated avoidance or temporal segregation mechanisms that reduce encounters with aggressive territory holders. As an ultimate consequence, home range behaviours may then involve the interaction of multiple social, behavioural and ecological determinants with direct ecological and evolutionary consequences for population dynamics and colonisation.

240

CHAPTER 10

Natal and breeding dispersal Dispersal is generally considered the movement of individuals away from their birthplace (i.e. natal dispersal), or from where they have once reproduced to another breeding site (i.e. breeding dispersal). Through simply moving from one location to another, dispersal is an important process because it has consequences not only for individual fitness, but also for population dynamics, population genetics and species distribution (Clobert et al. 2001, Ronce 2007, Clobert et al. 2009). Dispersal allows individuals to cope with spatially and temporally variable resources, and thus it is essential for the persistence of most species in a changing natural environment (Ronce 2007). Not so long ago, no one had studied the dispersal of the Eagle Owl by integrating different aspects of behavioural ecology and evolutionary dispersal strategies. However, analysis of the dispersal process from the perspective of the peculiar life histories of birds of prey and owls (generally less studied biological models because of the inherent difficulties in working with this group of species) may enhance our understanding of some additional aspects of one of the most complex ecological processes. By assuming the risks of marking and following juveniles, several studies have demonstrated that a greater understanding of the dispersal process in Eagle Owls may be achieved by approaching it under the perspective of animal movement analyses (Delgado & Penteriani, 2008, Delgado et al. 2009b, Delgado et al. 2010), thus revealing surprising and novel results (Delgado 2008, Aebischer et al. 2010). There is an amazing variation in behavioural search strategies not only among different individuals but also within the same individual. This plastic dispersal strategy depends on many different biotic and abiotic factors. Here we will start this fascinating journey by describing the effects of the abiotic and biotic environment, individual internal state and the organism’s cognitive and learning abilities on the natal dispersal of Eagle Owls. Then we will turn our attention to breeding dispersal, a process that is unexpectedly occurring in a species 241

The Eagle Owl

that has always been believed to have strong fidelity to both site and mate. We will discuss how often and under what conditions breeding dispersal may occur, and what might be its consequences. Finally, we will report some cases of seasonal migration that seem to be the result of dramatic seasonal changes in climatic conditions.

Natal dispersal Natal dispersal is the process through which immature individuals depart from their birthplace in search of a new breeding site (Clobert et al. 2001, Ronce 2007, Clobert et al. 2009). Even though dispersal has been frequently considered to be a fixed species-specific strategy, it is actually a complex process that can be subdivided into three sequential but behaviourally distinct phases (Andreassen et al. 2002, Clobert et al. 2004, Delgado & Penteriani 2008, Delgado et al. 2010): (a) departure, when an individual leaves its place of birth; (b) wandering or transience, when the individual explores other areas for a variable period of time before definitively settling in a new area; and (c) settlement, when the individual settles in a more stable zone, either as a temporary settlement during the dispersal process or as ascension to ownership of a breeding site. Delgado & Penteriani (2008) quantitatively derived these different phases on the basis of individual locations during dispersal (Figure 82). For each juvenile Eagle Owl, they plotted both the beeline distance from the natal nest to each location and the global mean distance covered by each individual during the dispersal period. When juveniles left the nest they usually remained in their parental home range for a while, and thus distances from the nest to each successive location fluctuated around a low value. The authors deemed dispersal to start (i.e. departure) when the distance from each location to the nest progressively increased (Figure 82A). Later, when owls reached the settlement phase, dispersal distances levelled off. They considered that owls settled in a stable settlement area when the distances between successive locations became smaller than the average distance of previous moves travelled by each dispersing owl (Figure 82B). It is worth mentioning here that several temporary settlement phases may occur during the dispersal process, which ended when the juvenile finally occupied a breeding site. The wandering or transience phase encompasses the movements between departure and the final settlement in a stable breeding area.

Departure Dispersal represents a risky stage in an animal’s life because of its multiple costs (Bonte et al. 2012). Several ultimate (including kin competition, inbreeding, resource competition and environmental stochasticity) and proximate causes (including, for example, environmental factors and interspecific interactions) may play an important role in selecting for dispersal (Johnson & Gaines 1990, Clobert et al. 2001, Bowler & Benton 2005). The importance of these factors will vary among species, and which mechanisms in fact stimulate juvenile Eagle Owls to initiate dispersal are not yet well understood. Dispersal is a life history process related to reproduction and survival, and thus it is important to determine the age at which individuals disperse. Given a particular environment, we would expect the optimal age-specific dispersal strategy to be the one that maximises individual fitness for a particular age class (Johst & Brandl 1997, 2000). Interestingly, the age 242

Natal and breeding dispersal

A

7000 Start of dispersal

Distance (m)

5250

3500

1750

0

B

75

97

125

145

194

210

360

379

400

425

443

2300

1725 Distance (m)

236 280 333 Age (days)

End of dispersal

1150

575

0

194

210

236

280

333

360

379 400 425 Age (days)

443

471

504

530

552

574

Figure 82. (A) Analytical determination of the beginning of dispersal. The black line represents the distances between successive movements. The reference (dotted) line indicates the average distance covered by a dispersing individual to its nest. (B) Analytical determination of the end of dispersal movements. The dark grey bars represent movement distances when owls were searching for a stable settlement area or breeding site; the light grey bars are the distances travelled after the end of dispersal movements. The reference (black) line indicates the average distance of the previous movement covered by an individual (from Delgado & Penteriani 2008).

at which individuals disperse seems to be a constant trait of Eagle Owl life history (Penteriani et al. 2014b). Why, in spite of the diverse ecological conditions and wide latitudinal gradient where this phenomenon has been recorded (from Mediterranean to boreal habitats), most departures occur in a relatively narrow time window (Table 15) is a pertinent and open question. The departure phase of dispersal in Eagle Owls is a good example of an ecological process modulated by the combination of internally and environmentally determined factors. For example, Penteriani & Delgado (2011) documented that the age of dispersal in Eagle Owls is affected by some environmental aspects related to birthplace and other idiosyncratic factors that vary among years. Later on, by analysing data from three different populations located in Spain, Switzerland and Finland, Penteriani et al. (2014b) showed 243

The Eagle Owl

that the decision to start the dispersal phase is also influenced by: (1) the age of individuals, i.e. dispersal mainly started when juveniles were ~6 months old; (2) sex, with males (age of dispersal = 161.3 ± 22.1 days, range = 116–222 days) showing more variation in their departure age than females (age of dispersal = 166.6 ± 16.9 days, range = 127–206 days); (3) the moon phase, with dispersal departures mostly occurring during full-moon nights (Figure 83); and (4) the interaction between age and moon phase: the effect of moonlight was highest when owls dispersed at the average dispersal age of this species. Table 15. Mean age of dispersal of juvenile Eagle Owls with respect to diverse ecological conditions and a wide latitudinal gradient. Some studies reported the mean age of dispersal in days (d), whereas others provided the week of the year (w) when dispersal started. Type of study*

Country

Mean age of dispersal

Range

Month of departure

References

Sweden

O

-

-

August/September

Olsson (1997)

Finland

R

150 d

116–185

October/November

Penteriani et al. (2014b)

Switzerland

R O

164.1 d 24 w

140–209 -

September/October -

Aebischer et al. (2005) Bezzel & Schöpf (1986)

O

24 w

-

September/October

Mrlikova & Peške (2005)

R

172 d

-

November/December

Wassink (2009b)

Czech Republic The Netherlands Austria

O

28 w

-

November/December

Leditznig (1999)

Spain

R R

166.8 d 170 d

128–222 -

August/September August/September

Delgado et al. (2010) Penteriani et al. (2012)

Czech and Slovak Republics

O

-

-

mid July–October

Cepak (2008)

*O: observational study; R: radiotracking study.

0.06

Figure 83. The effect of moon phase on Eagle Owl dispersal departures. When individuals reach their dispersal age, they mainly leave the natal area during bright nights (from Penteriani et al. 2014b).

Estimated effect

0.04 0.02 0.00 –0.02 –0.04 –0.06

0 1 2 3 4 5 6

Moon phase (radiance)

244

Natal and breeding dispersal

Wandering The length of the wandering phase is highly variable, lasting between 3 and 90 days (average: 29.8 ± 24.8 days, n = 16 individuals) in Switzerland (Aebischer et al. 2010), and between 3 and 353 days (average: 150 ± 109 days, n = 24 individuals) in Spain (Delgado & Penteriani, unpubl. data). It seems that juvenile Eagle Owls never travel during daytime, as all their flying activities take place between sunset and sunrise (Aebischer et al. 2010). Previous works studying dispersal in Eagle Owls have mostly reported dispersal distances based on findings of dead or injured ringed owls (Desfayes 1951, Glutz von Blotzheim & Bauer 1980, Mebs & Scherzinger 2000), which limits comparisons. A notable feature is that Eagle Owls show a striking variation in dispersal distances (Table 16). Most Eagle Owls disperse less than 50km from their place of origin, but a few records have also reported

Table 16. Mean dispersal distance (km) of juvenile Eagle Owls reported by studies in different countries. Country

References

Mean total distance (± SD)

Range

Mean net distance (±SD)

Range

Switzerland

Nyffeler (2004) Aebischer et al. (2010)

124.0 102.0

max = 230.0 20.0–230.0

38.7 46.1

3.0–95.0

Spain

Delgado et al. (2010) Penteriani et al. (2012)

25.0

9.0–53.0

6.0 13.1

1.5–34.3 4.7–35.0

Czech Republic

Mrlíkova & Peške (2005) Cepák (2008)

>100.0 -

-

61.5

-

The Netherlands

Wassink (2009b)

-

-

62.0

17.0–150.0

Olsson (1997) Reported in Melling et al. (2008) Reported in Cramp & Simmons (1980)

-

-

56.8 (1-year old) 47.7 (>1-year old) 52.0 50.0

0.0–528.0 max 25.0–229.0 max = 400.0

Austria

Leditznig (1999)

222.0

-

84.0

-

Germany/ Austria

Fiedler (2005)

-

-

100.0

Valkama et al. (2014) Saurola (2002)

-

-

52.5 (♂); 64.0 (♀) 71.9a 48.2b 83.0

max >200.0 10.0–162.0

Fremming (1986) Larsen et al. (1987) Reported in Cramp & Simmons (1980)

-

-

100 days old, Eagle Owl families travel considerable distances together (Delgado et al. 2009a), a behaviour that has also been observed in other birds (Matthysen et al. 2005, 2010, Sharp et al. 2008). This behaviour has the potential to influence both the dispersal direction of offspring and their close proximity after the family breakup (i.e. the excursions may have familiarised the offspring with the surroundings of their birthplace; Drent 1984, Matthysen 2002, Matthysen et al. 2005). Delgado et al. (2010) also observed a heterogeneous patch occupancy during dispersal (Figure 89A), in which few patches were highly visited by different individuals, whereas many others were visited by just one individual (Figure 89B). When comparing the actual routes of dispersal with optimised and randomised ones, they found that the routes followed by dispersing owls were an intermediate solution between minimising the number of occupied patches and randomly exploring the available area (Figure 90). These results offer a unique 251

The Eagle Owl

A

B

Number of patches visited

50

40

30

20

10

0

0 1 2 3 4 5 6 7 8 9 10

Number of juveniles

Figure 89. (A) The structure of the studied spatial network, i.e. the spatial patterns described by dispersing Eagle Owls in SW Spain, in which the actual routes followed by owls from the natal site to the final settlement/breeding area are depicted. Nodes represent the crossed habitat patches and their size is proportional to the number of different individuals that occupied them. The black nodes denote those patches that also contain nests in which juveniles were radiotagged. (B) The number of nodes occupied by dispersing owls clearly shows a heterogeneous patch occupancy, i.e. few nodes are visited by many juveniles whereas most nodes were occupied only by one juvenile during dispersal (from Delgado et al. 2010).

opportunity to support the idea that the Eagle Owl seems to follow a dispersal flow, polarised along both a specific axis and direction. Implicit in this directionally biased dispersal pattern is the fact that we are faced with an asymmetric dispersal scenario (Vuilleumier & Possingham 2006, Revilla & Wiegand 2008) with several important consequences on metapopulation and source-sink dynamics (Penteriani & Delgado 2009b). Because the number of connected patches essentially determines metapopulation viability in an asymmetric system, we can expect population decline if the most frequented patches connecting breeding sites to settlement areas disappear or are affected by environmental stochasticity (Penteriani & Delgado 2009b). 252

Natal and breeding dispersal

A

B

D

Number of occupied nodes

C

12 10 8 6 4

random real optimised

2 0

Dispersal routes

Figure 90. Comparison of the spatial patterns described by (A) the actual route followed by owls from the natal sites (black dots) to the final settlement/breeding areas, (B) the optimised routes and (C) the randomised ones. (D) The number of nodes occupied by either dispersing owls, randomised routes or optimised routes shows that the actual routes followed by Eagle Owls during dispersal represent an intermediate solution between optimised and random ones (from Delgado et al. 2010).

253

The Eagle Owl

During the wandering phase, dispersing owls may explore a wide range of different habitat types, from completely natural landscapes to very anthropogenic areas. The proportion of habitat used largely depends on its availability. Wassink (2009b), for example, found that half-open cultivated landscapes were present in 80% of locations, whereas forest, farmland and industrial plants were present in 89%, 52% and 13% of localities, respectively. Bodies of water were also present in 10% of the locations. This author also found that 74% and 47% of Eagle Owl locations in Limburg and Gaba (Germany) were in coniferous forest. However, Delgado et al. (2010) observed that juveniles in Spain generally preferred open landscapes, the majority being composed of shrubs with sparse trees or cultivations with trees. Further, juveniles may frequently prospect urban areas (Wassink 2009b, Delgado et al. 2010). More detailed information on habitat use by juvenile Eagle Owls can be found in Campioni’s PhD thesis (2012). Campioni, along with her colleagues, performed a series of studies to determine the potential effect of social status on the behavioural process of habitat selection (Campioni et al. 2010, 2012). The motivation behind these studies was that social status may be reflected in many individual behavioural and ecological aspects. Depending on the diverse tasks associated with different social status (e.g. breeders vs. dispersing individuals), the trade-off between costs and benefits driving individual behavioural decisions may produce divergent behavioural strategies, including habitat use (see Chapter 9). Despite the important consequences for the ecology of populations regarding this aspect, it has thus far been frequently overlooked.

Temporary settlements During the wandering phase, Eagle Owls search for temporary settlement areas (Figure 91). Individual differences in movement behaviour can thus lead to variation in settlement patterns. Juveniles with straighter movement trajectories may travel longer dispersal distances, but over a shorter time, before settling than individuals showing tortuous paths (Delgado et al. 2010). In Switzerland, Nyffeler (2004) found that the first Eagle Owls arrived at their temporary settlements on 6 September, whereas the last ones arrived on 10 December (median: 27 October). All individuals spent the whole winter at the initial settlement site. Eight birds left this temporary settlement area on average 3.7 ± 3.1 months later (range: 50 days to 11 months). Six other birds stayed in the initial area until they died or the tag battery was exhausted, i.e. for 3.5–10 months. Some birds, after dispersing about 100km, returned to the same temporary settlement areas, showing a high fidelity to them. Wassink (2009b) observed that owls flew on average seven days straight (1–13 days) before they remained at least two days at about the same location. This first stopover lasted an average of eight days (range: 2–21 days). Individuals had from 1–4 temporary settlement areas, none of them being the final destination. After spending some days there, juveniles left these temporary areas and moved further distances. Similarly, Spanish juveniles (Delgado & Penteriani 2005, Penteriani & Delgado 2012, Penteriani et al. 2012) attempted to settle as soon as possible within well-defined home ranges close to their natal population. Males and females showed similar permanence times in their temporary settlement areas, ranging from several days to approximately a year (Penteriani & Delgado 2012). The temporary settlement areas were relatively small (~2km2), being larger at the beginning of the dispersal process. Although the temporary settlement areas were slightly larger for males than females, this difference was not statistically significant (Penteriani & Delgado 2012). Moreover, dispersing Eagle Owls 254

Natal and breeding dispersal

CC

B B

A

N 2 0 2km

Figure 91. An example of the movement path of a juvenile Eagle Owl during dispersal. After starting the dispersal phase (A), the juvenile explores its surroundings for a certain period of time (B) before selecting a temporary settlement area (C).

Figure 92. Examples of the large overlap between the temporary settlement areas (estimated by the Minimum Convex Polygon method) used by juveniles in SW Spain (Delgado 2008).

were surrounded by conspecifics (Figures 87 and 92; Delgado & Penteriani 2005, Penteriani & Delgado 2012), the most frequent case being when individuals during the wandering phase shared areas with individuals already settled. On average, core areas (i.e. the smallest area used by individuals) spatially but not necessary temporally overlapped (Figure 92). 255

The Eagle Owl

Table 18. Estimates of movement parameters and space used for both dispersing individuals and territory owners. Juveniles (wandering phase) (Mean±SE)

Juveniles (stop phase) (Mean±SE)

Territory owners (Mean±SE)

1396.54±174.67

725.25±67.21

762.86±77.12

Roost site1 (m) 2

Foraging areas (%) Speed3 (m/hr) Fractal D4

0.12±0.009

0.10±0.007

0.09±0.007

874.98±54.26

801.06±35.82

641.75±37.07

1.06±0.005

1.08±0.005

1.09±0.006

Path length (m)

9958.56±614.63

9248.99±395.25

6676.09±359.73

Step length6 (m)

608.1 ±21.78

546.70±32.94

456.68±23.42

5

Source: Delgado et al. 2009b. Diurnal place selected by individuals; 2size of main areas used by individuals for hunting purposes; 3measured as the distance covered by individuals per unit time; 4measure of the tortuosity of the path followed by individuals; 5distance covered by individuals during the night; 6distance of each movement taken by individuals (see also Figure 85).

1

Delgado et al. (2009b) showed that moving within different temporary settlement areas and spatial learning are two interrelated processes: (a) changes in movement behaviour determine the learning of the spatial environment, and (b) information plays a crucial role in several animal decision-making processes like movement decisions. Juveniles at the beginning of dispersal travelled significantly further (net distances) during the night than both floaters within their temporary settlement areas and territory owners (Delgado et al. 2009b). That is, dispersing individuals with incomplete information concerning the environment travel faster, with longer step lengths, and have the longest and straightest trajectories (Table 18). However, when they find a temporary settlement area, their movements are similar to the movement patterns of a territorial owl (Delgado et al. 2009b). For example, individuals already established in a temporary area usually come back to a given roost site or area more frequently than do owls during the wandering phase. Indeed, dispersing owls in a temporary settlement area show an intermediate behaviour between wandering and territorial owls (Delgado et al. 2009b). Further, there are no significant differences in the relative size of the foraging areas used, even though the relative size of them gradually decreased from territory owners to the wandering phase of dispersal (Table 18). At the beginning of the wandering phase, individuals frequently travel across unfamiliar and often unfavourable areas. Uncertainty regarding the location of conspecifics, predators and resources may pose significant problems (Stamps 1995, Stamps & Krishnan 1999, Dall et al. 2005). But these costs may be reduced by becoming familiar with the spatial and social environment, e.g. searching actively for temporary settlement areas. Dispersing owls can benefit from living in a restricted area by gaining knowledge of the habitat and establishing dominance relations with other dispersing individuals and territory owners (Smith 1978, Stutchbury 1991, Bruinzeel & van de Pol 2004). Temporary settlement areas are generally unknown or difficult to detect. They may look very different from breeding areas, as they are usually characterised by high levels of anthropogenic disturbance. Consequently, less effort is typically devoted to the conservation of these apparently low-value sites, though they may be inhabited by the majority of dispersing individuals waiting for opportunities in breeder habitats. Population studies, 256

Natal and breeding dispersal

analyses of population viability and extinction risk assessments that ignore the dynamics of dispersers in settlement areas may fail to understand how and why animal populations decrease, and may consequently lead to inappropriate or ineffective conservation actions (Penteriani et al. 2005c, Oro et al. 2008, Penteriani et al. 2011b). In the absence of this knowledge there is a risk of underestimating threats to a species/population for which the main problem is not in the breeding area, but rather in settlement areas where the effects of mortality are sooner or later likely to have an impact on breeder numbers.

Settlement in a breeding site The time between when a disperser finds a settlement area and then becomes a breeder is very unpredictable in the Eagle Owl. Aebischer et al. (2010) observed that ten out of eleven individuals did not breed at the end of their first year of life. Only one female successfully reared a brood in her first year. The two birds that they could follow at the age of two years and another one that was still alive and tracked at the age of three years did not breed at these early ages. However, Delgado et al. (2010) observed that those juveniles that randomly crossed an empty nest site, or the ones that by chance arrived close to an available mate during the wandering route, settled and became breeders, even at the age of a year old. For example, they observed how a juvenile male and a juvenile female whose birthplaces were separated only by 0.6km established a pair bond in an empty breeding site located at a distance of ca. 3.4km. This happened in their first year of life (Figure 93). Indeed, in that NW Spanish population, 35% (n = 24) of the Eagle Owls found a stable settlement area in the middle of March (mean dispersal age at the settlement phase = 395 ± 109.9 days, range = 181–40 days). Ten (42%) out of these 24 settlers became territory owners and started breeding (Delgado et al. 2010). As they marked 103 juveniles, and in accordance with Swiss birds, the most common situation observed was still that dispersers settled in an area and remained there for several years without breeding (Fasciolo et al. 2016). The same pattern was indeed observed in Norway (Oddane et al., personal communication), where two (out of seven) juveniles radio-marked and followed during 2.5 and 3.5 years respectively never reproduced. Another population located in an archipelago in northern Norway (Oddmund et al., personal communication) has been monitored by DNA tools since 2012. Researchers analysed DNA from shed feathers and plucked feathers from nestlings. One of the aims of this DNA-based monitoring was to study natal dispersal and recruitment. In 2013 and 2014 they found that individuals born in the population returned the following year(s) but without breeding. In 2015, however, they documented that two females, one born in 2012 and the other in 2013, returned to the population to breed. A similar situation was also observed in Finland, where only one female and one male out of 44 radio-marked juveniles (from 2011 to 2013) reproduced (Penteriani, Delgado, Valkama & Saurola unpublished data). The female successfully raised her first two chicks in her second year of life. The following year she did not reproduce, but she again successfully produced two chicks in her fourth year of life. The male was captured in a breeding site in his fourth year of life. The average settlement distance in Valais (Switzerland; Aebischer et al. 2010) was 46.1 ± 31.9km (range: 3–95km, n = 18; Table 16). This distance is notably large when compared with the ones recorded in two areas of SW Spain (Sierra Morena: 1.5−34.3km; mean ± SD = 6.0 ± 4.2km, see Delgado et al. 2010; Doñana National Park: 13.1 ± 7.7km). The dispersal distance observed by Aebischer and colleagues did not correlate with the age of the young 257

The Eagle Owl

N 1000

0

1000

2000 km

Figure 93. Dispersal routes of two juvenile Eagle Owls of a Spanish population, differentiated by black and grey lines. They were born in March 2003, started to disperse at the end of July 2003 and reproduced in the breeding season of 2014. Circles with a black dot represent their first locations after leaving their birthplaces. Squares with a black dot represent their last locations in their new breeding sites.

at the start of dispersal nor with the date on which the young left parents. For 10 out of 18 young (56%), the settlement distance was shorter than the maximum distance flown from the nest during dispersal. That is, a progressive return towards the vicinity of their native place might be possible. The same pattern was indeed observed in other populations. For example, Olsson (1979) found that half of the recovered young birds were found more than 75km from their hatching place. All other recoveries revealed that the Eagle Owls remained very close to their hatching place. Seventy-five percent were found less than 50km away, and none farther away than 86km. He also showed that old birds were found closer to the nest in which they were born than young ones. Two birds from one brood were found, 16 years later, 16 and 21km away from their hatching place. Another 15-year-old bird was found only 15km from the nest in which it was ringed. These findings may indicate that either far-flying young died in distant areas or survived, gradually returning home later on. A similar pattern was observed in Finland (Penteriani, Delgado, Valkama & Saurola unpublished data), where 42% of the radio-marked juveniles progressively returned to the vicinity of their birthplaces. Sex-biased dispersal is a widespread pattern in vertebrate organisms (Greenwood 1980), with one sex being more philopatric and the other one more prone to disperse. While in 258

Natal and breeding dispersal

mammals dispersal is often male-biased (males have higher dispersal rates than females), the opposite pattern is generally found in birds (Greenwood 1980). The dispersal patterns of Eagle Owls observed in Finland (Penteriani, Delgado, Valkama & Saurola unpublished data) indicate that this is indeed the case. However, males usually disperse longer distances than females but they return more often (33%) than females (8.3%) to their natal site (Figure 94). This pattern could have been overlooked if observations about dispersal distances were only based on findings of dead or injured ringed owls. Mrlíková & Peške (2005) observed that at the end of December a juvenile they were tracking simulated true breeding behaviour with typical vocalisations and an extremely prolonged stay in a particular spot with an evident and continually renewed nest depression. Yet, it is important to stress that there are always some observations of individuals dispersing far away from their natal area without returning back to it. For example, one Eagle Owl in Sweden was seen flying across the sea to Öland (Larsson 1973), and another one is known to have reached Gotland in the middle of the Baltic Sea. As another example, out of the 24 Eagle Owls that were born in Valais (Switzerland; Aebischer et al. 2010), eight settled in that area, whereas five went to Italy, five to France, and six to the north of the Alps. Such observations seem to indicate that some individuals have somewhat potentially given rise to a spread to and nesting in surrounding areas, even in those areas where Eagle Owls are absent.

Figure 94. Examples of two juveniles during dispersal in SW Finland. The male (black path) travelled over 130km and then returned to the vicinity of its birthplace. The female (white path) also travelled over 130km and found a settlement area without returning to its birthplace. As a general rule for southern Finland, males exhibited the tendency to come back close to their birthplace after one year, whereas females never came back.

259

The Eagle Owl

Looking at the low rate of juveniles becoming breeders, a pertinent but unanswered question is: why a generalist species, which is mature at its first year, as is the case for the Eagle Owl, does not reproduce early more often than it actually does. Natal dispersal influences the demographic and evolutionary dynamics of spatially structured populations, and conversely ecological and evolutionary dynamics provide the context against which individuals make their dispersal decisions. The relatively long permanence period of Eagle Owls inside their temporary settlement areas, as well as their relatively short dispersal distances, suggests that they are not dynamic dispersers actively looking for short-term breeding opportunities, as is commonly the case in other bird species (e.g. Heg et al. 2000, Tobler & Smith 2004 and references therein). Data from some different Eagle Owl populations support a different dispersal scenario with regard to the traditional or common views under which dispersal only represents the mechanism by which dispersers move far from their population (that is, with the perception of dispersal as the process in which ‘when I move it is to go far’). In fact, it seems that dispersing owls remain floating close to their native population (Delgado & Penteriani 2008), with both the search and final settlement pattern strongly affected by the complex combination of dispersal behaviour tactics, physiological parameters and the spatial configuration of the matrix (Delgado et al. 2010). Fasciolo et al. (2016) importantly argued that this kind of dispersal scenario may be compared with an ecological trap (i.e., maladaptive behavioural or life-history choices made despite the availability of higher-quality options; Schlaepfer et al. 2002), which may result from a combination of some intrinsic properties of animal populations. In their case study, the dispersal trap is represented by an optimal settlement area that is difficult to leave: food is abundant, intraspecific competition seems to be low (Penteriani & Delgado 2012) and mortality is not any higher than in the breeding area. Individuals settle in these very attractive patches and do not show any prospecting behaviour aimed at filling empty breeding sites, despite the fact that this strategy is not the ‘best’ in terms of population growth rates and persistence (Delgado et al. 2011). The reasons behind the decision to leave an area may reside not only in the characteristics of the settlement areas, but also in what there is outside that place. For example, they argued that an ecological trap may emerge when: (1) there is a high density of nesting sites characterised by high fecundity; (2) mortality rates in breeding sites vs. the dispersal area are fairly similar; (3) trophic resources in breeding sites vs. the dispersal area are similar; (4) breeders frequently disperse to neighbouring breeding places, and this breeding dispersal seems to be the principal mechanism by which nesting sites from the core of the reproductive fraction of the population are reoccupied after they become vacant; (5) dispersers are recruited into the breeding population only when their dispersal paths cross an empty breeding site (which generally happens on the periphery of the breeding area); (6) after settlement, juveniles do not prospect for breeding opportunities in their first few years of life, remaining ‘trapped’ within their settlement areas; and (7) extremely low recruitment rates prevent empty breeding sites from being reoccupied promptly, especially in the core of the reproductive area, despite the high density of juveniles moving into the dispersing areas. The emergence of ‘evolutionary traps’ during dispersal (Fasciolo et al. 2016) does not feed off behavioural mistakes, but rather good choices (at least in the short term): dispersers may find and settle in a very good and safe environment that on the one hand interacts positively with their survival, but, on the other hand, has a negative effect on the need to prospect for breeding opportunities and, consequently, on individual fitness. It is neither uncommon nor novel that natural selection acting at the individual level may engender 260

Natal and breeding dispersal

negative effects on the population as a whole (reviewed in Delgado et al. 2011). The idea that individual behavioural choices can promote a decline at a population level has strong relevance to ecology but has rarely been observed in natural systems. This trap underlines some important aspects of the tight relationship between dispersers and the population as a whole: (a) the study of dispersal patterns can provide information about whether the population is responding favourably to the behaviours of dispersers; and (b) there is a lack of knowledge concerning the results of the interactions between dispersal rules and the ecological characteristics of populations. Yet, one contrasting explanation for the dispersal trap Fasciolo and colleagues observed may involve the non-mutually exclusive effects of (i) queuing for available/best breeding sites (Kokko & Sutherland 1998), and (ii) delayed reproduction, which is important for long-lived individuals that may have greater lifetime reproductive success than individuals that breed immediately (Cooper et al. 2009, Millon et al. 2010). These results illustrate how the analysis of the dispersal process from the perspective of unusual biological models may open up new horizons for scientific know-how because it may enhance our understanding of some additional aspects of one of the most complex ecological processes (Penteriani & Delgado 2009b). Only with a more comprehensive view of dispersal, incorporating a detailed analysis of individual variation in behaviour, can we improve our understanding of the crucial relationships between its causes at the individual level and its consequences at the population level (Penteriani & Delgado 2009b). Following this line of research, i.e. with the aim of linking dispersal strategies with population dynamics, Penteriani, Delgado and collaborators (Switzerland: A. Aebischer & R. Arlettaz; Norway: B. Oddane, O. Kleven, G.A. Sonerud, O. Gjershaug, T. Nygård, I.J. Øien & K-O. Jacobsen; Finland: J. Valkama & P. Saurola) started a project that is built on existing Eagle Owl data sets acquired in Spain, Switzerland, Norway and Finland. The collection of the Finnish data was part of an international research collaboration funded by the Junta de Andalucía (Spain). The Spanish data compiled a time series (2002–2008) on Eagle Owl dispersal and demography in SW Spain. The demographic data consisted of a total of 168 breeding attempts in 48 nest sites. Additionally, to characterise the breeding-site quality, there were census data on the main prey species, the Rabbit, and detailed data on the owls’ diets (pellets collected at each breeding site). The dispersal data consisted of the movement tracks of 103 juvenile owls equipped with VHF-tags. Each tagged juvenile was followed from the moment it left the natal area to the moment it settled in a new area, died, or the tag stopped working. All juveniles were localised at least weekly. Additionally, individuals were followed, one at a time, continuously throughout the nights, providing high-resolution data that allowed the identification of individual movement steps. During the movement phase, additional behavioural data (e.g. hunting behaviour) were recorded. All juveniles were measured at the age of 35 days for various morphological (e.g. length of tarsus, length of wing), biometrical (e.g. body mass), biochemical (e.g. cholesterol, urea) and blood (e.g. parasites, lymphocytes, haematocrit) parameters. During the study period, 29 breeders were also radiotagged and followed during the nights. The Swiss data consisted of a time series of Eagle Owl demography (1998–2008) and dispersal (2002–2005), acquired in the SW Swiss Alps (Canton of Valais). The demographic data consist of annual counts of breeding pairs and the number of fledglings. The dispersal data consist of 41 juvenile Eagle Owls, 21 being equipped with GPS transmitters and 20 with VHF-tags. Tagged birds were localised at least once every fourth night. Finally, similar data from a population located in NW 261

The Eagle Owl

Norway and another one located in SV Finland are available. There, 7 and 44 juvenile Eagle Owls were equipped with GPS transmitters respectively, and demographic parameters have been recorded as well. In this ongoing project, the researchers aim to quantify how dispersal strategies are influenced by the relative roles of external factors, the properties of the individual, and the surrounding population. Conversely, they seek to quantify how demographic population dynamics are influenced by the relative roles of external factors, the properties of the population, and the movements of individuals. As a final goal, the researchers aim to understand if the underlying factors shaping patterns of dispersal and population dynamics are consistent among landscapes (Mediterranean landscapes, Alps and boreal forests) and/or if they depend on the ecological context; that is, if the observed differences can be attributed to differential evolutionary selection gradients in the respective environments.

Breeding dispersal Breeding dispersal is the movement of breeders away from the place in which they have once reproduced to another breeding site (Greenwood 1980). It has always been believed that Eagle Owls were pair-bond monogamous (but see Dalbeck et al. 1998), and that they occupied the same nesting site during their entire life (Baumgart et al. 1973, Blondel & Badan 1976, Mikkola 1983, Cramp & Simmons 1980). However, in the long-term study in the Sierra Morena, Delgado, Campioni & Penteriani (unpubl. data) recorded eight cases (four males and four females) of breeding dispersal from radiotagged individuals (Figure 95), corresponding to 26.5% of the radiotagged breeders. In six out of these eight cases, a male or a female moved to a new area several kilometres away from the original breeding place after the loss of a mate. The remaining cases were otherwise divorces (for more details on divorce rates, see Chapter 7). The mean breeding dispersal distance recorded in the Spanish population was 3.6km (range = 0.6–10km), with females dispersing longer distances (mean = 6.6km; range = 3.3–10km) than males (mean = 1.3km; range = 0.6–2km). They also recorded breeding dispersal events into breeding sites that were not empty. In those cases, the replacement involved the killing of the owner: a male dispersed to another nest site, killed the owner and occupied its breeding place. In the first case, a radiotagged male moved to a neighbouring nest site, which was at a distance of ca. 2km. They found the bones of an Eagle Owl male in the pellets collected at that site, probably indicating that the former male was killed and eaten by the new male. In the second case, another radiotagged male moved to a neighbouring breeding site, which was located at the close distance of ca. 0.6km. Very surprisingly, we found the remains of the former male at a very visible calling post that was close to the nest. Its feathers were decorating the calling post in exactly the same way as is customarily done by Eagle Owls with their prey to signal their presence as active breeders (Penteriani et al. 2008). Some other cases of breeding dispersal have been registered in another Eagle Owl population located in Murcia (SE Spain; Mario León, pers. comm.). In 2008, a female was captured and radiotagged at her breeding site. The following year she moved to another breeding site located 1km to the north. That year she did not reproduce, but she successfully raised three chicks in 2010. In 2011 she came back to her former breeding site. While absent, two different females occupied and reproduced at her first breeding site. As another 262

Natal and breeding dispersal

6000

0

6000

12000 km

N

Figure 95. Breeding dispersal movements observed in an Eagle Owl population located in Sierra Morena (SW Spain). Each symbol type represents a different pair of nests. White dotted circles represent the nest sites surrounding the nests where breeding dispersal was observed.

case, a female captured and radiotagged in 2009 moved from her breeding site to another breeding site located 1km to the south and successfully reproduced. To our knowledge, no additional cases of breeding dispersal have been published in the literature, even though we believe that this process in this species may be more common than currently thought. The population-level consequences of breeding dispersal may be numerous and diverse. In the case of the Spanish population, for example, breeding dispersal is a key process for the persistence of the population, as the lack of prospection by dispersers for breeding opportunities prevents empty breeding sites from being reoccupied promptly, especially in the core of the reproductive area (Delgado et al. 2011). As breeders may disperse to neighbouring breeding places, breeding dispersal seems to be the principal mechanism by which nesting sites from the core of the reproductive fraction of the population are reoccupied after they become vacant (Fasciolo et al. 2016).

Seasonal migration Seasonal migrations are the consequence of dramatic seasonal changes in weather conditions such as severe and lasting frosts or major depressions of prey number. Seasonal migrations in Eagle Owls are generally poorly studied, even though these movements may have important consequences for the dynamics of populations, for example, by contributing to gene flow. Seasonal migrations have been reported for several subspecies inhabiting Central Asia. Here we briefly summarise the few records that have been published (for more details see Mitropolskiy & Rustamov 2007 and references therein). Bubo b. sibiricus appears irregularly in Central Asia only in winter, the common grounds of which do not extend to the south and do not stretch outside the area of N Kazakhstan. In Central Asia, where Eagle Owls are only observed in January–February, individuals come from the southeastern Caspian 263

The Eagle Owl

Sea Region (Gasan-Kuli), the lower areas of the Syr Darya River (Dzhulek) and Alma-Ata. The presence of Bubo b. yenissensis in Tarbagatai may also be related to winter nomadic movements. Seasonal migrations are common in Bubo b. turcomanus. Individuals migrate to the south from their breeding grounds in September–October. In autumn–winter seasons birds are observed in Kyzyl Kum, lower areas of the Amu Darya River, Central Kyzyl Kum and the piedmont areas of the Kopet Dag. However, the main nesting site of the species is the southeastern Caspian Sea Region. They appear in the Caspian Sea Region at the end of September; this multitude of birds is seen in October–February and departs in March–April. Individuals of Bubo b. omissus breeding in southern regions of Turkmenia, Uzbekistan and Tajikistan (Mitropolskiy & Rustamov 2007) also perform nomadic movements. Birds breed in the upper parts of the Kopet Dag Mountain Range, an area that they leave for foothills in winter. Finally, vertical migrations seem to be a common feature of populations of Bubo b. hemachalanus, which normally breed in piedmont areas in winter. In winter Eagle Owls of all high-mountain areas are common in piedmont areas, particularly in the Chuyskaya and Ferganskaya valleys, the Issyk Kul hollow, and the piedmont areas of the entire external boundaries of the Tien-Shan and Pamir Alai mountain ranges. During the winter season the species usually stays near urban areas. Long-distance winter vertical nomadic migrations are more common in females than males.

264

CHAPTER 11

Mortality and threats Mortality is an essential parameter for understanding the dynamics of any population, and Eagle Owls are no exception. Without understanding the speed and rate at which Eagle Owls are removed from a population it is not possible to model population dynamics or estimate any useful management parameters such as population persistence. There are two types of mortality: natural mortality, which is the loss of individuals from natural causes such as disease and old age, and human-related mortality, which, as the name suggests, is the direct loss of individuals due to human activities. Direct mortality resulting from human activities has important consequences, particularly when it is additive to natural mortality (Anderson & Burnham 1976). There is great uncertainty about the impacts of direct mortality on Eagle Owl populations (but see Penteriani & Pinchera 1990a, 1992, Sergio et al. 2004a, Schaub et al. 2010), although the causes of death of Eagle Owls have been observed and commented on for many years (Hagen 1950, Thiollay 1969, Choussy 1971, Görner 1977, Förstel 1977, Leibundgut 1981, Barkhoff 1987). For example, collisions and electrocutions of Eagle Owls at power lines have long represented a major conservation issue (Förstel 1983, Fremming 1986, Penteriani & Pinchera 1990a, Grüll & Frey 1992, Sergio et al. 2004a, Görner 2005, Bassi et al. 2011). Mortality at power lines may contribute to population declines of Eagle Owls, as demonstrated by studies documenting that power line-caused mortality can constitute a large percentage of the total mortality (Table 19). Similar consequences may be caused by other human-induced sources of mortality. This is especially important when mortality is considered together with other indirect stressors such as decreasing prey availability and habitat alteration. Some studies (e.g. Temple 1986) have emphasised that the circumstances that ultimately cause a population to go extinct may be completely different from those that first caused the population to become endangered. For example, some mortality factors 265

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that were once considered insignificant are now quite pertinent due to steadily increasing environmental stress (Bevanger 1998). Yet, more research is still needed to quantify the magnitude of human-related Eagle Owl mortality and its population-level effects. Some studies have illustrated spatio-temporal patterns in Eagle Owl mortality. For Table 19. Percentages of the most significant causes of Eagle Owl mortality across its distributional range (from north to south). In a few studies electrocution accounts for both electrocution itself and collisions with wires, but most studies properly separate these two causes. Electrocution Collision

Persecution

Area

Source

16.6

16.6

33.4

Sikhote-Alinsky

Russia

Ryabtsev 2005

19.8

12.9

5.0

SE Sweden

Sweden

Olsson 1979

16.0

13.3

17.7

39.0

20.0

23.7

10.9

26.2

26.9

40.8

30.4

33.0

27.0

31.0

30.0

33.0

29.0

35.3

5.9

12.5

Finland

Saurola 1979

Finland

Valkama & Saurola 2005

West Germany

Germany

Wickl 1979

2.8

Eifel

Germany

Radler & Bergerhausen 1988

24.0

Hesse

Germany

Brauneis & Hormann 2005

Switzerland

Aebischer et al. 2005

Switzerland

Koch 2005

Switzerland

Aebischer 2008

25.1

13.0

Bern

47.1

Central Massif

France

Choussy 1971

50.0

Provence

France

Blonde & Badan 1976

34.0

30.0

7.0

France

Nadal 2010

28.0

18.0

10.0

France

Nadal & Balluet 2010

42.8

14.3

14.3

Italy

Rigacci 1993

68.4

21.0

Italy

Rigacci 1993

2.0

14.0

63.0 47.1 42.0

4.0

66.3

4.1

66.3 47.1

9.7 9.7

10.0

11.2 11.8

13.7

51.7 6.0 3.7

17.9

Emilia-Romagna Alto Adige

Italy

Sascor & Maistri 1996

Belluno

Italy

Tormen & Cibien 1997

Lombardia

Italy

Leo & Bertoli 2005

Sondrio

Italy

Sondrio

Italy

Ferloni & Bassi 2008

Alps

Italy

Marchesi et al. 2002

Catalonia

Spain

Real et al. 1985

Spain

Hernández 1989

Spain

Hernández 1989

80.6 40.2

Madrid, Guadalajara, Toledo

Bassi & Ferloni 2007, Ferloni & Bassi 2008

78.6

Murcia

Spain

Martínez et al. 1992

22.2

4.0

Valencia

Spain

Martínez et al. 1996b

24.1

10.3

44.8

Albacete

Spain

Fernández 2000

41.7

8.3

25.0

Toledo

Spain

Ortego & Calvo 2001, 2004

Spain

Martínez et al. 2006

Spain

Molina-Lopez et al. 2011

20.1 25.0

266

Country

19.2 13.0

4.0

Catalonia

Mortality and threats

example, Martínez et al. (2006) analysed 1,196 records for which complete information was available for cause of death, region and year, showing significant interactions between region and year, region and cause of death, and year and cause of death. Low and high frequencies of persecution in both southern and eastern Spain, respectively, high frequencies of powerline collisions in the centre, as well as the relatively high frequencies of other causes in the south, were responsible for the significant interaction between region and cause of death. The significance of the year–cause interaction was due mainly to an increase in recorded powerline mortality. The significance of the region–year interaction was attributable to a higher number of casualties recorded in the south, east and some areas of central Spain in the period 1995–2003 than in previous years. Similarly, Koch (2005) found a marked increase in the number of dead Eagle Owls recorded in Switzerland since 1965, with a peak in the early 1990s and a slight drop afterwards until 2004. The seasonal distribution of mortality did not differ significantly between younger owls (i.e. from August of birth year until July of second year) and older owls. Whether these spatio-temporal patterns of mortality are a true representation of Eagle Owl death is, however, an open question, as mortality is a difficult parameter to determine accurately (Erickson et al. 2000, 2001). Biases associated with observer detection and scavenging rates can lead to biased mortality estimates (Morrison 2002). For example, it is obviously easier to find an Eagle Owl that died from electrocution than one that died from starvation (Bassi & Ferloni 2007). Furthermore, observers conducting searches for Eagle Owl carcasses may often not detect some carcasses for a variety of reasons, including inaccessibility to certain locations and dense vegetative cover. Carcass detection rates and scavenging rates of Eagle Owls do vary among sites, habitats and seasons. An Eagle Owl that died in a collision with a car may be more easily found than one dying from natural causes in a remote area (Aebischer et al. 2005), leading also to a potential spatial bias in detection rates. In addition, mortality rates due to persecution are also difficult to be estimated properly because most of the killed individuals are not reported. Comparisons of mortality rates that are not adjusted for these sources of bias can thus be very misleading. Many, if not most, of the studies of Eagle Owl mortality we perused for this chapter do not account for the potential biases described above. Thus, even though we have extensively reviewed the literature published on the causes of death in Eagle Owls, the information presented here should only be considered an indication of the relative importance of the various mortality factors, and not as a definitive representation of Eagle Owl causes of death.

Causes of death Many different causes of mortality have been reported for Eagle Owls (e.g. Penteriani 1996, Martínez & Zuberogoitia 2003a, Sergio et al. 2004a, Martínez et al. 2006, Penteriani & Delgado 2010). On the one hand, despite the fact that the Eagle Owl has been protected throughout most of its distribution range since the 1970s, in most cases where the cause was known, death was attributable directly to human activity (Figure 96; Table 19). These include deaths by shooting, trapping and poisoning, collisions with vehicles and wires, as well as electrocution (e.g. Olsson 1986, Kunstmüller 1996, Martin 2008, 2010a). Other causes are nest destruction, the effect of pesticides, pollutants and landscape changes. In a study performed in Switzerland by Koch (2005), at least 84% of all the recorded mortality cases were 267

The Eagle Owl A

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% Russia Sweden Finland Finland Germany Germany Germany Switzerland Switzerland Switzerland France France France France Italy Italy Italy Italy Italy Italy Italy Italy Spain Spain Spain Spain Spain Spain Spain Spain Spain

0%

B

electrocution

collision

25%

27%

persecution

other threats

31%

17%

Figure 96. Percentages of the different causes of death of Eagle Owls by (A) country and (B) globally. The reviewed references are cited in Table 19.

found to be connected directly or indirectly to anthropogenic factors. This corresponds well with the findings by Haller (1978), who showed that in his study sample all but one of the known mortality cases were due to humans and their facilities. In the former Eastern Germany and in Baden Württemberg (Germany), 66.8% and 88.0% of all recorded dead Eagle Owls died as a result of anthropogenic factors (Rockenbauch 1978, Piechocki 1984, 1985). In his study, Koch (2005) found that the cause of death in 37% of 24 first-year Eagle Owls and 31% of older individuals was due to traffic accidents. On the other hand, causes of natural death include starvation and diseases. Yet, infectious diseases may have serious negative consequences on animal health and can be an important threat to wildlife (Ortego & Cordero 2009, 2011). But let us analyse in more detail the main causes of death and threats for Eagle Owls.

Persecution One of the most powerful limiting factors for the Eagle Owl is its persecution, e.g. shooting and killing of nestlings (Frey 1973, Fremming 1986, Aebischer 2008, Robitzky 2010c, Martin 2010a, Bearzatto 2014). Persecution is especially a problem in those countries with a long tradition of small wild game hunting (Ortego & Calvo 2004) as Eagle Owls predate on small game species like Rabbits and partridges. The highest rates of known persecution are found in Spain (80.6% of all deaths; Table 19; Figure 96A), followed by France (up to 50% 268

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of all deaths; Table 19; Figure 96A), Russia (28.6% of all deaths; Table 19; Figure 96A) and Italy (21% of all deaths; Table 19; Figure 96A). In southwestern Finland, a surprisingly large number of chicks were found killed in 2010 and 2011 after they were ringed (Penteriani, Delgado, Saurola & Valkama, unpubl. data), when it was believed that the ringers were no longer checking the nests and, thus, the killings should have gone undetected. Spain is probably the European country with the most areas inhabited by small wild game, which are usually private lands. There, persecution is a common practice not only by hunters but also by game keepers, who are still entrusted to keep the areas free from any kind of predator. They may even receive an economic reward (which often absurdly consists of only a few coins) if they bring the claws of an Eagle Owl to the landowner. Martínez et al. (2006) provided valuable quantitative data on the causes of mortality of wild bird populations, particularly in regard to human-related causes. They showed that the killing of Eagle Owls is still a common practice throughout Spain, where the legal protection of birds of prey seems to have had a limited effect. It is generally believed that the killing of raptors is opportunistic, i.e. it takes place during the hunting season and is not deliberately aimed at reducing raptor predation (Viñuela & Arroyo 2002). However, their finding suggests that the shootings occurred outside the hunting season, indicating that killing Eagle Owls is proactive to a remarkable extent. In addition, adults, nestlings and eggs were intensely collected in several countries during the second part of the last century for the purpose of scientific research as well as to supplement biological collections (Nellis 2006). Hernández (1989) reported spoliation as the main cause of nest lose, which affected 69.5% of nests in a Spanish region. Another eight nests in that same area (17.3%) were lost because of disturbance during the reproductive period.

Unintentional human disturbance Human disturbance has always been an important factor in chick mortality in Eagle Owls. Unintentional disturbance can happen when people (e.g. mountaineers, hikers, birdwatchers and photographers) come upon the immediate vicinity of Eagle Owl nests in which the female is still incubating or chicks are very young. Disturbance may also lead to predation of eggs and chicks. Furthermore, several forms of human disturbance like the building of cabins and boathouses, roads and forestry practices have removed Eagle Owls from breeding sites or led to poor reproduction (Jacobsen & Gjershaug 2014). Lack of knowledge of Eagle Owl nesting sites among land-use planners, and failure to consider Eagle Owls when assessing potential disturbances, are indeed important causes of Eagle Owl population decline. The Norwegian Ornithological Society (NOF) began a national survey of Eagle Owls in 2008. They investigated to what extent Eagle Owls are vulnerable to human activity near their nesting sites early in the breeding season, finding that they easily abandon the nest when disturbed. If breeding areas suffer intense human activity due to, for example, the building of cabins, forestry practices, ramblers and rock climbers, Eagle Owl pairs may also abandon the nesting site. Even though the occurrence of livestock has had little impact in the decline of Eagle Owls thus far, it has been reported to be an important threat along some parts of the Norwegian coast. For example, sheep may lie in Eagle Owl nesting sites early in spring, thus displacing the birds from these sites. Overgrowing of the cultural landscape is also known to have resulted in a decline in the Eagle Owl population in the county of Rogaland (Norway), and in other countries as well. 269

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Electrocution and power-line collisions Quantitatively, electrocution on power lines is the main human-induced cause of mortality in Eagle Owls (Table 19, Figure 96B; Penteriani & Pinchera 1991, 1992, Tucker & Heath 1994, Rubolini et al. 2001, Sergio et al. 2004, Martínez et al. 2006, Tinto et al. 2010). While electrocutions may take place when a bird touches two phase conductors or one conductor and a grounded device simultaneously, particularly when the feathers are wet, power-line collisions occur when birds fly into wires. Electrocution occurs primarily at power lines with voltages between 2.4–60 kilovolts (kV), whereas collisions occur at both distribution lines and transmission lines – large power lines with voltages >60 kV. The lower impact of high or very high voltage power lines (60 to >150 kV) on raptors is illustrated by studies of avian mortality along portions of such lines in e.g. Germany (Hoerschelmann et al. 1988), France (Bayle & Iborra unpubl. data) and Italy, which show that birds of prey represent only between 0.1–0.4% of all birds killed on these lines (Bayle 1999). In another French study, of the raptors killed on medium voltage power lines, 93.5% were electrocuted and 6.5% collided with electrical wires (Bayle 1999). When the space between power-transmission lines or between those and a grounded device is small, the Eagle Owl may touch both simultaneously when it spreads its long wings. It will then be killed by the flashover. Transformers mounted on poles and poles that provide a link to an underground or submarine cable are particularly hazardous. Power lines carrying moderately high voltages (22 kV–132 kV) are the most dangerous due to their design. Irregular and unexpected electrocution accidents do take place as a result of the huge diversity in electrical installations and equipment (Kroodsma & Van Dyke 1985, Negro & Ferrer 1995). In Norway, pole-mounted transformers, pin insulators and a triangular conductor configuration were reported as the most dangerous electrical devices (Bevanger & Thingstad 1988). Eagle Owl mortality due to power lines in Europe represented the leading cause of death in the cases reviewed by Sergio et al. (2004a) and accounted, on average, for 38.2 ± 3.8% of reported deaths (range 9.7–75.0%). This value is similar to the 34% we obtained when including additional case studies published during the last ten years (Table 19, Figure 96B). The percentage mortality by electrocution reported by Sergio et al. (2004a) has increased over the past three decades. More specifically, power lines were responsible for 22.6% of Eagle Owl deaths in Sweden (Olsson 1979, Bayle 1999) and 39% in Finland (Saurola 1979, Valkama & Saurola 2005), 32.5% in Germany (Wickl & Bezzel 1979), 54.7% in France (Bayle 1992 and Bayle unpubl. data) and 16.3% in Spain (Hernández 1989). Power lines seem to have been the major limiting factor of the alpine population of Eagle Owls and may be responsible for the past decline of the species in the Swiss and French Alps (Haller 1978, Bayle 1992), as well as in the Italian Apennines (Penteriani & Pinchera 1991, 1992). Some specific studies on Eagle Owl electrocution provide an idea of the local frequency of this cause of death and its effects on breeding populations. Penteriani & Pinchera (1991, 1992) showed that 11 out of 15 deserted nesting sites (73.3%) in Central Italy (Abruzzo) were located less than two kilometres from a dangerous power line and only 20% of the occupied sites were relatively close to power lines. Sergio et al. (2004a) estimated that, although there was no effect on long-term breeding success, the presence of pylons within 200m of the nest increased the likelihood of partial or complete brood loss in the post-fledging period in an Italian Alps population. Moreover, they estimated that 17% of fledged young were lost to electrocution. The results of these Italian studies highlight some important points related to 270

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the phenomenon of electrocution in Eagle Owls: (a) even if the preference for low-altitude areas exposes Eagle Owls to high electrocution risk, individuals do not seem to actively avoid sites crowded with power lines, suggesting that they are incapable of recognising them as hazardous; and (b) electrocution has the potential to alter the temporal evolution of nest site distribution. Yet, breeding sites that were at lower altitudes, where most power lines were concentrated, were progressively abandoned, which is consistent with abandonment due to the electrocution of owls without subsequent replacement. Such patterns were confirmed by three cases in the Alps, in which the installation of new power lines near three owl nests was shortly followed by the electrocution of both adults, with subsequent nest site abandonment (Sergio et al. 2004a). Also, (c) a local threshold of landscape ‘power line load’ may exist (Bevanger 1994a,b), beyond which there may be a population effect on local distribution, nest dispersion and population trend. Koch (2005) found that the two most important mortality factors in Switzerland were electrocution (31%) and traffic accidents (27%), followed by collisions with a cable or fence (13%). The importance of electrocution and traffic accidents did not differ significantly over the time period 1975–2004. Furthermore, the distribution of electrocutions and traffic accidents differed significantly between the Alps and Plateau/Jura, with the relative importance of electrocution being higher in the Alps. Here, many power lines lead from hydroelectric power plants in this region through valleys down to supply stations. In the central-eastern Italian Alps, 16 out of 22 Eagle Owls for which cause of death was known perished by electrocution between 1993 and 2000 (Marchesi et al. 2002). Power lines may reduce population size in different ways, e.g. by affecting juvenile mortality (Marchesi et al. 2002), breeding pairs (widowing or death of both mates of a pair), or post-juvenile dispersal (Renssen et al. 1975, Real et al. 1985, Stolt et al. 1986). Thus, this type of mortality generally affects raptor population dynamics by regulating the density of the birds, targeting specific age classes and changing population size and structure (Schaub et al. 2010, Sergio et al. 2004a). As a consequence, this non-natural cause of mortality is capable of destabilising populations and could potentially cause local extinctions (e.g. Haas 1980, Penteriani 1996, Jenny 2011, Guild et al. 2011 and references therein). Bird electrocution on power lines has been the focus of extensive research worldwide (e.g. Arlettaz 1988, Janss & Ferrer 2000, Petrovics 2009b, Tinto & Mañosa 2010, Petrovics 2012). For instance, the elements related to power transmission seem to be the prime cause of Eagle Owl mortality in Norway. From 2009 to 2013 the Norwegian Institute for Nature Research (NINA) received economic support for research on power lines and wildlife from the Norwegian Research Council through the Clean Energy for the Future programme – RENERGI (Bevanger et al. 2009, 2010, Christensen-Dalsgaard et al. 2010, Bevanger et al. 2011, Bevanger et al. 2012, Nygård & Polder 2012, Øien et al. 2014, Jacobsen & Gjershaug 2014). The overall goal was to develop predicting tools for optimal routing of power lines from an environmental perspective and to assess technical and economic solutions to minimise conflicts with wildlife (Bevanger et al. 2009, 2010). Eagle Owls that were at risk were equipped with radio transmitters, and the study subsequently showed that more than half of them were killed by electrocution due to short circuiting and earth wires. A corresponding proportion of ringed Norwegian Eagle Owls whose cause of death is known were killed by electrocution or collision with power lines. Several studies in other countries confirm that electrocution is the greatest threat to the Eagle Owl (Figure 96A). The marked Norwegian owls spent about 120 hours per year per individual perching on electricity pylons, which certainly involves a high degree of risk. 271

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Several factors may affect the mortality of Eagle Owls by electrocution and collision, including: (1) biological factors, e.g., Eagle Owl age, manoeuvrability and vision in collisions; (2) environmental factors, e.g., topography, vegetation, and prey abundance; and (3) structure-related factors, e.g. line orientation and distance between wires for both threats, as well as exposure of and distance between energised and grounded parts in electrocutions. For example, Eagle Owls often perch on the highest elements of their surrounding landscape and, consequently, a particularly high number of bird casualties related to electrocution and collision have been recorded in open landscapes (Sergio et al. 2004a, Lehman et al. 2007, Lash et al. 2010). In the absence of trees in these habitats, the most suitable structures for perching are the electric poles which individuals mainly use for perching and roosting (Infante & Peris 2003, Sánchez-Zapata et al. 2003, Karyakin & Barabashin 2005). Tinto et al. (2010) found that pylons with a dominant position in the landscape, especially those placed on hilltops and surrounded by low vegetation cover (as also highlighted by Sergio et al. 2004a), had higher electrocution rates. These pylons are often chosen as perching points because they are good places from which to detect potential prey items (Benson 1982). Another reason why these pylons are actively selected is that low vegetation cover may hold a high abundance of several prey species, which are also more accessible in open vegetation. For example, Ortego & Calvo (2004) reported that pylons placed in areas with high Rabbit abundance accumulated more carcasses of those raptors that prey on Rabbits, as is the case of Eagle Owls in Mediterranean areas. Hence, landscape structure and individual behaviour, such as perching and roosting on poles or wires, are key to understanding why and how birds are electrocuted (e.g. James & Haak 1979, Bevanger et al. 2008, Mihelič & Denac 2011). Thus, when studying the mitigation measures for both electrocution and collision, it is important to keep in mind that environmental factors (e.g. vegetation and landscape structure) may strongly influence electrocution risk, as also observed by Guild et al. (2011) in Spain. Indeed, the strategic location of a pylon (in good hunting habitat or near a nest) may represent a central factor increasing the potential danger of a specific structure (Sergio et al. 2004a). For this reason, it is crucial to evaluate the characteristics of the landscape crossed by power lines to curtail their presence in large patches of open areas with limited availability of alternative perch sites. On the basis of their study, Sergio et al. (2004a) mainly suggested that the most dangerous power lines are those: (a) within 200–300m of known Eagle Owl nests and, more generally, within 300m of all cliffs below 1,000m in altitude; and (b) in which many pylons are located in more than 40–50% open habitat in a 100m radius. Important efforts have been made to mitigate the risk of electrocution on dangerous pylons, although as yet their effectiveness has rarely been tested (Janss & Ferrer 2000, Lehman et al. 2007). This source of mortality is reducible with the use of retrofitting measures or with the implementation of bird-safety standards at new constructions, and there are many safe and easy corrections to apply in order to protect dangerous electric power pylons (Harness & Wilson 2001, Lehman 2001, Sergio et al. 2004a, Pestov 2005, Volkov et al. 2005b). However, most mitigation measures used on wooden pylons do not seem to be very effective on metal pylons (Janss & Ferrer 2000). Studies conducted in southern Spain indicated that perches and perch guard devices do not significantly reduce the risk of bird electrocution, although the insulation of conductive wires and jumpers does lower bird mortality (Regidor et al. 1988, Janss & Ferrer 2000, Moleón et al. 2007). Because electrocution takes place at medium-voltage electricity poles, it is highly recommended that the most dangerous medium-voltage poles (e.g. pin-type insulators on a grounded crossarm) be replaced, or 272

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otherwise modified (e.g. by installing an insulating cap on pin-type insulators; Tinto et al. 2010). Pylon function and the number of insulators, as well as the number of phases above the crossarm, affect mortality (Calvo 1999, Guild et al. 2011). Furthermore, the design of the crossarm emerges as one of the most important factors affecting electrocutions. Pylons can be modified by adding a special perch above the wires. Although some European power companies are very satisfied with these devices and argue that they reduce avian mortality by 90–95%, most of them can only be considered expedients. Indeed, substitution of dangerous pylons or crossarm designs for safer ones may be preferable to insulation, which is not permanent and needs regular monitoring and repair (Tinto et al. 2010). Indeed, the degradation process of insulation provokes higher electrocution rates compared with noncorrected pylons (Guild et al. 2011). Thus, structural changes of pylons or crossarms are also required. Structural changes should focus on eliminating phases above the crossarm and increasing the distance between perch sites and wires, both of which influenced mortality in Eagle Owls. Alternatively, as far as medium-voltage power lines are concerned, an underground network is the only totally efficient solution to electrocution.

Collisions with fences, roads and trains Among the other known causes of death, it is worth mentioning collisions with game fences, trains and cars, the former reported as an increasing menace for the species (Table 19, Figure 96B; Gylstorff 1979, Grichtscbenko & Gaber 1990, Tucker & Heath 1994, Abel 1997, Aebischer et al. 2005). The frequency of collisions with game fences could be underestimated if some of the deaths attributed to traumas had in fact been caused by a previous impact with a fence. Eagle Owls may be prone to impacts with fences when flying low after prey (Muñoz-Cobo & Azorit 1996).

Pesticides and pollutants Eagle Owls may be subject to high levels of pollutants because of their position at the top of the food chain (Odsjö & Olsson 1975, Gerlach et al. 1984, reviewed in Toms 2014). Organochlorine pesticides, polychlorinated biphenyls (pcBs) and mercury (Hg) are among a number of persistent lipophilic compounds that may ‘bioaccumulate’ as they are passed up the food chain from one trophic level to another. Such compounds have been shown to reduce breeding success and increase levels of mortality (Newton et al. 1993). The mercury content of feathers of various species of birds found killed at an Eagle Owl’s nest close to the Baltic coast, in the Russian region of Ostergbtland, ranged between 2,000–3,000ng/g (Berg et al. 1966). Feathers from a breeding pair of Eagle Owls registered 13,500–41,000ng/g, suggesting a concentration factor of five or more. In Sweden, Hg compounds have been used as seed dressings in the form of inorganic salts from ca. 1920 and in the form of phenyl-Hg acetates and alkoxy-alkyl-Hg in the paper industry from ca. 1930 to the 1940s. These Hg compounds were mainly responsible for the increase of mercury levels in most bird species. In 1964, 352 raptors were collected and analysed for mercury content by the Swedish Veterinarian Institute (Olsson 1979, 1997). The livers of 60% of these birds contained more than 2mg/kg of mercury, and the livers of 12% contained more than 20mg/ kg. Because many of these raptors were Eagle Owls, it seems that the mercury content in the Eagle Owls of southeast Sweden was especially high during the early 1960s. In 1960 273

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the most fatal mercury compound was banned in Sweden (Westermark et al. 1975, Broo & Odsjö 1981), but even until 1968 the risks of mercury contamination remained very high. The high mercury contamination in these Eagle Owls probably derived from prey species of aquatic origin, for which the decrease in mercury content since 1966 had been much slower than in terrestrial ones (Broo & Odsjö 1981). The Eagle Owl nesting sites in which the birds had the highest contamination were deserted from 1971 onwards, even though it is not possible to know whether the birds died or not. Mercury levels may also differ between individuals living within the same relatively small area, probably due to diet composition (Lourenço et al. 2011c). Mercury levels in the feathers of Spanish Eagle Owls, which range from very low (0.03mg/kg) to relatively high (80mg/kg), are linked to diet, being greater in individuals involved in the predation of other top predators than in those feeding predominantly on favoured herbivores like Rabbits and Red-legged Partridge. Since Eagle Owls may switch to superpredation when the availability of preferred prey species is low (see Chapter 5), there is a potential increased risk from pollutants at a time when the population is already under stress (Lourenço et al. 2011c). Ortego et al. (2006) analysed mercury concentration in the breast feathers of Spanish Eagle Owl chicks from 27 nests in 2003 and from five nests in 2002. They found that mercury concentration was considerably low compared with other reports previously published. In another area located in southeast Spain (Murcia), Espin et al. (2014) also found that the mercury level detected in blood obtained from Eagle Owl chicks from 2006–2012 was in general low (mean concentration in blood = 36.83 ± 145.58µg/l wet weigh, n = 600). The use of rodenticides may represent another serious risk to many animals (MartínezHaro et al. 2008). The impact of Dichlor-diphenyl-trichlor-ethane (DDT) on wildlife has been well documented in the past (Gómez-Ramírez et al. 2007, Gómez-Ramírez et al. 2012). Mortality from pesticides and poisonous agents from 1950–70 has been observed in Sweden (Mysterud 1983), and suggests a strong negative effect at the population level. Like other organochlorine compounds, DDT accumulates as it moves up the food chain. DDT was widely used as an insecticide following the end of the Second World War and it was soon joined by other compounds, such as cyclodiene dieldrin, which was used as a seed dressing. Together with direct mortality, many of these compounds also produce sub-lethal effects, like the egg shell-thinning associated with DDE, a metabolite of DDT (Gómez-Ramírez et al. 2010, Gómez-Ramírez et al. 2012). DDE has been shown to inhibit carbonic anhydrase, an enzyme required for normal shell formation. In Norwegian coastal areas where Eagle Owls predate largely on seabirds, high toxicant loads may have been a contributory cause of their poor reproduction and decline (Jacobsen & Gjershaug 2014). Even though the old toxicants PCB and DDE are used in considerably reduced quantities nowadays, new ones, like brominated flame retardants, are constantly appearing and may be hazardous to a top predator like the Eagle Owl. The widespread industrial use of heavy metals, coupled with their relatively low chemical reactivity, has led to worrying metal contamination of the environment and the organisms living within it (Gómez-Ramírez et al. 2011, Kim & Jong-Min 2012). For example, in the 1960s in Novosibirsk Oblast (Russia) an Eagle Owl that predated European water voles that were poisoned by zinc phosphide grain bait was found dead (Pukinsky 1993). In two cases, in Leningrad Oblast (Russia), some Eagle Owls died as a result of the use of barium fluoride acetate for poisoning wolves. The danger of Eagle Owl poisoning is high due to the fact that Eagle Owls often predate poisoned, abnormally behaving rodents (Pukinsky 274

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1993; see also Chapter 5). Biomagnification is, again, responsible for the accumulation of such compounds in Eagle Owls and other top predators. However, various studies, carried out across Europe, have revealed that the presence of heavy metals in Eagle Owls is at levels that are typically well below the toxic thresholds that have been defined for each compound. What remains unclear, however, are the potential sub-lethal effects. Levels are often, but not always, greater in older individuals, suggesting accumulation with age, and they tend to be higher in areas of human activity. Lead (Pb) poisoning in raptors is usually caused by the ingestion of prey with lead shot embedded in their flesh (Scheuhammer & Norris 1996). This situation can be common in Eagle Owls inhabiting areas with high shooting pressure. For example, some areas are intensively used for pigeon, partridge and Rabbit hunting in Spain, and cases of lead poisoning have been described in several bird of prey species (García-Fernández et al. 1997, Mateo et al. 2003). Although the effect of the mortality caused by lead poisoning on Spanish raptor populations is not yet well known, this has been relevant as a potential cause of population declines (Wiemeyer et al. 1988). García-Fernández et al. (1997) reported high bone lead levels (15.4mg/g) in nine Eagle Owls from southeastern Spain. As stated above, Eagle Owls may frequently prey on animals with physical impairments (Fernàndez-Llario & Hidalgo de Trucios 1995; see also Chapter 5), and so further dietary studies of this species should pay attention to the presence of lead shot in pellets. In order to biomonitor lead contamination in southeastern Spain, Gómez-Ramírez et al. (2011) analysed 218 blood samples from 28- to 30-day-old Eagle Owl chicks born between 2003 and 2007. They found that chicks born in an ancient and abandoned mining site or in its vicinity showed high concentrations of lead and other metals. When Eagle Owls hunt weak or sick prey, they may accidentally get other poisons into their bodies as well. In Russia (on the bank of Zuevskoe Lake) a dead bird smudged with petroleum products was recorded in 1991 (Ryabtsev 2005). It has been established that the ingestion of petroleum oil in birds at the time of egg-laying may reduce the hatchability of their eggs. The manner whereby the Eagle Owl eats large parts of its prey surely facilitates ingestion of quite a lot of oiled feathers. Many other pollutants are known to be present within the wider environment and some may affect Eagle Owls. A few may be responsible for the death of the owl itself, perhaps a bird whose plumage has become coated in tar or a similar substance, while others occur at low levels within bodily tissues. Many pollutants are persistent in the environment and may pose a risk to owls over long periods of time. The longevity of owls and their position at the top of the food chain facilitates their use as biomonitors for the impacts of pollutants within a wider ecosystem.

Age- and sex-dependent mortality Some authors have concluded that juveniles of large raptors may suffer higher mortality rates than adults as a result of their inexperience (e.g. Bevanger 1998 and references therein). For example, in the case of electrocution risk, juvenile individuals are inexperienced flyers and less well-adapted at manoeuvring than adults, such as when landing and taking off, and they may even overbalance while perching on an electric wire and be electrocuted (Guild et al. 2011 and references therein). Other authors, however, have stressed that there is a high percentage of juveniles and subadults among collision victims because subadults normally 275

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constitute the majority of a population, particularly in autumn. Furthermore, it has been claimed that birds learn to avoid ‘air obstacles’ through experience. However, no data exist to support this idea and habituation seems particularly irrelevant in the case of electrocution. In most studies, the age of dead owls has not been recorded or reported, but still many researchers suggest that mortality in Eagle Owls may be age-dependent because they observed marked mortality peaks coinciding with the timing of juvenile dispersal and during the courtship period (Rubolini et al. 2001). The few studies based on individuals of known age at death seem to support this pattern. Koch (2005) reported that among 184 individuals for which age at death was known, 42 (22.8%) died during the first calendar year, 14 (7.6%) during their second calendar year, and the remaining (69.6%) at an older age. In Italy, Bassi & Ferloni (2007) found that 76% of dead owls were older than two years of age, which is similar to the data reported in Spain (Molina-Lopez et al. 2011) with 28 dead individuals less than one year old, 110 older than one year and 60 of unknown age. Very few works have reported differences between male and female mortality rates in Eagle Owls. The patterns are indeed not clear. While one study reported no differences between males and females (e.g. Bassi & Ferloni 2007), another found a higher proportion of males than females dead (Molina-Lopez et al. 2011), and two others observed a higher mortality rate in females (Koch 2005, León-Ortega et al. 2016). Yet, no differences in the causes of mortality between sexes have been observed (Koch 2005, León-Ortega et al. 2016), so the higher mortality of females might be associated with differences in sex-specific behaviour, habitat use or energetic investment during the reproduction period. For example, the higher mortality of female Eagle Owls may be associated with differences in habitat use by the two sexes, with females moving over larger home ranges than males (Lambertucci et al. 2012, Campioni et al. 2013), which may increase the interaction with human infrastructures. Thus, in some situations, females might face greater risks than males. However, it is also true that females remain close to the nest during a large part of the reproductive period, which may counterbalance these potential risks.

Diseases and parasitisms Other mortality factors appear to be natural hazards to wild animal populations. Infectious diseases have serious negative consequences on animal health and can be an important threat (Ortego & Cordero 2009, 2011). Descriptive epidemiological and morbidity studies (mortality reports of free-living raptors presented to rehabilitation centres) of wildlife are an important source of information about natural hazards to wild animal populations (MolinaLopez et al. 2011). Unfortunately, there are few studies on morbidity in Eagle Owls, and these have mostly focused on specific causes. In addition, the information reported by such studies has been mainly based on the proportion of cases of the total number of admissions and not in relation to the overall wild populations, which could provide a more accurate assessment of the potential impact of the causes of mortality in Eagle Owl populations. Some viral diseases that have been observed in Eagle Owls include those caused by Herpesvirus strigis, which causes necrosis of the liver, spleen and bone marrow as well as stomatitis, enteritis and esophagitis, or infections by bacteria such as Escherichia, which is responsible for gastric mucosal inflammation (Frey 1973). H. strigis is a pathogen for several species of owls in the order Strigiformes. In 1915, a spontaneous lethal infection by H. strigis 276

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was observed in Eagle Owls in Austria (Burtscher & Sibalin 1975). Detailed information on H. strigis is desirable because it may cause severe disease and, occasionally, high levels of mortality. Kaleta et al. (1980) detected antibodies against the egg drop syndrome 1976 (EDS 76) virus, which may cause problems of egg shell quality. Battisti et al. (1998) found Salmonellae havana in three Eagle Owls and Salmonellae virchow in three dead chicks. They also isolated, in 1994, a strain of S. hadar in a healthy 1-year-old female Eagle Owl. Another case of fatal salmonellosis in a male Eagle Owl in Bursa Province (northwestern Turkey) in June 2005 was described by Kocabiyik et al. (2006). The Salmonella enterica subspecies enterica serovar enteriditis (S. enteriditis) was isolated from the liver and spleen in pure culture and also from the intestine. The coccidia observed in the Eagle Owl’s intestine could have enhanced the severity of the Salmonella infection in this bird. Death associated with salmonellosis caused by Nontyphoidal Salmonella Serovars of S. enterica usually occurs in birds co-infected with other agents, especially coccidian parasites, presumably due to the increased ability of Salmonella to colonise the intestinal tract (Kocabiyik et al. 2006). Roh et al. (2012) recently reported the first case of Eagle Owls infected by highly pathogenic avian influenza (HPAI) viruses of subtype H5 and H7 in South Korea. They noted that these Eagle Owls were dehydrated and in poor body condition. Among other findings, they observed multiple distinct white spots on the pancreas and spleen, enlargement of the spleen and uric acid deposition in the kidneys. HPAI viruses induce fatal systemic infection in poultry (Lapshin 2005). First in 1946 and later in 1954, in the steppe-forest zone of Western Siberia and the forest zone of India respectively, a viral disease named ‘Omsk haemorrhagic fever with renal syndrome’ (OHFRS) was first discovered (Lapshin 2005) in 34 out of 84 species of birds (including five bird of prey and owl species). It seemed that the migratory birds that carried over the virus infections played an important role in forming the new centres of the pandemic virus (Lapshin 2005). In Germany (Berlin and Brandenburg), Schettler et al. (2001) tested 448 blood plasma samples from free-living birds of prey for antibodies against Newcastle disease virus (NDV), falcon herpesvirus (FHV), owl herpesvirus (OHV) and Chlamydia psittaci. Only antibodies for C. psittaci were detected in one out of four Eagle Owls. In a study performed in Croatia, in order to examine natural infections by Campylobacter, Salmonella spp. and Chlamydophila sp., in 107 free-living birds belonging to 25 species from 13 families, Vlahović et al. (2004) did not find any infected Eagle Owls. Eagle Owls are susceptible to infestations by a large number of parasites (Sanmartin et al. 2004), but the damage caused by these parasites, both directly and indirectly, often cannot be calculated accurately (Ortego & Espada 2007a). It seems that parasitic organisms that have a low degree of host specificity are more dangerous than those occurring only in owls (Pfister 1990). In 2004 ectoparasites were detected on an Eagle Owl at a laboratory of the Department of Parasitology in Turkey. Five Mallophaga specimens were collected from the bird, which were identified as Strigiphilus striges (Dik & Uslu 2007). A rare parasitic disease was reported by Pirali-Kheirabadi et al. (2010), who found an Eagle Owl in 2008 that was infected by Lucilia sericata and Lucilia cuprina (Diptera: Calliphoridae), thus having a myiasis infestation. In a study performed in Spain by Cabezón et al. (2011), the highest Toxoplasma gondii seropositivity by species was observed in the Eagle Owl. T. gondii is a zoonotic intracellular protozoan parasite, which is considered to be the cause of mortality in many birds. Investigation of T. gondii seropositivity in birds could be a useful way to assess environmental contamination with oocysts since some avian populations feed directly on the ground and are continuously exposed to oocyst ingestion. The authors found that the 277

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main risk factors associated with T. gondii seropositivity in wild birds were age and diet, with the highest exposure in older animals and in carnivorous wild birds (Cabezón et al. 2011). The only blood parasite found by Ortego & Espada (2007b) in nestlings of Spanish Eagle Owls was Leucocytozoon ziemanni. The prevalence of L. ziemanni increased with the age, showing an initial rapid increase of prevalence. However, they did not find an effect of sex on prevalence. In two other studies, Ortego & Espada (2007b) and Ortego & Cordero (2011) demonstrated the existence of complex and interrelated links between ecological parameters and three different measures of disease risk, highlighting the importance of immunological approaches to assess disease risk at an intraspecific level. They tested the hypothesis that disease risk in fledglings was higher in nests located in areas with greater length of and proximity to watercourses (as a higher abundance and viability of parasites and vectors occur in wetter areas), higher cover of forest (as forest moistness and humidity can favour higher vector and parasite proliferation), higher habitat diversity (as environmental heterogeneity increases the pool of potential vectors and parasites) and higher local owl population density (as disease transmission might be density-dependent). They measured immune defence combined with the presence or absence of three parasite types (a blood parasite L. ziemanni, a parasitic insect Carnus haemapterus and a tick Rhipicephalus sp.) and the count of each of these parasite species present. They found that the concentration of white blood cells, the number of parasite species and, weakly, the prevalence of C. haemapterus were all higher in nests closer to watercourses. The prevalence of blood parasites also increased with the cover of forested areas. Fledglings from nests located in more diverse habitats had higher white blood cell concentrations and showed higher prevalence of blood parasites. Finally, local host population density was positively correlated with the prevalence of blood parasites. In a further study, Ortego & Cordero (2009) found the presence of haematozoa parasites (Protozoa) isolated from nestling Eagle Owls in Toledo Province (Spain).

Other threats Eagle Owls may also be victims of other potential threats (Table 19, Figure 96B), but little information is available because they are not easy to survey. For example, in Switzerland starvation has been highlighted as among the main factors leading to high mortality in the first year of life (i.e. during dispersal), which can, in certain cases, reach 80% (Aebischer et al. 2005). The high mortality rate due to starvation in juveniles has also been underlined by Schaub et al. (2010) in a study supported by data obtained with radio-tracking where ten out of 21 cases of death (48%) were due to this threat. Adverse weather conditions can threaten eggs, especially if nests are very exposed (Sascor & Maistri 1996), or chicks, if the weather does not allow adults to bring them enough food. Frey (1973), for example, reported that heavy snow during the 1973 breeding season led to the loss of at least six out of 21 broods. In Spain, heavy rainfall during the breeding season in 2013 also caused the loss of almost all broods of the entire population (Penteriani & Delgado, unpubl. data). Few predators can threaten an adult Eagle Owl, although a female killed by a Lynx Lynx lynx has been reported in Finland (Jari Valkama pers. obs.); young, however, can occasionally be victims of mesopredators such as Red Foxes or martens (Pukinsky 1993, Ryabtsev 2005). Olsson (1979) mentions owls with perforated oesophagi from the claws of birds of prey or urchin spines previously ingested, and an individual that died of poisoning, in whose 278

Mortality and threats

stomach a Viper Vipera berus was found. Another peculiar case reported was that of an owl found dead with its stomach pierced by the talons of a bird of prey they had swallowed (Rigacci 1993). Sometimes a threat can be represented by collisions with hard vegetation during flight. An Eagle Owl was found dead following a collision with a pointed branch, which, after penetrating the eye and orbit, had reached the skull, impaling the animal. Another potential threat for Eagle Owls may be wind farms. Indirect effects such as physical alterations, habitat destruction and human disturbance will probably be the most important negative impacts of wind power development (Jacobsen & Røv 2007). However, further research to assess the direct impact of wind farms on Eagle Owls is still needed (Hötker et al. 2005, Bevanger et al. 2008). Habitat reduction and changes in prey populations have long been suggested to affect Eagle Owls indirectly (e.g. Mysterud 1983, Donázar 1988b, Marchesi et al. 2002, Ortego & Calvo 2004). Olsson (1989) confirmed the important effect of declining game levels and diminishing areas of agrosystems in Sweden. Profus (1992) reported that along the coast of Poland Eagle Owls disappeared in 1987 as a consequence of a newly formed connection between the small sandy islands where the birds nested and the mainland, which made Eagle Owl nest sites accessible to some mammalian predators and humans. Leditznig et al. (2001) reported that habitat deterioration was among the main factors determining the decrease, by a third in the last 10 years, of an Eagle Owl population located in the Danube Valley (Austria). There are several indications that the lack of prey may be an important reason for the decline in Eagle Owls in some areas. Eagle Owls may respond functionally and numerically to variations in the abundance of their main prey (Martínez & Zuberogoitia 2001, Martínez & Calvo 2001, Martínez et al. 2006). For example, Mysterud (1983) related the dynamics of the tetraonid population with the dynamics of the Eagle Owl population. He observed that when an increasing trend in the number of tetraonids was observed, Eagle Owls were reported to be present in previously abandoned breeding places in several inland areas of southern Norway. In a more recent study, Jacobsen & Gjershaug (2014) described that on the islands of Hitra and Frøya (Sør-Trøndelag, Norway), Eagle Owls only occasionally succeed in rearing more than one chick, and many emaciated chicks have been found. This can be a direct consequence of the fact that the populations of important prey species have probably declined considerably in this area. In many places along the Norwegian coast, the Eagle Owl used to nest near gull and tern colonies, but the decline in several species of seabirds has probably led to less prey being available to the Eagle Owl. Moreover, in some places on the outermost islands, the Eagle Owl lives largely on water voles, but after the wild mink became established along the coast it must be assumed that the vole population has been decimated in many places. This may have resulted in the Eagle Owl losing much of its main food supply.

279

CHAPTER 12

Vocal communication In this chapter we will report the main Eagle Owl calls and, then, we will describe the vocal behaviour of Eagle Owls.

Different types of vocalisations and their meanings Small chick, nestling and fledgling calls and sounds When chicks are still white and lie on their talons, their typical food begging–contact call is a rapidly repeated bibbering bibibi-bibibi-bibibi (4–10 kHz, Figure 97A). This same call may be heard when the chick is ready to break open the egg, as well as during hatching. This sound is also produced when chicks experience trouble or conflict among siblings (Pukinsky 1993). Upon hatching, the food call may be described as a thin high-pitched sri or djchü. During food transfer a rapid hihihihi-ia and kli-kli-kli (Hagen 1950) can be heard; at its shrillest, it resembles the squeaking call of adults (see Adult calls and sounds, page 282). Several distress calls (e.g. in disputes between siblings) can be given by young, all of them a variety of chirping, gackering and screeching sounds (Scherzinger 1974, Glutz & Bauer 1980). When owlets are more than two weeks old they may snap their bills loudly in anger or agitation (Blondel & Badan 1976). With age, food calls change into harsh and compressed calls, then into more drawn-out hissing, which at around two weeks of age become the more rasping and typical chwcitsch. At roughly two weeks of age, during the begging for food, two calls are used, i.e. the repeated bibbering bibibi-bibibi-bibibi and the typical rasping chwcitsch. Fledglings confront intruders with harking hia hia hia, perhaps the incipient alarm call of adults. Threatened young also bill snap and make cat-like hissing noises with gaping bill and tongue raised, as in adults (Scherzinger 1974). 280

Vocal communication

A 10

kHz

8 6 4 2

B

0.5 secs

10

kHz

8 6 4 2 0.5 secs Figure 97. (A) Five-day-old chicks may give an acute and plaintive call: bibibi-bibibi-bibibi. (B) Starting from when they are 20–25 days old, young Eagle Owls begin to emit their typical chwätch call (from Penteriani et al. 2005b).

The most common food begging–contact calls of nestlings and fledglings are the characteristic chwcitsch or chwätch (up to 6–8 kHz, Figure 97B), and a longer and faster chjüjöo; these hootings can be heard up to at least 1km away by humans (Frey 1973). Kranz (1971) calculated that owlets may make 2,000 to 2,500 food begging calls per night, with the maximum increasing with age: around 550 calls per hour in June, 600 calls per hour in July, and 900 calls per hour in August, when the owlets may utter a total of 5,000 calls per night. Although extremely noisy, nestlings and fledglings are extremely sensitive to the presence of intruders when calling, being able to stop vocalising immediately (and thus concealing themselves) if they consider that their presence and position may be perceived. However, they resume calling as soon as they deem that the potential danger is no longer threatening them. 281

The Eagle Owl

During feeding, young Eagle Owls may also emit several cries, which are described as a series of chü-eet, chui, chii, chechechueen, chauuu, tchuau and sharp cheeenn (Burnier & Hainard 1948, Choussy 1971, Delgranche et al. 2011c). At approximately four months of age, young start to give a hoarse hououou-hou (at the same time that ear-tufts enter the last stage of growth), followed by the main adult call at around five months (Piechocki & März 1985), which is almost perfect by the time young reach six months of age.

Adult calls and sounds Adult vocal behaviour is associated with intra- and intersexual territorial disputes, as well as with courtship behaviour (Penteriani 2002). Although Eagle Owl vocal activity can be disturbed by wind and rain (Ruiz-Martínez et al. 1996), and they may cease calling activity when wind and rain are very strong (März 1940), they do not necessarily remain silent under bad weather conditions (Choussy 1971), but this is always related to intra-individual variations (Fremming 1983). Several authors agree on the complexity of the vocal repertoire of the Eagle Owl (Géroudet 1979, Cramp & Simmons 1980, Doucet 1989, Mikkola 1983). For some of its vocalisations this complexity simply reflects the fact that they are difficult to transcribe, or difficult to classify correctly, as they are composed of a mix of several calls. In addition, it is difficult to attribute a specific and unique meaning to each type of call because many of them are used in very different situations, so that in some cases the meanings are somewhat arbitrary. For example, as we will see later in this chapter, the most typical hoot of adults may be used in both territorial and sexual contexts, whereas some calls that have been described as alarm calls may also function as calls related to situations of excitation. Here, we present a comprehensive list of the vocalisations that have been previously described in papers and books (mostly Géroudet 1979, Cramp & Simmons 1980, Glutz & Bauer 1980, Mikkola 1983, Piechocki & März 1985). The main Eagle Owl call for both males and females (defined as an advertising call in Cramp & Simmons 1980) is the typical deep and booming oohu (approximately 150–400 Hz, average 384 Hz for males, initial frequency mainly ca. 350 Hz, dropping to below 260 Hz at the end, Cramp & Simmons 1980; 250–350 Hz following Voous 1988), with emphasis on the first syllable and pitch descending to the second, which is easily distinguishable between sexes and generally performed as a monotonous series (oohuoohu-oohu…) of more or less regularly spaced calls (the time lag between single hoots is extremely variable, lasting from only a few seconds to up 10 seconds). This call is quite variable in pitch, volume, timbre and rhythm, allowing individual recognition (see Can population characteristics reduce the level of call identity?, page 302). Sex differentiation is possible because the sonorous, low-pitched booming of the male is easily distinguishable from the harsh, higher-pitched call of the female, which is also more distinctly disyllabic than the male call (Cramp & Simmons 1980, Mikkola 1983). This is the typical call used by males and females in duets, as well as during territorial displays, demonstrating the dual function of attracting females and rebuffing males. Sometimes, when mates are extremely excited, the intervals between separate hoots become shorter, and the voices of birds join together; then, the voice of the male may turn into the typical laugh (150–500 Hz, see later in this section) while the female continues to hoot, barely increasing the frequency of its rhythm (Pukinsky 1993). The laugh may last for 10 seconds at a time; after laughing, 282

Vocal communication

peculiar moans and groans may follow. Throughout the year, this hoot is the most frequent call uttered by Eagle Owls (Martínez & Zuberogoitia 2002, 2003b, Delgado & Penteriani 2007), which can be used in extremely diverse situations, ranging from territorial contests to sexual displays, as well as threatening situations (e.g. when the pair defends nestlings/ fledglings from an intruder, humans included). When an individual (male or female) loses its mate and remains alone in the breeding site, it generally intensifies its calling rate, probably to increase the probability of attracting a new partner (see also Grosjean 1976, Wilhelm Bergerhausen pers. comm.). Such intense activity may also continue during those periods (e.g. incubation) in which owls are generally more silent (W. Bergerhausen pers. comm.). It has been reported that the main call of adults may be heard by humans up to 3–4km from the calling individual (Glutz & Bauer 1980), although the general distance from which this call may be heard is around 1–2km (Mikkola 1983, Piechocki & März 1985, Pukinsky 1993, Ruiz-Martínez et al. 1996), depending on the acoustic properties of the surroundings of the calling owl. The main call can also be heard during daylight (Piechocki & März 1985), frequently in the central hours of the day and early afternoon. Glutz & Bauer (1980) and Rigacci (1984) report a nasal variant of this main call, given by both sexes. This variant is highly variable and difficult to render, usually described as a short sequence of soft, muffled, nasal or wheezy uh or uhju sounds, sometimes at close intervals. It has been heard during nest-showing, and during food transfer from male to female or from female to young. Single calls are also given by the male during and after mating (Schnurre 1936). The vocal repertoire of the Eagle Owl also includes a relatively long series of rapidly repeated and descending hohohoho (males) and a higher-pitched huhuhuhu (females) defined as an excitement call, often referred to as giggling or laughing (Grosjean 1976, Glutz & Bauer 1980, Piechocki & März 1985). This call is used in various heterosexual contexts associated with mating and copulation, mostly by males, as well as during food transfer and confrontations. Desfayes (1951), Grosjean (1976) and Rigacci (1984) cite a similar boubouboubou call, which can probably be regarded as a variation of this same excitement call, used as a contact call between mates (Rigacci 1984 reported this call for the male when joined by the female within the cavity of the nest). A rapid series of wheezy, squeaking wi-wi-wi (Glutz & Bauer 1980) is considered a squeaking-call, given in rapid rhythmic fashion by the male during copulation. During mating this call may represent a transition from low grunting cho through chwi sounds, which then changes to the higher pitched wi. This squeaking-call may thus be considered a high-intensity excitement call. A similar sound (pipipipipipie or kjikjikjikji) is given by females at the nest upon greeting food-bearing males (Piechocki & März 1985 report a similar ikkie ikkie sound). A typical contact call attributed only to females is a harsh sound, difficult to render, similar to the Jay Garrulus glandarius call or the sudden tearing of coarse cloth (Desfayes 1951, Choussy 1971), which has been rendered, for example as kvék, gwach or gräck, gwiing or chria (Hagen 1950, König & Haensel 1968, Glutz & Bauer 1980, Fremming 1983, Piechocki & März 1985). The call has been observed in series near time of laying, combined with the main male call in duets. A more rapid, perhaps variant, series appears to be associated with food transfer and copulation (Glutz & Bauer 1980). Females arriving to feed young give similar grä grä grää sounds as well (Piechocki & März 1985). The kvék and gräck sounds, combined with wäha, are also considered alarm calls, frequently emitted as 283

The Eagle Owl

single calls or in a more or less short series of calls (Choussy 1971, Piechocki & März 1985, Delgado & Penteriani 2007), although they may also function as contact calls between mates (see also Choussy 1971). These additional calls may be performed as single or evenly spaced calls, as well as repeated in fast sequences of several calls (e.g. kvék kvék kvék). Long call series such as kvék kvékkvékkvékkvék may have comparable meaning (e.g. mate duets, alarm or excitement calls) to the uäuäuäuäuä series, performed by males (as also stated by Rigacci 1984). Both the kvék and uä series have been frequently described in the literature as ‘Eagle Owl laughing’, amusingly associated with the image of the devil, probably because they bear resemblance to a ‘scary’ sound. Continuous series of clucking tucka sounds have also been reported, glugg-glugg-glugg (Leibundgut 1973) and ugg-gugg-gugg-gug (male; Hagen 1950), as well as rapid cackling like klukk-klukkklukk and kodékodékodé, which are considered soliciting calls, given by both sexes (mainly females) in various situations to invite close approach, particularly during copulation, nest-showing, food solicitation and to encourage young to take food (Schnurre 1936, Cramp & Simmons 1980, Glutz & Bauer 1980, Fremming 1983, Piechocki & März 1985). Adults proffering food to young accompany this call with a bowing display (the adult faces the young and bows up and down; Blondel & Badan 1976). One of the most typical alarm calls is the sharp croaking grack, similar to the call of the grey heron (Glutz & Bauer 1980), the male call being deeper than that of the female. Females may also give a loud, barking chwa or kwa, similar to that of the fox; other alarm sounds include ka-ká-ka ka-ka, ke-ke-ke and a more raucous gra-gra-gra (given while perched or in flight), used to warn young of approaching danger and silence them (Choussy 1971) and during attacks on intruders. Grichik & Tishechkin (2002) describe a peculiar, not very loud and hoarse vä-kaä, used during a particularly stressful and threatening situation (i.e. chick defence against humans). More difficult to describe is a sound that has been considered to be a specific warning call, i.e. a succession of high-pitched, drawn-out piping sounds with a wavering, finely vibrant quality, reminiscent of the distress cry of small mammals (Grosjean 1976, Cramp & Simmons 1980). It is often described as a high-pitched whistle. When flying, or in some warning contexts, the species may emit an extreme noise in the form of loud buzzing (Pukinsky 1993). This same author also describes some other peculiar calls such as a hoarse ap or aak in situations of anxiety, which sound similar to the screech of the grey heron. Sometimes when Eagle Owls are disturbed, they produce a ‘yelp’ that is comparable to that of the Ural Owl Strix uralensis, even if quieter and duller. A ‘hen’s cackling’ is often produced at the moment of food delivery during the feeding of chicks. Among other calls and sounds, (i) Choussy (1971) and Scherzinger (1974) mention a sort of cat-like hissing call, probably related to warning situations or a threat, which frequently accompanies bill clapping; (ii) Doucet (1989) cites a powerfully exhaled häi, which could also indicate alarm, threat or anxiety; (iii) a strange sound similar to rrro rrrorrrorrrorrro, in which alternate units are lower pitched, like the noise of a small saw on wood, heard from females at the nest during the feeding of young (Burnier & Hainard 1948); and (iv) different types of growling, grunting, jarring, rattling, moaning and screeching, likely to be mostly the same, or variants of, the sounds listed above. Finally, and although it cannot be considered a vocalisation, it is worth mentioning here the typical bill snapping, which is generally accompanied by excitement and hissing calls. Both adults and young produce this sound, generally associated with stress and menace. 284

Vocal communication

It can be also heard (i) from the nest during mealtimes, which is typically very noisy and animated, accompanied by food-calls and wing noises, as well as (ii) from closely threatened fledglings, and accompanied by a forward-threat posture (as in adults). Although rare, during courtship a sort of wing-clapping may also occur (König & Haensel 1968).

Patterns of daily and seasonal vocal behaviour Nestling and fledgling calls Patterns of daily calls of young have been systematically analysed from the age of 70 days, when their typical chwätch call starts to be easily detected, to the start of natal dispersal (Penteriani et al. 2005b). This call is in fact audible from about the 40th day of life, but at this time the call is only detectable at close range and the frequency during the night is low. For listening sessions (from 1h before sunset to 1h after sunrise), the period in which the young stay in the parents’ breeding area has been divided into five 20-day blocks, i.e. when the young were 70, 90, 110, 130 and 150 days old (Penteriani et al. 2005b). The total number of calls per night per young ranges from 318 (at 70 days of age) to 1,106 (150 days; see also Table 20). There is a positive relationship between the duration of call bouts and the number of calls per bout, and the duration of call bouts and number of calls per bout are generally positively correlated with the age of juveniles (Figure 98). Young vocal behaviour displays two main patterns by age (Figure 99): first, between 70 and 110 days, call activity is primarily concentrated near sunset and sunrise, juveniles remaining quite silent during the middle of the night (especially from 70 to 90 days). Calling in the middle of the night increases from 110 days onwards, showing the highest rates when juveniles are 130 and 150 days old. Finally, the young show quite a cyclical vocal activity during the night, characterised by four peaks of intense calling, two of them coinciding with 1h after sunset and 1h before sunrise (Figure 99). Such a pattern is mainly evident among the oldest age-classes. During the post-fledging dependence period (e.g. from fledging to the start of natal dispersal, when young owls are still in the paternal breeding area and more Table 20. Vocal behaviour (mean ± SD and range within brackets) of juvenile Eagle Owls from 70 to 150 days. Vocal features

Age (days) 70

90

110

130

150

232.90±374.15 (1–1208)

126.08±254.97 (1–1190)

107.88±123.42 (1–372)

126.85±263.11 (1–1780)

170.56±240.94 (1–1072)

Calls per bout

31.80±50.30 (1–164)

13.83±20.57 (1–75)

15.24±19.30 (1–64)

11.87±20.55 (1–131)

16.76±22.46 (1–94)

Max time interval (min)

64.24±120.26 (1– 370)

20.16±49.03 (1–225)

22.22±51.48 (1–237)

9.38±16.08 (1– 85)

7.18±15.60 (1–78)

Min time interval (min)

5.69±7.33 (1–20)

4.68±4.25 (1–15)

8.34±10.89 (1– 48)

6.75±9.28 (1–55)

4.92±9.60 (1–59)

Bout duration (s)

Notes: The longest bouts and the largest number of calls correspond to the youngest age-class (70 days old), when calling activity is shortest and concentrated close to sunset and sunrise (see also Figure 99). Time intervals are calculated between two neighbouring bouts, for both the entire night (max time interval) and per hourly block (min time interval).

285

12000

10000

10000

8000

8000

6000

6000

4000

4000

2000

2000

Call duration (sec)

12000

0

70

90

110

Age (days)

130

150

Call numbers (x10)

The Eagle Owl

0

Figure 98. The duration of vocalisations (black line) and call number (grey line) of juveniles per night, increase with age (from Penteriani et al. 2005b).

or less dependent on them), the vocal activity of adults is lower than in the previous and following months (see Adult calls, page 289). In addition, many offspring vocalisations occur at times other than sunset and sunrise (when adult calls are mainly concentrated), when their parents are absent (Figure 100). At later-age stages, first and last calls are also closer to sunset and sunrise, respectively, than before (Figure 101). In particular, during the last three age-classes, the first call occurs only between 9 and 22 minutes after sunset, whereas at 70 and 90 days it occurs 81 and 83 minutes after sunset, respectively. Generally, young always give their last call before sunrise (range 9–47 minutes). To summarize, patterns of young

Call duration (sec)

3000

2000

1000

0 1 2 3 4 5 6 7 8 9 10 11 12 13 Night hour (from 1 hr before sunset to 1 hr after sunrise)

Figure 99. Call behaviour of juveniles displays two different patterns by age (grey dotted line = 70 days; grey broken line = 90 days; solid black line = 110 days; solid grey line = 130 days; black dotted line = 150 days): between 70 and 110 days the calls of young are close to sunset and sunrise, whereas later in development main vocalisations occur during the middle of the night. Four main peaks of vocal activity characterise the call behaviour of young: at 1h after sunset, at 1h before sunrise and at the 5th and 8th hours of the night (from Penteriani et al. 2005b).

286

Vocal communication

call behaviour mainly demonstrate that: (1) the duration of bouts and number of chwätch per night increase with age, especially from 110 days of life, even if the longest bouts and greatest number of calls per bout are recorded when the young are 70 days old; (2) starting from 110 days old, young vocalisations are for the most part uniformly distributed over the whole night and start shortly after sunset; (3) age-classes of 110, 130 and 150 days show a quite generalised cyclical pattern of vocal activity throughout the night. The vocal activity of young represents a useful method for detecting occupied nests: on the basis of the recorded call rates, surveys focused on young begging should start when the young are at least 110 days old, when call activity increases and becomes evenly distributed over the whole night. Before this stage, passive auditory surveys should be planned only close to sunset and sunrise. Because young begging may be performed during the day, concerns were expressed about the fact that this call was used for adult feeding alone (Penteriani et al. 2000). Actually, at least during the day, when it was possible to observe adults near young that were calling, the adults appeared to ignore them. Indeed, many calls were performed by young when adults were either not calling or absent, supporting the hypothesis that young begging calls may also represent a way of communicating within family groups (i.e. sibling contact calls). In fact, increasing frequencies of this call coincide with the period when young move several hundred metres from the nest and when the mean distance between siblings increases. 70 days

90 days

110 days

Call duration (sec) Call duration (sec)

500 500 500 500 400 400 400 400 300 300 300 300 200 200 200 200 100 100 100 100 0 0 0 1 3 5 7 9 11 13 0 1 1 3 5 7 9 11 13 1

Call duration (sec)

130 days

500 500 400 400 300 300 200 200 100 100 0 0 1 1

3

3

5

5

7

7

500 500 400 400 300 300 200 200 100 100 0 9 11 13 0 1 9 11 13 1

3

3

5

5

7

7

9

9

11 13 11 13

150 days

500 500 400 400 300 300 200 200 100 100 0 3 5 7 9 11 13 0 1 3 5 7 9 11 13 3 5 7 9 11 13 1 3 5 7 9 11 13 Night hour (from 1hr before sunset to 1hr after sunrise) Night hour (from 1hr before sunset to 1hr after sunrise)

Figure 100. Temporal distribution of young (grey line) vs. adult (black line) vocal activity. Even if there is a temporal correspondence between adult calling and young begging, vocalisations of the latter also occurred at one-hour periods other than sunset and sunrise and in the absence of their parents (from Penteriani et al. 2005b).

287

The Eagle Owl

A

22.10

Hours

21.42 21.15 20.47 20.20

B

70

90

110 Age (days)

130

150

70

90

110 Age (days)

130

150

7.50

Hours

7.30 7.10 6.50 6.30

Figure 101. With increasing age, the first call of young (A) is closer to sunset and last call is always before sunrise (B). Calls are represented by black dots. Solid lines indicate actual times of sunrise and sunset (from Penteriani et al. 2005b).

6000

450

Male call duration (sec)

350

4000

300 250

3000

200

2000

150 100

1000 0

Female call duration (sec)

400

5000

50 Pre-laying

Incubation

Nestling

Fledging/post-fledging

0

Figure 102. Total duration of vocal displays of males (black line) and females (grey line) during the four main periods of the year: pre-laying, incubation, nestling and fledging/post-fledging periods (from Delgado & Penteriani 2007).

288

Vocal communication

Finally, nestlings and fledglings may also be vocally active during the day (Penteriani et al. 2000). The duration of vocalisations and the number of calls are different in the various diurnal hourly blocks, with peaks occurring 3 hr after sunrise and 3 hr before sunset. Such a diurnal vocal activity is relatively high, if we consider that (a) the mean (± SD) number of calls per series is 65.6 ± 127.1 (range = 1–259) and (b) the duration of vocalisations in a single series ranges from 1–1,130 sec (mean ± SD = 808.4 ± 891.4 sec). The maximum time interval between two neighbouring series was 40 minutes (during the hourly block corresponding to 5 hr after sunrise). The mean interval between calls is 10.5 ± 6.02 sec (range = 2– 28.7). Such high rates of diurnal vocalisations in Eagle Owls may simply result from young owls practising their calls, just as high rates of other diurnal activities may represent muscular exercise (e.g. flight training).

Adult calls A study conducted in the Sierra Morena (Delgado & Penteriani 2007) revealed how the duration of both male and female calls varies considerably among the four different periods of the annual breeding cycle, i.e. incubation (mid-January–mid-March), nestling (late March–early April), fledging and post-fledging dependence (mid-April–August) and territorial/courtship (the pre-laying period, i.e. the period between the beginning of juvenile dispersal to egg-laying; September–early January in the study area; Table 21 and Figure 102). The data are presented per biological period and not per month for two main reasons. First, months do not have a real biological meaning for animals, whose activities are determined by their biological cycle. Second, call rates per month are often so detailed that many suffer from local variations in call behaviour, which may be extremely large. Because call behaviour depends on several internal (e.g. individual quality and age) and external (e.g. conspecific density, prey availability, moon phase) factors (Hardouin et al. 2007, Penteriani et al. 2014c), Table 21. Characteristics of Eagle Owl vocal activity (n = 15 pairs) throughout the year. Incubation

Nestling

Fledging

Pre-laying period

Period Males

Females

Males

Females

Males

Females

Males

Females

371.7± 266.9 (26.0–761.5)

4.3±3.7 (1.5–8.5)

198.6± 180.4 (8.0–567.5)

175.0 (175.0)

177.7± 93.0 (1.0–243.8)

15.8±14.9 (0.9–39.7)

330.5± 284.8 (24.0– 1099.5)

34.9±35.9 (0.1–103.4)

Number of calls per bout series

36.1±25.6 (4.5–73.0)

1.3±0.3 (1.0–1.5)

21.6±18.7 (1.5–52.5)

11.0 (11.0)

10.23±7.1 (1.0–18.0)

1.4±0.3 (1.0–1.9)

32.8±30.3 (2.1–117.7)

8.06±8.6 (0.1–23.3)

Minimum time interval (min)

20.1±52.9 (1.0–345.0)

0.0±0.0a (0.0–0.0)

35.8±81.5 (1.0–415.0)

14.6±23.0 (1.0–65.0)

28.0±60.7 52.3±135.3 (1.0–401.0) (1.0–669.0)

Duration of vocal displays (s)

266.0± 56.3±91.4 202.1 (1.0–340.0) (56.0–447.0)

Notes: Duration represents the mean±SD, as well as the minimum and maximum (within brackets), call length of the different periods of the annual breeding cycle. Time intervals are calculated as the latency between two successive series of bouts. a There is no minimum time interval because during nestling we only recorded one series lasting 175s and consisting of 11 calls per series of bouts.

289

The Eagle Owl

a representation per period is useful to mitigate those monthly differences that are mostly due to local conditions and individual variations. In another investigation carried out in two different study areas in France, Defontaines (2009) highlighted how monthly variations in the number of calling individuals may determine relevant differences in the patterns of Eagle Owl vocalisations. It is worth mentioning here that occasionally meaningful differences in vocal behaviour are marked even between pairs breeding in close proximity to each other (Grichik & Tishechkin 2002, Penteriani et al. 2004), not all pairs vocalising with the same intensity in the vicinity of the nest site. A 1200

Sunset

Sunrise

Call duration (seconds)

600

400

400

200

1200

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Night hour Sunset

Sunrise

0

D 1200

1

2

3

Sunset

4

5

6 7 8 9 10 11 12 13 Night hour Sunrise

1000

Call duration (seconds)

Call duration (seconds)

1000 800

800

600

600

400

400

200 0

Sunrise

800

600

C

Sunset

1000

800

0

1200

Call duration (seconds)

1000

B

200

1

2

3

4

5 6 7 8 Night hour

9 10 11 12

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Night hour

Figure 103. Total duration of male (black line) and female (grey line) calls during the whole night for the (A) incubation, (B) nestling, (C) fledging/post-fledging and (D) pre-laying periods. Vertical lines indicate sunset and sunrise. The longest and most widely distributed call displays were recorded during the pre-laying period (September–early January in the study area). The duration of the night (i.e. night hours) is different due to seasonal variations (from Delgado & Penteriani 2007).

290

Vocal communication

In our study area, the longest vocal displays occur during the pre-laying period, whereas the shortest vocal activity was recorded during the incubation and fledging periods for females and males, respectively (Figure 103). This pattern has also been reported by Mikkola (1983) and recorded by Ruiz-Martínez et al. (1996), Muscianese (2006) and Defontaines (2009). It is during the pre-laying period (Figure 103) when we can observe: (a) the most widely distributed call peaks during the entire night; and (b) the shortest time interval between two consecutive series of bouts (Table 21). The first and last calls of the males we observed were always uttered relatively close to sunset (ranging from 29 minutes before to 145 minutes after sunset) and sunrise (ranging from 60 minutes before to 55 minutes after sunrise), respectively, as also observed in East Germany by König & Haensel (1968). However, during the fledging and post-fledging dependence period, the last call was always recorded before sunrise (from 2 to 60 minutes before). Females also displayed their peak of vocal activity during the pre-laying period, when duets were also more frequent. Unlike males, vocal events of females were less related to sunset and sunrise (Figure 103). Although the level of vocal activity of females was lower than that of males throughout the year, there was a positive association between vocal display patterns of both sexes (Figure 102). Duetting plays a role in the establishment, or re-establishment, of pair bonds (Marshall 1967, Klatt & Ritchison 1993). Because females displayed their peak of vocal activity during the pre-laying period only, whereas males exhibited high call rates throughout the year, we can hypothesise that territoriality is largely demonstrated by males, whereas female calling appears mainly related to mate communication. The nightly distribution of calls during the incubation and the fledging/post-fledging periods is marked by an important decrease of vocalisations in the central hours of the night (Delgado & Penteriani 2007; Figure 103), as also observed by Choussy (1971) and Frey (1973). On the contrary, during the pre-laying and nestling stages, peaks of vocal displays are distributed in a more homogeneous manner during the whole night (Figure 103). During pre-laying, the relatively homogeneous distribution of vocalisations is probably due to the intense vocal activity that characterises this period, when both territorial and mating displays are extremely frequent at almost every hour. Peaks of vocal activity during the nestling phase are, at least partially, the result of feeding at the nest: when the male arrives with a prey, he starts calling from a perch site close to the nest to contact the female (who frequently approaches the male to take the prey and then feeds the young). Close to the time of egglaying, Eagle Owls may call up to 600 times a night (Desfayes 1951, von Lossow 2010); Ruiz-Martínez et al. (1996) reported that the average number of calls recorded between September and June was around 20 calls per hour. Noga (2014) recorded an average of 334 male calls (range = 72–907) and 211 female calls (range = 1–1221) per night at the beginning of March. Finally, there is a difference in the duration of the so-called alarm calls during the prelaying period (41.62 ± 55.46 sec) compared with the whole year (1.25 ± 4.90 sec). A total of 70.7% of the alarm calls were preceded and/or followed at a ≤ 1-min interval by a single call or a series of main calls, whereas the remaining 29.3% of alarm calls represented isolated vocalisations. The high frequency of alarm calls during the pre-laying period and the fact that most of these calls were always preceded and/or followed by pair contact suggest that the meaning of this call is not only an alarm call related to stress or defence but also to high excitement during mate contacts. Indeed, during reproductive seasons, aggressive calls may also express sexual conflicts (Davies 1989). 291

The Eagle Owl

Detailed information on the frequency of Eagle Owl responses to vocal stimulation (playback) is relatively rare and may depend on the local condition of conspecific density, period of the year and intrinsic characteristics of focal individuals (bold vs. shy individuals). Some studies have suggested that playback may not be an effective method everywhere, all the time and with every individual (Mysterud & Dunker 1982, Penteriani & Pinchera 1990a, 1991, Zuberogoitia & Campos 1998, Martínez & Zuberogoitia 2002), whereas others used playback with apparent success (Martínez et al. 1992). This apparent inconsistency of Eagle Owls to taped calls may engender biased estimation of population density and/or breeding site occupancy, and some nesting sites can be mistakenly classified as unoccupied by a pair or a single individual when taped calls are broadcast (Penteriani & Pinchera 1990a, 1991, Martínez & Zuberogoitia 2002). In addition, it seems that the probability of detecting a male is greater for paired than for unpaired individuals (Martínez & Zuberogoitia 2002). These results suggest that the male reaction to an intruder may also depend on its mating status: paired males continued to advertise their presence while most unpaired owls became silent or reduced their hooting rate when stimulated by playback (Martínez & Zuberogoitia 2002). The tendency for mates to respond together to an intrusion may indicate that territorial defence is cooperative in Eagle Owls, as also observed in Scops Owls (Galeotti et al. 1997) and Tawny Owls (Appleby et al. 1999). Again, the reaction of males and females to the vocalisations of an intruder may vary as a function of the individuals and the sex of the intruder (Penteriani et al. 2007a), as well as the local condition of the population. For example, we recorded very low response rates of females to both male and female intrusions in southwestern Spain (see also Penteriani et al. 2007a), which is not the case in southwestern Finland, where females are more aggressive, primarily to female intrusions (Jere Toivola pers. comm.), or as observed by Haller (1978), where a territorial female chased and pursued an intruding female for more than 1km. Thus, as stated by Martínez & Zuberogoitia (2002), Eagle Owl surveys using calls should be carried out via both passive listening sessions (aimed to record spontaneous vocal activity) and playback, rather than using either of these methods alone. Actually, the effectiveness of passive listening sessions might also depend on the pair density: in a German study performed at the beginning of the reproductive season, 30% of the occupied nest sites were missed with one visit, and 13% were still missed after two visits, at least four visits being needed to detect all occupied nest sites (Bergerhausen & Willems 1988). On the contrary, in the highest-density populations in southern Spain breeding males call almost every night during this same period. Finally, it is important to highlight here that the majority of studies concerning daily activity patterns have been conducted in locations where daylight hours are followed by dark nocturnal hours on a predictable daily basis, and at high latitudes (i.e. close to or above the Arctic Circle), where the continuous daylight may be a challenge to the circadian rhythms and behaviours of nocturnal species (Eriksen & Wabakken 2018). Preliminary analyses of the vocal behaviour of Eagle Owls at high latitudes (the Arctic Circle of coastal northern Norway, Eriksen & Wabakken 2018) have shown that, despite the presence of light, they continue to vocalise mostly late in the daytime: 23 out of 31 acoustic observations occurred between 20:00 and 01:00 (i.e. before the sun is at its lowest point), and the remaining eight observations of calling individuals took place between 01:00 and 05:00. Thus, Eagle Owls appear to remain prevalently ‘nocturnal’, with most activity occurring when the sun is at its lowest (around midnight), and with minimal activity during a 12-hour period in daytime. Hence, it seems that Eagle Owls do not respond to the relatively constant light conditions by expanding their activity into the daytime hours. 292

Vocal communication

Features and spatial distribution of call posts In long-lived monogamous species, in which pair bonds and breeding places are maintained year-round and can persist over several years, space use and vocal displays may show strong interactions. Such interactions may increase as the result of constant pressures represented by the year-round presence of both neighbours and intruders (e.g. Hall 2000, Penteriani 2002). In fact, conspecific density is an important factor affecting territorial behaviour and spatial patterns of movements and habitat use (e.g. Parker 1974, Danielson 1992, Ruxton et al. 1999, Penteriani 2003). Delgado & Penteriani (2007) demonstrated that the high density of the study population (~40 pairs/100km2) determined close distances of call posts during vocal displays. Indeed, pair call posts were located at a mean distance of (i) 232.1 ± 244.4m (range = 1.5–1,133.6m) apart, (ii) 332.4 ± 202.7m (range = 26.2–1,140.5m) to the nest and (iii) 1,156.2 ± 358.5m (range = 125.0–2,515.3m) from the call posts of the nearest-neighbour male. The area delimited by the different call posts (29.5 ± 24.7ha) was significantly different from the total extension of the home range (174.8 ± 117.2ha, calculated using the Minimum Convex Polygon method), representing only 16.9% of the extension of the latter. Call posts tended to be concentrated within the most used portion of male home ranges (Figure 104). Thus, not all areas of the home range are of equal importance to its inhabitants, and certain places are used more frequently than others: core areas, which contain essential resources

0

0.5

1 km

500m

Figure 104. Within an Eagle Owl home range, call posts mainly delimit core areas (shown by the straightedged shapes). Contours represent 50, 75 and 95% (from finer to bolder lines, respectively) of the core area utilisation distribution, and dots indicate call-post locations. The adaptive kernel contouring method was used for describing internal range structure (Worton 1989) (from Delgado & Penteriani 2007).

293

The Eagle Owl

for its owner, were clearly surrounded by call posts. Such core areas (Samuel et al. 1985) receive the bulk of territorial activity because they surround the nest or are within hunting areas. These areas of intense use usually contain refuges and the greatest availability of food (Burt 1943, Ewer 1968). Although home-range overlaps were common in this study area (Hayward et al. 1987, Nicholls & Fuller 1987, Belthoff et al. 1993, Melis et al. 2005), core areas of neighbouring individuals largely showed exclusive owner occupation (Ewer 1968, Samuel et al. 1985). Moreover, the position of call posts was found to be inversely correlated with the distance to the call posts of the nearest-neighbour male; that is, the more distant the call post of a male from its core area, the closer this call post to the call post of the nearest male neighbour (Figure 105). Finally, call posts tended to be higher (31.4 ± 16.5m) than the surrounding area, as reported by Ruiz-Martínez et al. (1996) and Frey (1973). This explains why few or no calls are generally emitted from diurnal roosts, which are located in less dominant and more concealed places than call posts (see also Ruiz-Martínez et al. 1996). The spatial arrangement of call posts within the home range is thus mainly related to the location of core areas and neighbours (see also Frey 1973), as well as both visual and vocal signalling needs. In fact, the location of call posts mainly reflects a trade-off between territorial defence and within-pair communication inside core areas and efficient communication with neighbours (both vocal and visual). A similar trade-off was shown by Cerasoli & Penteriani (1996) for Buzzards, which select their aerial meeting points both close to nests and in specific environmental conditions favouring conspecific communication. Eagle Owls generally select the highest points of their home range as call posts, probably to enhance signal transmission (Marten & Marler 1977, Beck & George 2000) towards neighbouring territorial males and, possibly, dispersing individuals floating in the vicinity of the nest site (i.e. to avoid dangerous physical contacts during intrusions). The selection of such posts serves to increase their visibility during communication and hence to increase the effects of territory defence (Eason 1992): the greater the visibility, the greater the capacity to evict intruders. Because social interactions represent a crucial aspect of animal communication, individuals need to enhance their potential to transfer messages to conspecifics by selecting the best location for this purpose. In this context, not only are the highest points crucial, but they are also the posts that enhance communication between neighbours. This may explain why some Eagle Owl call posts are closer to the call posts of neighbours than core areas, as also observed by Frey (1973). However, and contrary to what Ruiz-Martínez et al. (1996) reported, call posts may also be very close to the nest; although Blondel & Badan (1976) have never heard series of vocalisations emanating from the roost site or from the nest, some calls can be uttered from these places, even during the daytime (see also König & Haensel 1968, Frey 1973). However, Eriksen & Wabakken (2018), working in northern Norway, suggest that call posts may be situated in more concealed locations if Eagle Owls suffer from harassment by other predators (e.g. White-tailed Eagle Haliaeetus albicilla) during calling. These authors mentioned that when Eagle Owls were detected acoustically, they were rarely seen. This finding lends support to the fact that Eagle Owls might avoid exposing themselves to Whitetailed Eagles by using less exposed call posts, in contrast to previous studies reporting that Eagle Owls often select highly visual call posts in order to enhance signal transmission (Delgado & Penteriani 2007, Campioni et al. 2010). However, those study sites had dark hours when Eagle Owls could call without threat from potential avian predators like eagles. The use of inconspicuous call posts may suggest a trade-off between the need to defend the 294

Vocal communication

Figure 105. In a social system in which an owner needs to communicate with conspecifics, signallers were located according to best visual and vocal signalling. Black dots represent the locations of the owner of the territory; black contours show the core areas (Kernel 50%) of that owner; grey contour shows the core area (Kernel 50%) of the neighbouring owl. The 3D representation illustrates how the signaller (S) generally selected a dominant and highly visible position to communicate with its nearest neighbour (R, the receiver) (from Delgado & Penteriani 2007).

S

R

1000

0

1000 m

territory and the need to avoid revealing their position to White-tailed Eagles that can pose a threat to the caller itself or to nearby chicks. The use of call posts during the night is very dynamic, with the calling individual generally using several of them during its displays. Both males and females can repeatedly change their location when calling, and they can also call when flying from one post to another (see also Frey 1973, Blondel & Badan 1976, Mikkola 1983, Ruiz-Martínez et al. 1996). In general, the location of successive call posts used by males at sunset is an ordered sequence en route to hunting areas, so that after the last call the owl is generally closer to the areas in which it hunts than when in the diurnal roost. Actually, males typically begin vocal activity in the vicinity of the nest and, with the progression of crepuscule, they displace themselves farther and farther away, until they stop calling and fly towards the hunting areas. In some cases, when the diurnal roost of the male is relatively far from the nest (from one to several km), the individual first approaches the nest site, moving from one call post to another (or just flying directly from the diurnal post to a call post close to the nest), and then starts to approach the hunting grounds, repositioning himself from one post to another. When calling, the Eagle Owl may assume different postures, ranging from an almost completely horizontal position (with the head, body and tail on the same axis, rocking backwards and forwards on its extended feet to the rhythm of its bitonal song; Blondel & Badan 1976) to a completely horizontal posture, where body and feet form a 90° angle. Finally, the location of call posts also appears to change during the breeding season, being more frequently located in the vicinity of the nest when close to egg-laying (Muscianese 2006). This pattern is also supported by the non-exclusive but different meanings that 295

The Eagle Owl

vocalisations may take during the long pre-laying period (see The function of vocal behaviour), which generally lasts from the beginning of young dispersal to egg-laying (up to 5–6 months). Nevertheless, Eagle Owls generally show an extreme degree of fidelity to certain call posts, which can be used night after night during the same year, as well as for several years. Pedrini (1989) refers to the case of an individual using the same post for 89% of the calls uttered from December to March (this same call post was used 74% of the time in other months). This marked fidelity to call posts and, in general, to the posts Eagle Owls select within their home range, is frequently attested to by the presence of nitrophilous lichens (e.g. rose, yellow and orange Caloplaca and Xanthoria lichens; Gilbert Cochet pers. comm., Penteriani 1996), which appear in these locations as a result of the nitrates (and, more generally, nutrient enrichment) derived from the faeces left by the calling/roosting individuals. Because of the long-term permanence of these lichens on the rock substrates, they may also serve to indicate past posts belonging to old breeding sites.

The function of vocal behaviour: territorial defence and intra-pair communication It is widely accepted that many vocal displays have evolved through intersexual (e.g. mate attraction and stimulation, mate-guarding, extra-pair copulations) and intrasexual selection (e.g. conspecific repulsion, delineation of territorial boundaries; Krebs et al. 1978, McDonald 1989, Catchpole & Slater 1995, Kroodsma & Miller 1996). In addition, because animals also communicate in the interest of managing a social environment, we can hypothesise that they act differently if the message needs to be received by a mate, other conspecifics or both. In fact, the frequency of female responses to male calls is higher closer to egg-laying (from January to mid-February in Penteriani 2002) than in the October–December period (Figure 106). Similarly, more copulations following mate duets are observed at dusk from January to mid-February (when egg-laying approaches) than in the period October–December (Figure 8 7

Frequency

6 5 4 3 2 1 0 Oct Nov Dec Jan Feb

Month Figure 106. Monthly frequencies of female responses to male calls (grey bars) and copulations following call duets (black bars) (from Penteriani 2002).

296

Vocal communication

106), which agrees with the observations of Defontaines (2009), who recorded copulation during the 1.5 months before egg-laying. The choice of call posts also differs between months, as males mainly choose their posts on the most dominant part surrounding their nest in the period October–December. It is possible to identify two different patterns of call displays associated with these two periods (Penteriani 2002): during the October–December period males generally start dusk vocalisations on an elevated post located near the nest and then move successively to several other habitual call posts in the area surrounding the nest (from several hundred metres up to 2km away). Mate duets are rare during this period. During the period closer to egg-laying, although male–male calling or males calling in groups persisted (i.e. territorial function of vocal displays), female calls and mate duets increased, and males spent more time in closerange courtship dialogues (lower volume vocalisations near the female and on posts which were not necessarily high up) and finally mating (usually preceded by duets). Close to egglaying it is common to observe close range male–female courtship dialogues followed by copulations. The changing vocal behaviour between these two periods is also reflected by the significant increase in the duration of male vocalisations from October–December (mean ± SD = 1,145 ± 876 sec, range = 30–2,100 sec) to the period closer to egg-laying (2,469 ± 1,675 sec, range = 280–4,472 sec). The above-mentioned patterns of male and female vocal behaviour at dusk highlight the presence of two different call periods, relatively well separated in time and function: in line with the territory-establishment hypothesis (Møller 1988), the territorial function seems to be predominant at the beginning, whereas late in the season, when the egg-laying period approaches, calling related to mating prevails. In this latter period the behaviour of both males and females changes: both mate duets and the number of subsequent copulations increase, and the choice of song posts and male vocal behaviour are different (Penteriani 2002). These results do not indicate that the two functions and contexts are really distinct and well separated but that, depending on the stage of the breeding season, one of the two main call functions may be prevalent or added to the other (e.g. when approaching the female fertile period). Because all the individuals included in the study were already mated, calls were not used in mate choice per se, and probably were not used in mate evaluation because of the low rate of extra-pair copulations and the absence of divorce in the breeding season (Dalbeck et al. 1998). In addition to reinforcing territorial defence, the higher call rates observed during the sexual period could have the function of (i) mate guarding (Møller 1991), (ii) the stimulation of ovarian development and copulation behaviour by the mate (femalereproduction hypothesis; Morton et al. 1985, Møller 1991), and (iii) the maintenance and consolidation of pair-bonds (Klatt & Ritchison 1993). In addition, the vocal displays at dusk may correspond with a peak of female fertility (Mace 1988, Møller 1991). The male-announcement hypothesis (Møller 1991) predicts that in birds unable to guard their mates adequately to prevent extra-pair copulations (because females rely on their mate for food throughout the fertile period) the announcement of female fertility could be disadvantageous. The observed calling patterns of the Eagle Owl, as well as the patterns displayed by other bird species (e.g. Hanski & Laurila 1993, Rodrigues 1996, Gil et al. 1999), do not appear to agree with this hypothesis concerning the announcement of female fertility. The contemporaneous call activity of breeding males could itself prevent extrapair copulations between neighbours, whereas the relatively strong pair-bonds characterising Eagle Owls could reduce extra-pair copulations: in several species the female tends to avoid 297

The Eagle Owl

copulations with other males (e.g. Whittingham et al. 1992, Korpimäki et al. 1996). Another possibility is that males announcing their high quality by longer-lasting long-distance calling suffer fewer territorial intrusions, as they are recognised by neighbours as good defenders of their mates.

The effect of density of conspecifics and quality of breeding site Conspecific density is an important factor affecting several individual and population features including territorial behaviour, the evolution of species characters, species distribution, extrapair copulation and cuckoldry rate, fecundity, as well as vocal behaviour. An individual’s vocal behaviour may thus depend on whether it is surrounded by conspecifics or it is relatively isolated. An example of the effect of conspecific density on Eagle Owl vocal displays is provided by a study carried out in Provence (southern France; Penteriani 2001, 2003, Penteriani et al. 2002b), where it was possible to discriminate two subsamples of different density within the same population: eight males breeding in a high-density situation (32 pairs/100km2), where the distance between two nearest neighbours was less than 1km, and nine males breeding in a lower-density situation (20 pairs/100km2), where the distance between two nearest neighbours exceeded 3km. Here, from early October to mid-February (pre-laying period), the call activity of the 17 males was recorded from 1h before sunset to 2h after sunset. The 17 males exhibited high variability in terms of mean call duration, mean number of calls per series (a series was judged to have ended if more than 1 minute of silence elapsed before the next call) and mean number of series per listening session (Table 22): the shorter the nearestneighbour distance, the higher the call duration of males. High call rates in situations of high density of breeders are also reported by von Lossow (2010). Density also affected the time of the first call at sunset: males in the high-density situation started their first dusk call earlier (mean ± SD = 11.2 ± 19.8 minutes after sunset, range = 14–68 minutes) than males in the low-density one (19.6 ± 19.1 minutes after sunset, range = 10–73 minutes). This latter result is even more interesting when compared with the data on the dusk chorus of Eagle Owls obtained from an even higher density population (40 pairs/100km2) in southern Spain (Delgado & Penteriani 2007, Penteriani et al. 2014c; see also Patterns of daily and seasonal vocal behaviour, page 285): contrary to the relatively high-density population in France, where vocal displays generally start a few minutes after sunset, in the very high-density Spanish population the first call was generally before sunset. Similarly, in a low-density population (6.5 pairs/100km2) in northwestern Italy (Casanova & Galli 1998), most of the dusk calls were concentrated after sunset. Thus, we can hypothesise that conspecific density has the potential to shape characteristics of the dusk chorus: the higher the density, the earlier the beginning of the vocal displays. In situations of high density, when the social environment calls for longer vocal displays, the earlier starting time of vocalisations might be a strategy to save the night time for other vital activities (e.g. hunting). In some cases, the difference in the beginning of sunset vocalisations between the Spanish and the French populations is more than one hour, which may also have important implications during Eagle Owl population censuses. Excitation or stress due to conflict has been reported to increase call frequency in a number of passerine species (Brémond 1968, Van der Elzen 1977, Ueda 1993), not necessarily to 298

Vocal communication

Table 22. Characteristics of vocalisations during the pre-laying period for males in situations with highand low-density of conspecifics. Number of calls in each series

Call duration (s)

Number of series

Min–max

x±SD

Min–max

x±SD

Min–max

x±SD

Overall simple (n = 17)

30.0–4472.0

884.7±1105.4

5.0–387.0

82.7±97.2

1.0–8.0

3.1±2.0

High-density

(n = 8)

174.7–4472.0

1182.6±1018.2

59.1–387.0

96.2±96.2

3.5–8.0

3.7±3.7

Low-density

(n = 9)

30.0–497.2

257.4±180.7

5.0–48.2

33.2±30.5

1.0–2.6

2.1±2.0

communicate something, but perhaps as a result of a broader motivational context (Owings & Morton 1998). Among owls, an increase in call activity has been associated with high conspecific density for the Tawny Owl (Galeotti 1994) and the Long-eared Owl Asio otus (Tome 1997). A high investment in vocal signalling in the absence of competition would be a waste of time and resources: accommodation allows a male to minimise the costs of aggressive calling by adjusting his territoriality threshold to local conditions. Assessment forms the foundation upon which communication systems are built, and individual behaviours partly depend on evaluative reactions due to experience and source of feedback sustaining or disrupting activities that produce it (Owings & Morton 1998). Such flexibility allows males to balance the costs and benefits of territorial behaviour and maximise their fitness. Communication itself can be considered a social behaviour when a species: (i) is not homogeneously distributed across the landscape (e.g. some patches have a high concentration of individuals, while other patches have a few, relatively isolated, individuals), (ii) is strongly territorial, and (iii) has well-established communication networks. As a

Call duration (sec)

2000

1500

1000

500

0 1000 2000 3000

NND (m) Figure 107. Relationship between a proxy of pair density, the nearest-neighbour distance (NND) and Eagle Owl call duration: the lower the NND, the longer the call duration (from Penteriani et al. 2002b).

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consequence, failing to incorporate conspecific density in call surveys of territorial species may reduce the accuracy of population estimates (Penteriani et al. 2002b). Monitoring and estimating Eagle Owl (and, more generally, bird) populations using their call behaviour are common: birds often reveal their presence vocally, and many species of birds are best detected from their vocal displays. Penteriani et al. (2002) showed that the detection rate of Eagle Owls nesting in low-density situations was highly variable and lower than in highdensity contexts (Figure 107). Under these circumstances, it is possible to underestimate the number of breeding pairs. The influence of density on call duration and timing in Eagle Owls may be so dramatic as to reduce the detection of a considerable portion of the breeding population. Although in this study we did not test for the influence of conspecific density on the response rate of Eagle Owls when stimulated by playbacks, we consider it important to point out that a higher rate of elicited responses has been obtained in situations of high density than when the species is dispersed over a large area (mean nearest-neighbour distance of about 10km; Penteriani & Pinchera 1990a,b, 1991). Finally, the amount of time that Eagle Owls allocate to vocal behaviour can also be influenced by the extent of good hunting grounds in the vicinity of their nests (Martínez & Zuberogoitia 2003b). In an area of eastern Spain were the Rabbit is the main prey of the Eagle Owl (Martínez & Zuberogoitia 2001), call activity was related to the percentage of scrubland surrounding nest sites, probably because the amount of this type of habitat is a good estimator of the presence of Rabbits (Moreno & Villafuerte 1995). The authors of the study suggest that: (i) Eagle Owls breeding in prey-rich areas may engage in prolonged vocal displays in order to advertise their resource holding potential to females, i.e. their ability to obtain and defend resources; and (ii) the high availability of energetically profitable prey may outweigh the possible costs incurred by prolonged vocal activity.

The effect of conspecific density on the honesty of vocal displays Vocal displays are generally considered to be costly, as a result of the time spent in this activity instead of others relevant to individual fitness and the energy demands for song production (e.g. Brackenbury 1980, Alatalo et al. 1990, Eberhardt 1994, Catchpole & Slater 1995, Gaunt et al. 1996, Kroodsma & Miller 1996). Therefore, it has been proposed that the duration of vocalisations represents an honest signal of the phenotypic and genetic quality of individuals (and of the quality of their nesting site), because high-quality males, or males in high-quality sites, can bear singing costs better than low-quality individuals, or individuals in low-quality sites (e.g. Hutchinson et al. 1993, Eberhardt 1994, Catchpole & Slater 1995, Hoi-Leitner et al. 1995, Johnstone 1995, Kroodsma & Miller 1996). By affecting the vocal displays of mated males (see previous section), conspecific density can disrupt the honesty of call behaviour as a possible signal of male and/or nesting-site quality. If we accept the general assumption that call displays are costly, males without neighbours should invest relatively less in such displays than those in high-density situations, making it difficult to evaluate correctly, through the characteristics of their vocal displays, their individual quality or that of the site they occupy. Males in low-density situations would derive less advantage from paying the socially imposed costs of signalling. In fact, for the French population characterised by high- and low-density sectors (Penteriani et al. 1999, 2003, Penteriani 2001, 2003), call duration was explained by a 300

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Figure 108. Mean call duration of males in relation to the distance to the nearest neighbour and number of fledged young (productivity). The increase in the duration of vocal displays was positively correlated with productivity in the males nesting in high-density situations only, where male–male competition is considered to be higher (from Penteriani 2003).

proxy of the density (the nearest-neighbour distances among breeding pairs), as well as its interactions with the productivity of the population (expressed in this case by the number of fledged young, which can be considered a proxy of individual and/or nest-site quality) and the percentage biomass of mammals in the diet. That is, call duration increased with density of conspecifics, and the increase in the duration of vocal displays was positively correlated with productivity for males in the high-density portion of the population (Figure 108). Thus, the density effect masked the honesty of call duration as a possible signal of male and/or breeding-site quality in the whole population: call duration was related to productivity only for the males in the high-density area. Additional support for the hypothesis that density can affect vocal displays, masking their honesty as a signal of individual and/or breedingsite quality, is provided by the fact that the number of young fledged per breeding pair, percentage of open land, distance to open land, Shannon index for landscape diversity and diet richness did not differ significantly between males in high- and low-density situations. That is, the detected differences in mean call duration for the overall sample are unlikely to be attributed to differences in individual or breeding-site quality. The call display as an honest indicator of male fitness persists only in situations of strong male–male competition (i.e. high breeding Eagle Owl density). For the males in this situation, one parameter of individual fitness was related to call duration: males calling for longer had higher productivity. This call pattern is similar to those of songbirds, for which the cost of vocal displays is an increasing function of the time spent calling and a decreasing function of male quality (e.g. Møller 1991, Catchpole & Slater 1995, Kroodsma & Miller 1996). In the low-density situation, no correlation was detected between call duration and male or breeding-place quality: high-productivity owls were relatively silent. Males that sing longer are not only more likely to be in better condition, but also more strongly motivated than others (Galeotti 1998). 301

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Can population characteristics reduce the level of call identity? The structure of songs and calls, or part of it, generally allows for individual acoustic recognition based on individual differences that are significant variable traits within a population and show an appreciable consistency in the same individual (Lambrechts & Dhondt 1995). On the one hand, the call of any particular individual must have certain acoustic features that are unique to that individual; on the other hand, the acoustic features of the call should enable the identification of the individual as a conspecific. That is, while all the individuals of a population give calls that are typical of the species, selection would tend to favour some degree of inter-individual variability and intra-individual consistency if calls are important for individual recognition (Lengagne 2001). To date, recognisable individual variations have been reported for few species of owls, the first works published on Eagle Owls appearing at the beginning of this century (Lengagne 2001, 2005). These first two studies focused on two main issues: firstly, to assess vocal individuality within the oohu male call structure (this part of the study was conducted on a wild population in south-central France), and then to explore the stability of vocal individuality over successive years (a study carried out in captivity). To carry out quantitative analyses, a spectrographic representation was performed for each call uttered by nine wild males and five captive birds. The recorded hooting of Eagle Owls was composed of a single note which lasted 373.54 ± 14ms (mean ± SE) repeated monotonously. All the parameters used to describe the call showed great individual constancy, especially for those variables concerning the temporal patterning of syllables (i.e. duration of the call and duration of the ascending part). Call parameters also revealed an important inter-individual variation. That is, all parameters appeared useful for individual identification. On the other hand, the parameters used to describe the territorial call of Eagle Owls emphasised no differences in call structure from one year to another (i.e. calls are particularly consistent over years; Lengagne 2001). This individual signature in vocalisations may be extremely important in the social structure of a population, where territorial individuals (i) may be able to discriminate their neighbours from unfamiliar individuals and (ii) can recognise them individually and spatially. Indeed, such discrimination tends to reduce boundary conflicts because when a neighbour is identified as a territorial owner it immediately ceases to represent a potential intruder, as has been demonstrated for many passerine birds (Catchpole & Slater 1995, Kroodsma & Miller 1996) and Tawny Owls (Galeotti & Pavan 1993). Lengagne (2001) suggests that a fixed individual signature in calls over years could be advantageous for breeders: (1) long-term recognition of individual neighbours might provide immediate benefits in reproduction, as the establishment and defence of territorial boundaries take time that males could otherwise spend in other activities; and (2) in long-lived (generally) monogamous birds, individual recognition between mates is especially relevant (although this may not only rely on the voice), as partners usually pair for long periods of time. On the basis of Lengagne’s results, spectrographic measures of the main Eagle Owl call might be a useful, non-invasive technique to monitor populations, allowing the assessment of the demographic evolution of and yearly turnover within the breeding population, site and mate fidelity (e.g. breeding dispersal) and survival rates of breeders. Similar results were obtained a few years later by Geidel (2007) in Germany, who analysed 24 owls, including 18 in captivity and six wild birds. Spectrographic analyses supported Lengagne’s findings, with inter-individual variation call properties being on average greater 302

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than the existing variation within the calls of a single individual. In addition, the study showed that the number of correctly classified calls decreases significantly with increasing sample size. The importance of individual recognition by voice for Eagle Owl population monitoring has also been stressed in a study carried out, over two consecutive years, on nine nesting sites in central France (Grava et al. 2008). For the scope of this chapter, it is interesting to point out here that this study, involving both males and females, was conducted in a relatively low-density population, characterised by the presence of 2.5 pairs/100km2 (Balluet & Faure 2004) and a minimum distance of 5km between breeding sites. The results of Grava et al. (2008) were consistent with those obtained by Lengagne (2001, 2005), this time with the inclusion of females in the analyses: the parameters used to describe the calls of both sexes showed a greater inter-individual than intraindividual variability, again supporting the potentiality of call analyses for individual identification. This more recent study added new information to the previous investigation: although the vocalisations of individuals can be identified, the accuracy of discrimination is higher for males than for females (Figure 108), as also observed by Geidel (2007). Intriguingly, different results were obtained when a similar study was performed on a different population in southwestern Spain (Delgado et al. 2013), characterised by a very high density (ca. 40 pairs/100km2) and extremely reduced distances among breeding sites, i.e. from 250m up to 1km. Why are we stressing these differences between the French and Spanish populations? We have to bear in mind that individual variability can be generated by differences in environmental conditions or genetic background, and there are several examples of individual variability in life-history traits such as age of maturation, clutch size, reproductive success and dispersal strategies. Vocal behaviour is a prime example of the type of behaviour that is largely dependent on both the features of an individual (e.g. social status, physical condition) and the characteristics of the population (e.g. density, level of fragmentation) to which they belong (Otte 1974). In addition, there is a consensus that the cost of producing calls and songs that transmit fitness-related information (individuals are able to ‘announce’ their own quality or the quality of the breeding place they occupy, e.g. Hardouin et al. 2007) is only offset if the environment is heterogeneous and, consequently, when it is really important to discriminate, during vocal signalling, either the quality of the breeding site or the quality of the owner (Catchpole & Slater 1995, Bradbury & Vehrencamp 2011). In fact, bird songs may convey information about individual features that play a relevant role in advertising territory ownership and mate attraction (Galeotti & Pavan 1993, Catchpole & Slater 1995, Appleby & Redpath 1997, Galeotti 1998). Owl calls have been shown to confirm this assertion and, additionally, individual recognition and variation among populations may exist (Galeotti & Pavan 1991, Galeotti et al. 1993). However, studies analysing individual call identity have not taken into account the crucial role that the environmental context may play in the evolution and maintenance of traits that reveal individual identity (Tibbetts & Dale 2007). Thus, Delgado et al. (2013) explored the possibility that individual vocal identity in Eagle Owls may be reduced by a given ecological scenario, i.e. a very high density of the breeding population and the fact that individuals live in an environment characterised by high abundance and availability of resources, leading to a relative lack of heterogeneity among breeding sites in terms of their quality and productivity. Given that the three previous studies showed that Eagle Owl vocalisations are individually distinctive, Delgado et al. (2013) were expecting to be able to recognise, over the years, each 303

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Seconds Figure 109. Four spectrograms of the call uttered by different males in southwestern Spain. The high similarity among Eagle Owl calls is apparent even by visual inspection. Owing to the considerable overlap observed, individuals could not be discriminated on the basis of the information concerning their vocalisations (from Delgado et al. 2013).

territory owner within their population by the characteristics of its call sonograms. For this reason, the main call of 15 males and 10 females at 15 breeding sites was recorded. Some individuals were also recorded over several different years, namely six males from the 15 breeding sites that were also captured and radio-tagged. In-depth analyses of the characteristics of this Eagle Owl population demonstrated: (1) that the population was characterised by its stability and high fecundity, i.e. Delgado et al. (2013) were faced with a relatively homogeneous population, with breeding sites of similar quality showing very similar annual variance in productivity; (2) a lack of significant differences in genetic structure; and (3) even though the acoustic variables of the calls showed some inter-individual variation, it was not possible to discriminate among the different individuals, the decrease of individual distinctiveness being due to the similarities between individuals in their vocalisations rather than possible variations within individuals over time (see Figure 109 for an example of the visual comparison between sonograms of different individuals). Just to give a practical example, the variation between individuals observed in this study (ranging from 0.09 to 0.24 of the inter-individual coefficient of variation) was negligible compared with that reported by Lengagne (2001), who found values between 7.1 and 42 (the values from these two studies are directly comparable as they were estimated from similar acoustic parameters). In addition, any acoustic similarity among close neighbours was detected, as would be expected if communication was limited by distance or if birds matched their songs to local neighbours. Thus, the lack of significant differences observed between the calls of Eagle Owls in Delgado et al.’s study population (2013) did not allow individuals to be discriminated on the basis of the information concerning their vocalisations. 304

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Following the idea stressed by Tibbetts & Dale (2007) about the important role that the social and environmental context can play in the evolution and maintenance of traits that reveal individual identity, it can be hypothesised that the decrease of individual distinctiveness in vocalisations in the Spanish area may be attributed to the peculiarities of the study population. Two main factors may have determined the similarity in call structure: (1) the population density, and (2) the relative lack of heterogeneity (in terms of prey availability and breeding success) among breeding sites. First, the density of the population under study is among the highest ever reported for the species (but see also Pérez-García et al. 2012). Densities were lower in the populations for which it has been possible to distinguish individuals by features of their calls. Second, our long-term study of the Spanish population showed that (i) fecundity was relatively high and rather similar for the whole population, and (ii) all pairs bred successfully almost every year. These two features are not typical attributes of Eagle Owl populations, which instead are usually characterised by their heterogeneity in quality and fecundity among breeding sites (Penteriani et al. 2002a, 2004). Indeed, prey availability is extremely high throughout the whole Spanish study area (Campioni et al. 2013), which may explain both the high density and fecundity. In addition, (iii) genetic results showed that there is no genetic structure between individuals in this population: they seem to form one unique panmictic population with no substructure, and with no spatial genetic autocorrelation. Similarity of Eagle Owl call types found across the entire range of nest distances might be consistent with a peculiar type of social synchronisation of vocalisations among all individuals (not only neighbours), matching each other and forming a wide communication network (Smouse & Peakall 1999). Owing to the unusually high density of breeders and, consequently, the extremely short distances between displaying individuals, a cascade effect between individuals can induce the population to behave as a larger network than the typical one in which only the closest individuals within a population form separated clusters. Actually, the density of breeders in most studies of avian networks is much lower, so that each calling individual has just a few neighbours with which to interact and from which to learn (Catchpole & Slater 1995). There are at least two biological benefits of the network identified in this study. A first obvious benefit at the individual level is that possessing similar individual calls can be advantageous during interactions with neighbours. Several researchers have attempted to address the question of why, in numerous territorial species, males interact with neighbours by partially sharing or matching some portion of their song repertoire, a phenomenon that has been termed the ‘dear enemy’ effect (Fisher 1954). Neighbouring territorial animals are often intense rivals; however, many studies have found that territorial birds may respond less aggressively toward neighbours than to strangers by overlapping songs to counter-sing with established neighbours. Reduced aggression toward familiar neighbours, especially in a situation in which all individuals inhabit a habitat that is uniformly good and where there is no apparent reason to compete, may decrease the likelihood of escalated contests whose outcome could involve a threat of takeover and a high risk of injury, particularly in a predatory species that has weapons able to inflict damage during conspecific contests. A second benefit of the observed communication network is that the reduced aggression towards neighbours may lead to the appearance of a high social stability, i.e. territory owners may decrease boundary disputes by having similar calls. This stability may prevent the attraction of floaters to the area (Beletsky 1992). Indeed, floaters can potentially use the detection of social instability as a strategy to establish territories. Before starting the dispersal period, we observed that owlets 305

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spend several months under parental care (i.e. a post-fledging dependence period; Delgado et al. 2009a), providing ample time to learn much about the population and the local area. Social stability might be one of the causes of the low recruitment rate of dispersing individuals to their natal area which we recorded (Delgado et al. 2010). It is worth noting that the fact that we found a reduced individuality in Eagle Owl calls does not imply that individuals are not able to recognise each other. In fact, the approach that allows us to describe vocalisations and to identify individuals by their calls (e.g. sonograms) may not be consistent with the manner in which individual birds perceive and recognise each other: more subtle mechanisms may be involved in neighbour recognition. The effect of distance among conspecifics in birds can have a strong effect on vocal communications (Laiolo & Tella 2005), demonstrating that gaps within the individual spatial distribution may hinder cultural transmission of call/song types over great distances, resulting in an increased differentiation between those individuals that lack many interactions. Following this line of reasoning, we consider it important to conclude by suggesting that there are two non-mutually exclusive explanations for the structural call patterns we detected: (1) similarity in calls may be principally a consequence of the homogeneous structure of the population; and (2) high density may encourage all individuals to match each other in a cascade effect, leading to a widespread and unique communication network. Even though it has been frequently overlooked in ecological and behavioural studies, a decrease of individual call identity may have relevant ecological and evolutionary consequences at the individual and population levels.

The dusk chorus is related to individual and nest site quality Animal populations are most commonly considered from a demographic point of view (e.g. density, fecundity, survival and population viability), whereas less attention is paid to the internal behavioural phenomena and patterns that may emerge from the interaction between individuals. In fact, the perception of spatial and temporal relationships among conspecifics may have repercussions on both behaviours and population structuring, primarily when individuals live at high densities: high densities of conspecifics may force individuals to find new ways to live together in the space available to them, which may determine the emergence of typical social relationships (i.e. behavioural patterns that are uncommon at low densities but likely common in aggregations of individuals typified by high density). Interactions between multiple senders and receivers have been defined as a communication network (Bradbury & Vehrencamp 2011), which may show emergent properties that would never be observed in isolated pairs of interacting individuals. For example, bird dawn and dusk choruses function as communication networks, where (i) various individuals are involved in interactive communication (Burt & Vehrencamp 2005, Hardouin et al. 2008); (ii) the frequency of interactions depends on relationships among males (Foote et al. 2008); and (iii) the timing of vocal displays varies consistently among individuals (Otter et al. 1997, Foote et al. 2011), and is related to male quality in some species (e.g. Otter et al. 1997, Poesel et al. 2006, Murphy et al. 2008). As we know, Eagle Owls have vocal display peaks at sunset and sunrise, when neighbouring individuals form communication networks, primarily when they reach high densities. In fact, when neighbours are in close proximity, most of the vocalisations are 306

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directed towards neighbouring conspecifics, and they do this nearly every night (Penteriani 2003, Delgado & Penteriani 2007). When observing the dusk chorus of neighbouring males in a dense population in southwestern Spain, we detected a phenomenon never reported before in an avian communication network; that is, a temporal succession in the order in which individuals start their vocal displays during the dusk chorus (Penteriani et al. 2014c). To understand the reasons behind this intriguing pattern in vocal communication, we tried to identify which factors might be playing a role in determining the order in which individuals start their vocal displays during the dusk chorus. In particular, the main objective was to understand whether this emergent vocal pattern is related to: (1) the quality of nest sites, with individuals living in nest sites surrounded by high food abundance calling earlier (and, consequently, for longer) than individuals in low-quality sites. The quality of the nest site was measured as mean egg-laying date, mean number of fledglings, abundance of Rabbits (which is the main prey of this population; Campioni et al. 2013) and individual diet, expressed as the percentage of biomass of Rabbits and rats (the latter represents the second most common species in the diet of the studied Eagle Owl); (2) the spatial characteristics of breeding sites, with individuals (a) surrounded by many conspecifics, and/or (b) breeding on slopes exposed to sunlight for a shorter duration, calling at higher rates than those on the border of the network or in more sunny places; and (3) the characteristics of the individuals involved in this type of communication network, with better-conditioned individuals calling earlier (and more frequently) than low-quality individuals. We estimated the physiological condition of an individual by measuring (a) haematocrit (an indicator of nutritional status, given that nutritional deficiencies result in anaemia due to shortages in essential aminoacids); (b) the body condition index, estimated from body mass and wing-length (Delgado et al. 2010); and (c) brightness (i.e. total reflectance) of the white feather patch on the male throat, which is positively correlated with individual quality (Bettega et al. 2013; see also Characterising visual signalling traits in young and adults, page 315). The call activity of 14 males breeding in the highest density area of the population (mean ± SD of the nearest-neighbour distance between breeding sites = 1,007.3 ± 480.4m, min = 250.0m, max = 1,759.7m) was recorded during 67 sunsets (from 1h before to half an hour after sunset) and for the entire duration of the pre-laying period (from the moment juveniles left their birthplace to start natal dispersal to the beginning of female incubations), which in the study area lasted from late September to the beginning of January. This period represents the main vocal activity peak of Eagle Owls during the year (Penteriani 2002, Delgado & Penteriani 2007). Finally, in order to document the features of the vocal display of neighbouring individuals calling together and, thus, detect a possible temporal, sequential order in the vocalisations of the different owls, the 14 individuals were arranged into four neighbouring groups (two groups of three neighbours and two groups of four neighbours). Such neighbouring groups were formed on the basis of: (i) the proximity of the individuals belonging to each group, i.e. they were always neighbours; and (ii) the location of suitable listening places from where the observer was able to listen to the individuals of each subgroup with the same ease. As a general pattern of vocal behaviour, the starting time of male calling ranged from 56 to 1 minute before sunset (average ± SD = 22.8 ± 9.9 minutes), except for two males that frequently started after sunset. The earliest males to stop vocalisations did so two minutes after sunset (average ± SD = 18.6 ± 3.5 minutes after sunset; no individuals ceased 307

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vocalising before sunset), but some males were still calling half an hour after sunset. There was a noticeable difference in the mean time of the starting call before sunset within the four neighbouring groups (from group 1 to 4: 41, 17, 30 and 31 minutes), whereas the average ending time of call displays after sunset was relatively homogeneous (from group 1 to 4: 18, 22, 20 and 26 minutes). That is, an early start of the call activity did not imply an early end of vocalisations. The individuals that started calling early were also the individuals that called for longer. Furthermore, once the first individual of a neighbouring group started calling, the second one did not always begin vocalising immediately afterwards (mean ± SD = 21.0 ± 4.2 minutes after the first individual started to call; min = 1 minute; max = 69 minutes). The commencement of the dusk choruses was clearly started by the same individual of each neighbouring group in most of the call displays. For groups 1 to 4, the same individual started first in 93.8%, 52.9%, 50.0% and 36.4% of the dusk choruses, respectively, and in three out of the four groups there were one or two individuals that never called first. An order in the sequence of call display was also evident for the individuals that occupied the second (53.9%, 66.7%, 40% and 36.4% of the cases) and third (45.5%, 50.0%, 64.7% and 42.9%) rankings in their respective groups (average vocalisation starting times are represented in Figure 110). B

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Figure 110. (A) The spatial location of the vocal communication network composed of 14 individuals divided into four neighbouring groups (defined by the identification letters of the nest): (1) AJ, V, N and S; (2) AU, B and E; (3) Q, A and T; and (4) F, AB, K and L. (B) The average time at which every male (dot labels are the identification name of the nest sites) started vocal displays at sunset (the dotted horizontal line). In addition to the observed calling order within each group of neighbours, there is also an evident calling order within the owl system as a whole.

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The individuals that usually called first in their group of neighbouring owls vocalised every sunset except for three of them; intriguingly, when these individuals remained silent (three nights), the whole group of neighbours did not show any call activity. The number of sunsets when the ‘followers’ remained silent ranged from 1 to 5 (mean ± SD = 2.8 ± 1.6 sunsets). The nights in which the ‘usual early’ caller was not first, the overall order did not change drastically, as the ‘usually second’ caller became the first (the third and fourth individuals did not change their order). The habitual order typically changed because the usually first owl started to vocalise later. Finally, the males exhibiting both the highest mean fecundity and the highest proportion of rats in their diet were the ones that started call displays earliest. Nest site exposure did not appear to affect the call order, whereas males with the highest haematocrit and badge brightness values started vocalisations earliest. On the basis of such results we can state that vocal interactions among individuals can shift from an individual-level behaviour, the individual calling, to a population-level phenomenon, i.e. the order in which such individual displays occur. The owl communication network thus determined the spontaneous emergence of a population-level pattern, i.e. a hierarchical vocal communication based on the characteristics of individual quality and the quality of nest surroundings. Results support the contention that calls can carry information related to both the individual’s state and features of the place that individuals inhabit. The information contained in calls may be considered an honest signal of individual and breeding place quality (Penteriani et al. 2002c, Hutchinson et al. 1993, Hoi-Leitner et al. 1995): similar to that suggested by Montgomerie (1985) for the dawn chorus, energy reserves of nocturnal species should be lowest at sunset and may impose a handicap in the timing and amount of vocalisations. Experimental evidence supports the idea that birds in food-rich breeding places show higher call rates than individuals in poorer breeding places (Hoi-Leitner et al. 1995) and, more generally, that high display rates are associated with physiological costs (e.g. Oberweger & Goller 2001), which only better conditioned individuals are able to afford (Hardouin et al. 2007). The ability to maintain a high song output may reflect the ability of the male to acquire the resources needed to allow time for earlier/longer vocal displays. This efficient resource acquisition is probably mediated by the occupation of high-quality breeding places (Reid 1987, Hutchinson et al. 1993). We can hypothesise that the detected order in individual call displays might send information regarding the ‘social’ rank of each territorial male to the whole population audience (i.e. individuals that are present during, but do not take part in, vocal interactions, including the eavesdropping non-breeding individuals living in the vicinity of the breeding portion of the population, i.e. floaters). A quality-mediated chorus might support the assertion that vocal displays arbitrate social relationships with territorial neighbours through interactive communication, as proposed by the social dynamics hypothesis (i.e. males sing to adjust their relationships with neighbours; Staicer et al. 1996, Foote et al. 2008), and also hypothesised for the Little Owl Athene noctua (Hardouin et al. 2008). Actually, the dusk chorus may represent an optimal opportunity for information exchange in a network context because all territorial males (or most of them, at least) participate and their attention is not divided among other tasks, compared with later in the night when males may differ in their motivation to interact. The Eagle Owl dusk chorus can thus emerge as a result of multiple external stimuli, such as sunset (Penteriani & Delgado 2009a), a high density of conspecifics and signals from early callers, where focal individuals orchestrate a sort of behavioural synchrony within the communication network. 309

CHAPTER 13

Visual communication In daylight, animals show an extreme diversity of communication strategies, visual signalling being one of the most obvious and widely used. Variability in coloration is a particularly common signal, and bird plumage is one of the best examples. In diurnal birds, this type of information is generally conveyed through a wide range of carotenoid- and melanin-based colours, as well as structural colours (e.g. blue, violet, ultraviolet and white patches). At sunset, however, colours become progressively more indistinguishable, and thus apparently useless for signalling. Scientists have long believed that owls and other crepuscular and nocturnal birds forgo such visual signals and rely solely on sound (but see Scherzinger 1986). The Eagle Owl, however, seemed to indicate otherwise. Indeed, its unpigmented throat feathers are ideal for signalling at twilight, when contrast becomes more important than colour. Moreover, while owls have famously acute night vision, some evidence suggests that their perception of colour is limited at best, so white feathers would seem a fitting way for them to communicate during the night. That insight in Provence inspired our subsequent research into the Eagle Owl’s evening displays. Contrary to common wisdom, our research has shown that Eagle Owls use a surprisingly large repertoire of visual signals to communicate in a variety of contexts. There is every reason to suspect that visual signalling is more widely employed by nocturnal animals than previously thought. Like Eagle Owls, a number of other nocturnal bird species bear patches of white feathers and make twilight displays. In fact, many nocturnal species have been found to be habitually active around sunset and sunrise (Martin 1990), when specific conditions of ambient light could facilitate visual communication by white patches and flash-marks, and several other crepuscular species present achromatic visual signals associated with crepuscular displays, such as Burhinus spp. (Martin 1990), Great Snipe Gallinago media (Höglund et al. 1992), Little Bustard Tetrax tetrax (Jiguet & 310

Visual communication

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Figure 111. The difference between the white patch around the mouth of a young bird (A) and the white badge on the throat of a full-grown individual (B) (from Bettega et al. 2013).

Bretagnolle 2001) and Nightjars Caprimulgus ruficollis (Aragonés et al. 1999). The need to convey information to conspecifics by visual communication in nocturnal species may have promoted convergent evolution towards white visual signalling at crepuscule in distantly related groups of nocturnal species. Whether they, too, communicate by means of visual signals remains to be studied. In any case, the general rule seems to be that avian conversations, whether between early birds or night owls, involve visual, as well as vocal, dialogue. Eagle Owls have two different, age-dependent patches of white feathers. The first patch forms a large white border tracing the edge of nestling and fledgling mouths (Figure 111A). This patch begins to be clearly visible at approximately 30–35 days of age and gradually disappears in full-grown individuals (i.e. before the start of natal dispersal). These white patches appear to play a role in parent–offspring communication and may serve to signal the quality of the young, to assist parents in feeding, or to increase a fledgling’s visibility in the dark. Full-grown individuals develop a second white badge, the previously mentioned throat patch (Figure 111B), visible on the throat only during call displays. It is interesting to point out that this white badge is a coverable patch (i.e. it may be facultatively concealed or exhibited), thus representing a good compromise between communication (when needed) and crypsis (e.g. during daytime, when it can be costly to be visible to other birds; Lourenço et al. 2011b). Here we will begin a tour of the different methods Eagle Owls use to communicate by visual signals. Some of the visual signals they use are very conspicuous, like the white patch they have under the feathers of the breast, which appears every time owls give a call, or the white edge around the bill, which becomes evident when they are nestlings and 311

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almost disappears before dispersal. Some other signals are more ‘cryptic’, such as the use of their white faeces or the brightest feathers or fur of their prey. But let us start from the beginning of the story.

The white badge of adults Achromatic plumage patches, i.e. those showing pigment-free white feathers and/or variability in the amount of melanin, have attracted the interest of evolutionary biologists due to their role in signalling individual status, or predicting mating preferences. Achromatic plumage has been shown to present individual variability in size or design, as well as statusrelated variability and sexual dichromatism in the amount of total intensity of the light spectrum (320–700nm, also known as total reflectance or brightness). Sexual dichromatism is expected to evolve under the pressure of sexual selection (Owens & Hartley 1998): the evolution of secondary sexual traits, such as plumage, may also be due to mutual inter- and intrasexual selection taking place in both sexes, i.e. mate choice and male–male or female–female competition, respectively. This would be particularly expected in sexually monomorphic species with monogamous breeding systems, where males and females have similar roles in reproduction (as in Eagle Owls). The white patch of the Eagle Owl throat is repeatedly exposed (inflated and deflated) at each call and its visibility is increased by the typical posture that Eagle Owls assume when calling. Both males and females display this apparently identical trait, which can be divided in two parts: a small one, just below the bill, and a larger one on the throat (Figure 112). Eagle Owls show crepuscular (i.e. sunset and sunrise) peaks of activity, thus any visual signal used around twilight should maximise the use of the scarce light available. Therefore, the Eagle Owl’s white badge has the potential to be a good candidate to present sexual

A

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Figure 112. (A) The white patches of Eagle Owls include a small one, just below the bill, and a larger one on the throat. The latter becomes evident when Eagle Owls call (B).

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Wavelength (nm) Figure 113. The reflectance spectra of Eagle Owl white badge feathers (from Penteriani et al. 2006).

dichromatism and/or other overlooked patterns in the total amount of reflected light. The first step to understanding this peculiar way of communication was to analyse the size and variability in brightness of the Eagle Owl’s white badge (Penteriani et al. 2006). The potential patterns of variation related to sexes and seasons were explored for a better understanding of the functionality of this trait. To do that, feather samples (39 specimens, 20 males and 19 females) of the white throat of specimens belonging to the two largest bird collections in Spain (Estación Biológica de Doñana in Seville and National Museum of Natural Science in Madrid, both from Consejo Superior de Investigaciones Científicas – CSIC) were taken. The coloration of throat feathers was quantified using a spectrometer and measurements were made in the range 320–700nm (Figure 113). Total reflectance or brightness was the sum of reflectance data in the interval 320–700nm: such a variable can be considered an index of whiteness, with the highest values showing the purest white colour. The UV range of reflectance (320–400nm) was included in the brightness measurement because several avian species are able to see UV light (Cuthill et al. 2000). In fact, a large proportion of the Eagle Owl specimens showed badge feathers with peaks under the 400nm UV threshold (Figure 113). Because of the typical patterns of seasonal variability of sexually dimorphic traits subject to sexual selection, higher values of total reflectance during the territorial– mating period should be expected; in contrast, in the case of a possible sexual dichromatism, a priori predictions on the sign of the difference would be more difficult because of the lack of knowledge about how selective pressures (mate choice and intrasexual competition) may act in this species. Feathers from the throat are similar to down, with two distinguishable parts: a very delicate and light periphery and a more consistent central area near the rachis. The analyses mainly showed that: (1) the size of the white badge did not vary with sex or season; (2) when considering the tip of the feathers, total reflectance varied with sex and season, i.e. brightness was season-dependent, with higher values in the territorial–mating period (Figure 114A); (3) when considering the centre of the feathers, brightness was sex-dependent, with females possessing higher brightness values (Figure 114B). Finally, since females showed higher levels of reflectance, and assuming the possibility that the white patch might signal individual quality (i.e. larger females can accumulate greater fat reserves prior to incubation, 313

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Figure 114. Differences between seasons (A feather tip) and sexes (B feather centre) in total reflectance of the white patch (means ± 95% confidence intervals) (from Penteriani et al. 2006).

with subsequent advantages during the incubation and brooding periods; Massemin et al. 2000), we examined the relationship between individual variability in brightness and body size. We thus correlated the size of the forearm with the total reflectance measured in the feather centre: although sexual size dimorphism is subtle, the forearm is a significant predictor of sex in this species (Delgado & Penteriani 2004). Although males did not exhibit a significant relationship, females showed a significant positive correlation. Hence, the white badge of Eagle Owls has the same extension on the throat of males and females, but its reflectance properties are sex and season dependent, with higher values in females and during the pre-laying period. The difference in the results obtained from the peripheral and central areas of the feather may be due to a stronger effect of abrasion on the former. Thus, lower reflectance values during the breeding months may be due to: (1) higher frequencies in this period of those behaviours that can potentially wear down the feather badge (e.g. incubation, chick protection and feeding), or (2) a longer period from the last moult. Whatever the proximate explanation of this result, it is important to highlight that the period in which badge reflectance was higher corresponds to the time when territorial and courtship displays are at their peak. Such temporal coincidence supports the hypothesis of a possible function of this trait in territorial and sexual behaviour. Eagle Owls showed female-biased dichromatism in the brightness of their badges. The Eagle Owl is a resident, territorial and apparently plumage monomorphic species characterised by relatively strong mate bonds and female contribution to territorial defence. Such characteristics suggest that higher reflectance in females may be due to intersexual selection (male mate choice). In fact, in some monogamous species with biparental care, males seem to discriminate their mate in the same way that females do, as the fitness of the former is also affected by female quality (e.g. Jones & Hunter 1993, Roulin 1999b, Roulin et al. 2000). As an additional or alternative explanation to intersexual competition, sexual dichromatism in Eagle Owls could also have emerged from female–female competition for both a mate and a territory. In general, female–female competition has received less attention than competition among males. However, the former can attain high levels of competition when females require exclusive ownership of a territory and a mate. Moreover, female aggression represents the main explanation for the evolution of female plumage 314

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coloration in some avian groups (Irwin 1994). Empirical evidence supports the validity of these considerations in the case of Eagle Owls: even if in a weaker manner than that of males, females also defend their territory against both intruding males and females, performing aggressive displays (Penteriani 1996, 2002, Penteriani & Delgado unpubl. data). In Eagle Owls, due to the subtle size sexual dimorphism, colour variability in the white badge may be used as a signal of female quality related to body size. In fact, significant correlations were detected between female forearm size and brightness. The result is even more relevant when considering that the typical low density of Eagle Owl populations may prevent birds from evaluating a high number of potential mates. The Eagle Owl, like many other raptor species, shows a reversed size dimorphism, as females are bigger than males (in agreement with differences in white badge reflectance). Several hypotheses suggest that the larger size of females could have evolved from intrasexual competition for males (see Size dimorphism and sexing, page 22, in Chapter 1, and review in Massemin et al. 2000). In this way, the variability in the female white badge could have arisen as a means to evaluate rivals. Since female body size has been related to fertility in different raptor species (i.e. larger females produce more and larger eggs; Massemin et al. 2000), males could also benefit from choosing mates based on this trait. Along the same line, previous research on another owl species, the Barn Owl Tyto alba, showed how plumage coloration (number of black spots) was positively correlated to female quality in terms of immunocompetence (Roulin et al. 2001a,b). The same authors suggest that the trait may be under intersexual selection (male mate choice) because: (a) males mated to highly spotted females obtain more immunocompetent offspring (Roulin et al. 2000), (b) males consistently choose to mate with such females in different breeding seasons, and (c) their male offspring also preferred highly spotted females as mates (Roulin 1999b). Because of Eagle Owl crepuscular (i.e. dawn and dusk) patterns of activity, any visual signal used during this period should maximise the scarce light available. In the Eagle Owl, variability in the total amount of light reflected by their white throat could be exploited as a high-contrast signal in dark backgrounds (e.g. vegetation, crepuscular sky) both at twilight and on bright nights. Empirical evidence has already demonstrated that: (a) birds use very specific light environments for their displays where plumage characteristics are maximised because of the ambient light and background properties (e.g. Endler 1993, Endler & Théry 1996), and (b) light environment plays a role in the evolution of colour patterns and signals (Endler & Théry 1996, McNaught & Owens 2002, Gomez & Théry 2004). Finally, the fact that the white badge can be concealed or exhibited facultatively with each calling bout suggests that it may have evolved as a ‘coverable badge’, signalling threat or territory ownership only when it is required (Hansen & Rohwer 1986).

Characterising visual signalling traits in young and adults Bettega et al. (2013) examined the characteristics and patterns of the brightness of white feather patches of young (n = 59) and full-grown individuals (n = 30). First, they identified the relationships between the brightness of the two white feather patches and several internal (e.g. an individual’s physical condition) and external (e.g. availability of main prey) factors with the potential to affect the properties of reflectance. Second, they examined brightness differences between the white feathers of young and breeders. Because the white plumage of young birds and breeders serves different functions, they 315

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Groups Figure 115. Sex-specific variation (males = dark grey; females = light grey; means and confidence intervals are provided) in the total reflectance of white feather patches in young and full-grown individuals (from Bettega et al. 2013).

hypothesised that the reflectance properties of their plumage should differ. Third, they investigated whether breeding pairs mated assortatively based on plumage characteristics. Although assortative pairing can occur for many reasons other than mate choice, assortative mating by plumage colour may occur when mate choice is directly related to phenotype (Bortolotti et al. 2008). Finally, Bettega et al. (2013) tested whether plumage brightness could be used to identify siblings and their parents by comparing the feather brightness of owls within family groups. The total reflectance of white feather patches was significantly lower in young (mean ± SD = 1,371 ± 293.4nm) than in adults (1,625 ± 282.4nm; Figure 115); in young individuals, the total reflectance of white feather patches varied significantly among brood sizes, whereas in full-grown birds the total reflectance is related to one trait of the quality of the male, namely haematocrit. Such a result is in agreement with a previous study (Penteriani et al. 2007a) in which the analysis of the relationship between the percentage of reflectance of the white badge of the adult throat and fecundity of eight radio-tagged territorial males showed that breeding output (number of fledged young) was positively correlated with the total reflectance of the male badge. Similar to the study carried out on museum specimens (see The white badge of adults, page 312), females (1,785 ± 162.5nm) showed higher brightness levels than males (1,556 ± 297.4nm). Although the available sample in Bettega et al. (2013) was small (n = 6 pairs), variability in the total reflectance of white feather badges was higher among breeding pairs than within pairs. Finally, it was not possible to detect variations in feather brightness between young and their parents or between siblings, and differences in feather brightness among families were not sufficient to identify individual families of owls. These results provide the first evidence that the white plumage in both young and adults may play an important role in animal communication among crepuscular and nocturnal species. In male breeders, the brightness of the white throat patch is positively correlated with 316

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haematocrit values. Haematocrit may be considered an indicator of phenotypic condition, with high haematocrit values associated with a better nutritional state and lower levels of infection. Signals may be reliable indicators of individual quality if they are costly. Following Kose & Møller (1999), we can suggest at least two potential costs associated with using white plumage for signalling. First, melanisation strengthens feathers; non-melanised feathers are more likely to break due to their greater structural weakness. Second, if feathers without melanin are particularly susceptible to breakage, it is also possible that feather parasites may display a preference for the melanin-free parts of feathers. Although the costs associated with the brightness of white feathers is a topic in need of further research, these results suggest a definite relationship between bright, white plumage and an individual’s physical state. This relationship is a prerequisite for the use of white markings as a signal of the quality of an individual, supporting previous evidence from Gustafsson et al. (1995) and McGlothlin et al. (2007). Although the available sample was small, the potential assortative mating pattern found by Bettega et al. (2013) may also support the role of white feathers as a signal of individual quality. Assortative mating, which may occur through a variety of behavioural mechanisms (Bortolotti et al. 2008), has been observed in both structurally coloured species and species showing white marks. For example, assortative mating may be the result of individuals’ mutual preference for similar phenotypes (Burley 1983) and/or by intrasexual competition for nest sites. This competition may result in high-quality individuals gaining access to the best nest sites and pairing with high-quality mates. The fact that there were no differences in the brightness of white feathers between male and female young is noteworthy, especially when considering that, in some owl species, young brothers and sisters differ in colour (e.g. Roulin et al. 2012). Possibly, differences in the brightness of the white signal may only appear in full-grown birds because it is only after dispersal that these individuals need to signal their state to conspecifics (e.g. during territorial confrontation or mating). As there were no sex differences in the feather brightness of young, these results suggest that parents do not discriminate the sex of their offspring using the brightness of white plumage. Finally, this new evidence also supports the possibility that the brightness of a young bird’s white feathers signals its quality. By detecting a positive correlation between brightness and brood size, Bettega et al. (2013) suggest that this may indicate that because larger broods are associated with better nesting sites (e.g. sites rich in high-quality food sources) and/ or better parents, feather brightness may also be a consequence of the quality of the nest’s surrounding environment and/or physical characteristics of the parents.

Experimental evidence of the importance of visual cues in adult conspecific communication We have previously seen that visual behavioural displays during specific conditions of ambient light (e.g. crepuscular light) may act as additional cues for social communication in nocturnal species. But only an experimental approach may reveal whether white feathers actually play a role in owl communication. The Eagle Owl represents an excellent species to truly understand a potentially overlooked way in which nocturnal animals may communicate among themselves. Thus far in this chapter we have seen that: 1) Eagle Owls have a white badge on the throat that is especially visible during vocal displays, when the throat is repeatedly 317

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inflated and deflated (Penteriani et al. 2006); 2) this badge has similar dimensions in both sexes, but its total reflectance (i.e. brightness) is sex- and period-dependent (Penteriani et al. 2006); 3) the period in which badge brightness is highest coincides with the time when territorial and courtship displays are at their peak (Penteriani 2002); 4) such displays are mainly performed by males, but females also show a peak in display rate during the prelaying period (Penteriani 2002, 2003); and 5) display activity peaks at sunset and sunrise (Penteriani 2002), when achromatic plumage patches (i.e. pigment-free white feathers) are the best candidates for crepuscular signalling. To try to understand if the white feathers of the throat patch could function as a signal towards conspecifics, Penteriani et al. (2007a) evaluated the function of this white badge during contests by simulating territorial intrusions. They analysed the reactions of territory owners towards a taxidermic mount with a control badge (i.e. normal brightness treatment) or a brightness-reduced badge, with both male and female territorial calls. If the brightness of the owl badge is a status-signalling trait reflecting individual quality and fighting ability (status-signalling hypothesis; Rohwer 1975), allowing opponents to assess the relative likelihood of winning a contest (e.g. owner vs. intruder asymmetries in fighting abilities; Maynard Smith & Parker 1973, 1976), different responses to the mount would be expected during different trials. That is, contests should range from ritualised displays of different duration without physical contact (e.g. call displays) to direct attacks, reflecting the highest intensity of aggression. Above all, different reactions to different badge treatments would indicate the importance of visual cues in owl conspecific communication. The experiment was conducted at 30 nest sites, where the authors analysed the response of both territorial males and females to a single taxidermic mount, positioned in a highly visible location and close to the nest (at a distance of ca. 100–200m), for which two different types of throat badges were prepared: normal-brightness and brightness-reduced badges (brightness reduction of the badges applied to the decoy was carried out by smearing the plumage with a mixture of duck preen gland fat and UV-absorbing chemicals; the latter was applied because part of the reflectance spectrum of the badge belongs to the UV range; Penteriani et al. 2006). In each nest site, the mount was presented four times: 1) with a normal-brightness badge and an associated male playback call, 2) with a brightness-reduced badge and an associated male playback call, 3) with a normal brightness badge and an associated female playback call, and 4) with a brightness-reduced badge and an associated female playback call. Each trial started 1h before sunset, coinciding with one of the two peaks of Eagle Owl calling activity, and lasted 30 minutes. Behavioural responses to the simulated intrusion varied widely among males, ranging from absence of contest engagement to overt physical aggression (some individuals attacked not more than four minutes after the start of the trial). In the latter case, attacks were always initiated from behind the mount’s back, striking the back of the head with the claws and throwing down the mount, sometimes to a considerable distance from its original position. We consider that such aggression would have caused significant injuries or, in many cases, death to the intruder. When analysing male aggressiveness towards male intruders, owners approached the brightness-reduced male-like mount more closely. In fact, attacks were exclusively performed on the brightness-reduced male-like mount and the time that elapsed between the first call of the owner and the attack was 10.4 ± 6.2 minutes (range = 4–20 minutes). The males that attacked the mount started calling before the males that only performed vocal displays. The probability of being engaged in vocal displays was higher 318

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Sex of the owner vs. sex of the intruder Figure 116. Percentage of Eagle Owl vocal engagement in contests, depending on the badge (white bar = normal-brightness badge; grey bar = brightness-reduced badge) and call (male or female) characteristics of the territory owner, as well as on the sex of the mount (male or female). The male normal-brightness mount inhibited the call display of 40% of territorial males, whereas females showed lower aggressiveness than males (from Penteriani et al. 2007a).

towards male-like and brightness-reduced mounts. The normal-brightness male-like mount completely inhibited the response of eight males (40%; Figure 116). Females were engaged in only 15 contests out of the 80 trails (Figure 116). If we assume that the absence of vocal activity did not imply female absence from the experiment area (as supported by radio-tracking information), we can conclude that the sex or brightness of the mount did not affect the probability of being engaged in vocal activity (Figure 116). Only five times (6.3%) did females call alone (four towards the female-like mount). When in a duet, females always started calling after males and, usually, at a distance of 50–100m to their mate. The results of this study provide the first experimental support for the hypothesis that the white badge of Eagle Owls plays an important role in visual communication during contests, being the first time that it has been possible to establish an active role of visual signalling in a nocturnal species. Male responses could be summarised as follows: 1) brightnessreduced male-like mounts were always approached more closely and were the only group to be directly attacked; 2) vocal displays were frequently performed towards low-brightness intruders, whereas some birds remained silent when confronted with a normal-brightness intruder; and 3) there was less vocal behaviour and activity when the intruder was a female. Meanwhile, females did not show differences between treatments, displaying less frequently than males and mainly supporting their mate (e.g. territorial duets) or responding to the intrusion only when males refrained from contests. The dynamics of the contests were consistent with the idea that the brightness of the white badge was used as a status-signalling trait. The responses ranged from ritualised calls to direct attacks. Consequently, the Eagle Owl white badge might be considered a phenotypic signal that reliably informs opponents about their asymmetries in fighting skills, minimising the risk of wasteful and potentially injurious fights. Evaluating the chances of defeating an opponent before deciding whether 319

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to retreat from, approach, or physically attack an intruder could be considered an ‘assessor’ strategy (Maynard Smith 1982). Such a strategy should minimise the costs of unfavourable contests (i.e. when asymmetries are favourable to the opponent). Under this scenario, and in line with expectations from theoretical game models (Maynard Smith 1982), highquality individuals may decide to respond to a high-quality intruder because of their similar badge properties (i.e. small asymmetries in fighting abilities). Thus, direct attacks should be more probable as the signalling status of the opponents becomes more similar (‘likes-willfight prediction’ in status-signalling hypothesis; Rohwer 1975). Immobile mounts cannot escape, and therefore, a model resembling a subordinate should receive a greater number of aggressive attacks. Our results showing a high level of aggression to low-brightness mounts agrees with such a scenario. Animals may adopt submissive or neutral behaviours when their chance of winning a fight (or the benefit/cost ratio) is low (Bradbury & Vehrencamp 2011). This may explain why the high-brightness badge inhibited the vocal response of 40% of the males. These males may be lower-quality individuals and, possibly, also owners of poor-quality breeding places. In fact, the value of the defended resource may also affect the willingness to accept contest escalation and its consequences (Riechert 1998). The results regarding owl badge characteristics as a signal of individual quality fit well with the remarks of Johnstone & Norris (1993) who stated that badges that serve to settle conflicts should also constitute honest indicators of individual condition. As far as we know, there is no evidence concerning the direct physiological cost of producing achromatic traits. However, honest signalling may also be preserved by honesty-maintaining mechanisms (i.e. costs induced by social interactions), which would prevent cheating because only highquality dominant individuals could stand the cost of aggression (Rohwer 1975, Møller 1987). In the case of the Eagle Owl, the high cost associated with injuries during direct attacks should preclude the spread of cheaters in the population. Only those birds able to sustain the escalation would exhibit high-brightness badges.

Eagle Owls vocalise when their white patch contrasts most We have previously seen that achromatic plumage patches are the best candidates for crepuscular signalling, as contrast is more important than colour. In fact, the setting or rising sun forms the best light angle for using a white patch as a high-contrast signal against a dark background (Endler 1993, Endler & Théry 1996). Not surprisingly, brightness contrast is a very common animal strategy for enhancing conspicuousness (Bradbury & Vehrencamp 2011). We also know that Eagle Owls perform dusk and dawn chorusing behaviour, when the white throat badge is visible because during calling the throat is repeatedly inflated and deflated. Under this perspective, Penteriani & Delgado (2009a) proposed a novel explanation as to why Eagle Owls call at twilight. They propose that white plumage patches and the timing of vocal/visual signalling have coevolved to maximise the effectiveness of social communication such as the dusk/dawn chorus, when several favourable conditions for conspecific communication coincide (e.g. specific light conditions, proximity of individuals to their nest sites – territory owners generally have their diurnal roosts close to the nest). To test this possibility, a field experiment was carried out on 25 Eagle Owl breeding males during the pre-laying period. In order to measure the brightness contrast of the white badge with respect to both the owl’s body and the background surrounding calling individuals 320

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during the period of sunset displays, a stuffed Eagle Owl was placed in each owl nesting site and was photographed with its badge exposed as it normally would be during call displays. This decoy was always positioned to ensure that the dusk vocal displays of the territorial male could be clearly heard, taking care to avoid placing the mount where it would be directly visible to the bird, as this might interfere with spontaneous calling activity. Moreover, because light conditions can vary on a small scale, the decoy was placed on exposed places under light conditions similar to those of the calling male. This was possible because the position of calling males was generally predictable, as Eagle Owls use habitual call posts at sunset, and these were located during prior studies. Because ambient light at sunset and sunrise are similar and Eagle Owls perform dawn and dusk choruses of similar intensity, only the dusk chorus was considered. During each of the 25 photographic-listening sessions, the start and the end of the call activity of each owl was recorded in order to define the temporal range of their vocal activity. For each of the 25 calling owls, pictures of the decoy were taken every five minutes, from an hour before sunset to 15 minutes after the last call of the dusk chorus. The three supplemental pictures that were taken during the 15 minutes following the last call allowed the collection of information regarding brightness after the conclusion of vocal displays. Brightness values of the white badge, body and external environment were measured (more details in Penteriani & Delgado 2009a). The results clearly demonstrated that the white throat badge contrasted most with the surrounding background and body during the owls’ twilight chorusing (Figure 117). Similarly, the contrast between the badge and the body was significant between the calling and post-calling periods, but not between the pre-calling and calling periods. This evidence supports the supposition that Eagle Owls perform their dusk vocal displays under the light conditions that best allow them to visually communicate with conspecifics through their badges. Due to incomplete knowledge of the physiological functioning of Eagle Owls, there are several different proximate mechanisms that can also contribute to the timing of calls (e.g. peaks in testosterone before sunrise), but most of the main hypotheses previously proposed to explain the dawn and dusk chorus in birds (Staicer et al. 1996) do not seem sufficient to fully explain the twilight behaviour of Eagle Owls, at least when employed without the support of a visual signalling hypothesis. In fact, because this owl performs dawn and dusk choruses throughout the year, it is difficult to believe that such calling solely represents a method of regulating daily hormone levels depending on the immediate social situation (i.e. self-stimulation hypothesis), attracting females (i.e. mate-attraction hypothesis), stimulating reproductive development (i.e. mate-stimulation hypothesis) or guarding mates (i.e. mateguarding hypothesis). These hypotheses seem unlikely because the mate fidelity of territory owners is generally expected to be stronger in Eagle Owls than in songbirds (where all these hypotheses have been verified and/or suggested), and mate attraction, stimulation or guarding during or after the dawn chorus should not occur because after sunrise individuals retreat to their diurnal roosts to rest. Finally, since the Eagle Owl is a top nocturnal predator, it does not have to call under low light conditions to better avoid predation or because foraging is limited and low light levels could interfere with the ability to search for prey (as appears to be the case for passerines; low predation and inefficient foraging hypotheses, respectively). We could further discard the inefficient foraging hypothesis because the activity peaks of the Rabbit, the main prey of the Eagle Owl in Mediterranean regions, partially overlap with the Eagle Owl’s dawn and dusk choruses. Therefore, these findings concerning the patterns of vocal signalling allow us to hypothesise that Eagle Owls may principally 321

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periods: 1=pre-calling; 2 =calling; 3 =post-calling Figure 117. Three examples of the way the Eagle Owl white patch contrasts most with the surrounding background and its body during the dusk chorus. A picture of the stuffed owl for each of the three periods (i.e. pre-calling, calling and post-calling periods) is shown. Box-plots illustrate the brightness contrast during the three periods for the overall data; means and confidence intervals are provided (from Penteriani & Delgado 2009a).

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vocalise at twilight because this timing of signalling maximises the visual contrast of the white feathers associated with their vocal displays. Under such a viewpoint, the evolution of a white badge that operates in conjunction with call displays at dawn and dusk may be relevant to Eagle Owl social dynamics. The need for visual signalling and social interactions under the best conditions of the Eagle Owl daily cycle could represent an alternative, though not mutually exclusive, to the proposed but controversial acoustic transmission hypothesis (Brown & Handford 2003), which predicts that birds are merely taking advantage of better sound propagation close to sunrise and sunset. In view of the clear evidence that vocal and visual signalling act at the same time and are intrinsically related in adult Eagle Owls, it would be more appropriate to discuss vocal/visual displays when defining their call behaviour. To conclude, a growing body of literature has recently added new information supporting the role of visual communication in nocturnal birds (e.g. Parejo et al. 2010, Bortolotti et al. 2011), particularly through pigment-free white feathers, demonstrating that this method of communication is more widespread than previously thought.

Visual communication and moonlight The discovery that Eagle Owls also communicate by visual signals opens a wide range of possibilities and novel, previously overlooked, ‘avenues of exploration’. Clearly, a visual signal like white feathers needs a source of light to work, as they are more or less conspicuous depending on the amount of light available for reflection. But diurnal light (twilight included, i.e. any light directly dependent on the sun) is not the only source of luminosity on this planet. Actually, moonlight represents a powerful source of illumination that cannot be neglected from the perspective of visual communication in nocturnal species. The luminance of a full moon is approximately 25 times greater than that of the quarter moon and 250 times greater than that of a clear moonless starry night sky. As a consequence, white plumage patches and the timing of visual signalling may have co-evolved to maximise effectiveness of the signal. Under such a scenario, we can expect that Eagle Owls will call more during a full moon, when the lunar light favours communication via visual signalling and, if lunar brightness facilitates owl visual communication, displaying individuals should select higher call posts during a full moon, because higher positions increase the conspicuousness of the signal. Indeed, call displays of Eagle Owls are strongly related to the moon phase (Penteriani et al. 2010b), as silent nights are more frequent among darker nights compared to brighter nights (Figure 118). Furthermore, the intensity of vocal/visual displays is strictly related to the available amount of lunar light, given that the longest series of calls has been recorded during the gibbous phase of the moon or during a full moon (Figure 118). It is also worth noting two additional facts. Firstly, the frequency of crepuscular calls (sunset/sunrise displays) is not related to the frequency of nocturnal vocal displays; that is, Eagle Owls that call at sunset/sunrise did not continue to call (or called infrequently) in the absence of moonlight. Secondly, call posts chosen by displaying owls are higher on nights with moonlight than without: under lunar brightness, higher positions would increase the conspicuousness of the white throat (Figure 119). Although anecdotal, during a night of radiotracking we witnessed a very peculiar phenomenon. A total eclipse of the moon occurred during the night of 20/21 February 2008. 323

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Nights without and with vocal/visual displays (%) Figure 118. Call displays of Eagle Owls are strongly related to the moon phase: silent nights are more frequent among darker nights (e.g. new moon) compared to brighter nights (gibbous phase and full moon) (from Penteriani et al. 2010b).

Figure 119. During intense moonlight, Eagle Owls prefer to perform call displays from call posts that are more prominent than their surroundings, compared to posts used during the rest of the lunar cycle. This should increase the conspicuousness of the white badge. The figure shows the exact position of calling owls during moonlit (A) and dark (B) nights (from Penteriani et al. 2010b).

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The event, from start to finish, lasted about three hours and twenty-six minutes. The major phases of the eclipse occurred as follows (all times are Greenwich Mean Time, GMT): the partial eclipse commenced at 01:43 GMT; totality began at 03:01 GMT and lasted until 03:51 GMT; and the partial phases ended at 05:09 GMT. This lunar eclipse provides a clear example of the direct effect of moonlight on Eagle Owl call displays. The vocalisations of radio-tagged 324

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Figure 120. The total eclipse of the moon that occurred during the night of 20/21 February 2008 and corresponding vocalisations of radio-tagged male #314, which exactly reflected the eclipse pattern (see text for more details; from Penteriani et al. 2010b).

male #314 exactly reflected the eclipse pattern (Figure 120): he reduced the intensity of call displays as soon as the partial eclipse commenced, did not call during the period of total eclipse, and started vocalising once again after the end of the partial phases. Light levels of moonlight are similar to the light levels at dawn and dusk (Martin 1990), when Eagle Owls perform the most vocalisations. This may suggest that nocturnal birds simply take advantage of any source of natural light to increase the effectiveness of their visual communication. It is important to highlight that, if moonlight affects communication, the specific effects may be entirely dependent on the ecology of the species concerned. That is, birds that respond to moonlight conditions may show an opposite pattern to that shown by Eagle Owls. For example, although there is no detailed information about call display patterns for most owls, the Mexican Spotted Owl Strix occidentalis lucida called more during the last quarter and new moon phases (Ganey 1990). While this is contrary to the present findings, it should be noted that Mexican Spotted Owls do not display white plumage while calling. Moreover, it would not be advantageous for them to call more with moonlight because moonlight calling could increase the risk of predation of this small owl by the bigger Great Horned Owl Bubo virginianus. Finally, and still related to the effect of the moon on vocal/visual displays, a consequent effect of the lunar cycle was also detected in the movements Eagle Owls perform within their home range (see also Chapter 9): the total distance moved during the night, the speed of displacements and the total number of movements per night were influenced by the lunar cycle (Penteriani et al. 2011a). Together these results suggest a higher movement activity level around the time of the full moon than around the new moon: the reasons for the highest activity on the brightest moonlit night may be related to both an increase in the time needed to find prey and the time devoted to vocal displays during the full moon phase. These activities are not mutually exclusive, as breeders have to both contend with 325

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less active/more concealed prey during bright nights and ensure greater conspicuousness of their visual displays in moonlight. Indeed, because vocal displays also involve frequent and rapid movements from one call post to another (Delgado & Penteriani 2007, Campioni et al. 2010), some of the important activity at the time of the full moon is also due to the more frequent vocalisations of breeding individuals on moonlit nights. It also appears that the main behaviour of dispersers is not really affected by moon phases (Penteriani et al. 2011a), which can be partly explained (at least) by the fact that non-territorial individuals do not perform vocal/visual displays and, thus, their behaviours and rhythms of activity are less moon-constrained.

Young may signal with white mouth feathers The discovery of the role played by adult white feathers in intraspecific communication represented the very beginning of a series of studies aimed to understand the different forms of visual communication used by Eagle Owls. The following step was to investigate the white feathers around the bill that appear in nestlings and successively disappear when the birds acquire adult plumage (before natal dispersal). Until quite recently, the only recognised way in which owl chicks communicated with parents was the vocalisations associated with begging. Begging is an activity designed to solve family conflicts over parental feeding, and it regulates both parent–offspring conflict and sibling competition. As a general trend, the more intense the begging (and the signals associated with this behaviour), the more resources provided by parents. However, in most avian species begging may involve several different signals such as posturing and plumage features (Leonard et al. 2003, and reviews in Budden & Wright 2001 and Wright & Leonard 2002). The combination of vocal and visual components of begging might provide parents with additional information concerning the state of the offspring and/or reflect different aspects of offspring condition (Johnstone 1996, Leonard et al. 2003). The different elements involved in begging displays may interact synergistically and, in particular, visual cues may increase parental response to vocal cues (Rowe 1999). Finally, offspring detectability by parents can be enhanced by brightness contrast between the white fleshy borders of the gape and its dark surroundings (Kilner & Davis 1998, Hunt et al. 2003). Intriguingly, a white border of feathers appears at the edges of Eagle Owls’ mouths just before fledging (approx. 30–35 days of age; Figure 121), and it becomes considerably less apparent upon dispersal (approx. 150 days of age), when juveniles and adults have a similar appearance. Similar to the white patch of adults, this white border of feathers also reflects light in the UV range. Penteriani et al. (2007b) tested the hypothesis that such white feathers play a role in parent–offspring communication during the post-fledging dependence period. They hypothesise that if parents adjust food allocation based on the visual signals of offspring, then experimentally reducing the brightness of the mouth feathers should engender poorer physical condition in the young (e.g. because they will receive less care from parents). Again, a field experiment was carried out, during which: (1) each owlet, from the time it was 35 days old, was visited every seven days during a period of three weeks; (2) on the first visit, owlets were randomly allocated to an experimental group (either control or brightness-reduced, ten and nine chicks respectively); to produce two different levels of 326

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Wavelength (nm) Figure 121. (A) Reflectance spectra of the natural variation of brightness of owlet mouth feathers (n = 19 nestlings). Bold lines represent the maximum and minimum recorded values. (B) The white mouth feathers of an owlet during the post-fledging period (approx. 75 days old) (from Penteriani et al. 2007b).

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reflectance the same procedure as in Penteriani et al. (2007a) was followed (see Experimental evidence of the importance of visual cues in adult conspecific communication, page 317); and (3) during this first visit, morphometric measurements and blood samples were taken, whereas during the second visit, T-cell-mediated immune response of fledglings was evaluated by means of phytohaemagglutinin skin tests. One week after the immunity test (i.e. two weeks after the first sample was taken) young were again weighed and measured, and blood samples were drawn for the last time. Brightness-reduced individuals exhibited a lower uric acid concentration, a weaker T-cellmediated immune response and a higher proportion of parasitised lymphocytes. Cholesterol was higher in control birds, but it did not differ significantly between the two treatments. At the end of the experiment, body mass and body size did not differ significantly between treatments. These results suggest that the white border of the mouth of fledgling Eagle Owls plays a role in parent–offspring communication during feeding, as control owlets showed a better physical condition than owlets with brightness-reduced feathers. Actually, high uric plasma levels in control birds could reflect their higher protein intake (Lumeij & Remple 1991). Such a result would be consistent with their stronger T-cell-mediated immune response. In fact, the nature of this particular immune defence is associated with 327

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the amount of dietary protein among different avian species (e.g. Lochmiller et al. 1993). The higher intensity of parasitised lymphocytes in brightness-reduced fledglings completes this scenario. Thus, it is possible to hypothesise that control birds could have received more or better food, allowing them to mount stronger immune responses and avoid parasite infestations. Several studies have established that mouth colours associated with begging signals can indicate a need for food as well as other components of the offspring’s general state (i.e. condition-dependent traits) and, consequently, influence parental decisions in favour of offspring with higher reproductive potential. Therefore, we can speculate that the white feathers of this owl have evolved under selective pressure on parents to maximise their fitness during parental care. However, we cannot ignore the possibility that parental food allocation simply followed a fixed mechanism in which parents passively fed the offspring presenting the greatest stimulus (i.e. highest brightness). Our results may also lend further support to the hypothesis that mouth colours of nestlings also act to enhance offspring detectability. In fact, the white border appears when offspring need to be fed quite far from the nest and siblings can be relatively distant from each other (Penteriani et al. 2005b). In the dark, white feathers may aid parents in locating their offspring during begging. If this is the case, poor reflecting nestlings could have an increase in energy expenditure (e.g. through increased movements due to a reduction in parental feeding), leading to the observed changes between experimentally reduced-brightness and control individuals. It is worth noting here that the shift in the traits related to visual cues between young and adults may be related to a substantial modification in vocal signalling. In fact, the smaller white mark of young is associated with their ‘soft’ call, mainly used for short-range communication (e.g. with siblings and parents), whereas the bigger badge of adults plays a role in long-range communication. Also, the former is always visible, whereas the latter is only visible during call displays.

Breeders use faeces and prey feathers to signal current reproduction Many animals mark focal elements of their home ranges with different kinds of materials and use conspicuous visual and/or olfactory objects as a defence against predators and to attract potential mates. This territorial marking represents an extended form of display for some species. Moreover, many species of mammals demarcate their territory by faecal marks: faeces may represent an ideal substance for marking because it has a minimal energetic cost to the signaller (Gosling 1981), and can continue to indicate possession of a territory when the owner is occupied in activities other than territorial defence. However, because faecal marking is constrained by faecal production, territorial individuals should prioritise the marking of positions that have the highest value as territorial signals (Brashares & Arcese 1999). During the pre-laying period and throughout the nestling period, large quantities of extremely visible white faeces and prey feathers appear on posts and at plucking sites in the proximity of the nest site. Although breeding owls use vocal displays to convey important territorial information, these signals have temporal limitations since they need to be actively produced and, therefore, require the owls’ presence. Thus, it would seem useful to have additional, longer-lasting signals that could continue to indicate possession of a territory when owners are far from the nest. The presence of highly visible signals like white faeces and 328

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Figure 122. Temporal pattern of the appearance of defecation sites within Eagle Owl home ranges. The number of posts marked by faeces increases during the pre-laying period, and peaks just before the month in which most of the females start incubating.

bright feathers at plucking posts may indicate occupancy of nest sites. This could prevent floating individuals or neighbours from unintentionally approaching nest sites, thereby reducing the risk of potentially fatal aggressive encounters between competing males. Such territorial markings may have the additional advantage of indicating nest site occupancy to potential intruders even when territory owners are far from the breeding site or are not involved in territorial displays (e.g. when they are hunting). Posts marked with faeces and prey feathers generally start to appear in late September (Penteriani & Delgado 2008), during the renewal of territorial displays, increase until the nestling period and then decrease during the fledgling period (Figure 122). Because defecation posts and plucking sites are not refreshed after fledging, it seems unlikely that a large quantity of faeces and feathers simply built up at these locations because the owls, for a variety of reasons, are selecting resting points near the nest on which faeces and feathers accumulate by chance (i.e. they are not using excrements or feathers as signalling items). Post refreshing after fledging or during the post-fledging dependence period was never observed, when the following hold true: (a) both parents do not show important modifications of their space use and continue to stay relatively close to the nest; (b) small differences in space use due to young displacements are not followed by the appearance of new marked posts; (c) diurnal roosts continue to be generally located in the area surrounding the nest (i.e. territory owners start and end their activity close to the previously marked posts); and (d) such posts are still located within the 50% core area of owl movements. Eagle Owls generally hunt and eat their prey at a distance from the nest, but all plucking sites are generally concentrated close to the nest. Similar to the locations of defecation posts, plucking sites are located at the highest, most visible points of the valley slopes. Immediately after a breeding failure, the conspicuous plucking sites disappear and the marked posts are not renewed, even though the parents continue to move and roost close to the nest. The plucked prey or faeces that are observed after a breeding failure are generally on 329

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the ground, and not in prominent locations. In addition, faeces-marked rocks or prominent plucking sites are never found in the settlement areas frequented during dispersal by juveniles. Non-territorial individuals do not leave their faeces or prey remains on visible posts, even though they frequently use the same posts within their settlement home range areas. Remarkably: (i) faeces are generally recorded on the darkest substrates, even though bright rocks prevail in the breeding site; (ii) defecation posts are largely located on the highest points of valley slopes and on positions that make them easily detectable from neighbouring nest sites or by non-territorial individuals moving across the main valley; (iii) Eagle Owls with nearby neighbours and/or in situations of very high breeding-site density seem to mark more posts compared to owners of nest sites located relatively far from their nearest neighbours; and (iv) due to the distance at which Eagle Owls generally squirt their faeces (mean ± SD = 20 ± 11cm), isolated defecation rocks with abrupt and vertical shapes (the most common type of marked posts) should not show faecal marks if marking is due to coincidence (i.e. Eagle Owls leave their marks intentionally on given posts). Additionally, the same posts are refreshed during the breeding season or from one year to the next, if they disappear (e.g. because of the weather). Owls respond rapidly to changes to faecal marks on defecation posts: the mean time elapsed between the experimental covering of the faeces and the appearance of new faecal marks on the same post was of 2 ± 1.8 days (but for 30% of defecation posts, new faeces appeared in less than 24 hours). The prey species plucked on the most visible posts were always the brightest taxa. For example, Rabbit remains were never found at plucking sites, although they were observed on the ground, in low-visibility locations. All prey species found at plucking sites were birds endowed with highly visible (white or bright) feathers; these included the Azure-winged Magpie Cyanopica cyanus, Barn Owl Tyto alba, Larus spp., Little Egret Egretta garzetta, Redlegged Partridge, Short-eared Owl Asio flammeus, and Wood Pigeon Columba palumbus. The low frequencies of the species recorded at plucking sites in the diet of the studied Eagle Owl population (mean range = 0.8–6.9%) clearly show that they are among the least common (but brightest and most feather conspicuous) prey in the study area. Some birds are capable of masticating vegetables to paint a saliva–plant mixture on their bowers (Bravery et al. 2006), or arrange feathers to decorate their nests in a nonrandom manner with respect to their reflectance (Veiga & Polo 2005). We now know that Eagle Owls use faeces and prey feathers to signal their breeding status to conspecifics. As we would expect for a signalling behaviour that has evolved to maximise signal strength relative to the background environment (Endler 1992), the data in this study suggest that Eagle Owls preferentially leave white faeces on the darkest and most detectable surfaces, and preferentially leave prey species with conspicuous plumage at highly visible plucking sites. Granted, these signs might also contain useful information for predators. However, Eagle Owls can likely afford to give away the location of their nest sites in such a prominent manner because adult Eagle Owls essentially have no natural predators, and females are present in the nest with their offspring throughout most of the nestling period. Some of the faecal marks are only visible from the nests, suggesting that they may also signal the owl’s reproductive state or function in mate–mate communication (e.g. choice of nest placement), in a manner similar to that seen for scent marks in several mammals. In such a context, we cannot exclude the possibility that the faecal markings provide a signalling function similar to that conveyed by the transport of green material to the nest (especially for owls that do not carry nesting materials), which in some bird species serves as an intersexual 330

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signal for nest occupation. In mammals, the location of marked defecation posts allows them to be detected at some distance, alerting animals to the presence and location of an occupied breeding site. Moreover, the owls’ defecation posts and plucking sites are situated relatively close to each other, potentially maximising their chance of detection by intruding individuals. These markings may therefore function as an effective deterrent to neighbouring owls. Scent marking, defecation posts and plucking sites enable the signaller to leave messages that are long-lasting and can be read later by conspecifics (Alberts 1992), suggesting that they could have evolved as broadcast signals used for network communication. Finally, similar to transient mammals with no mate or territory (Gese & Ruff 1997), Eagle Owl dispersers do not mark their settlement areas with faeces or prey remains. Faecal marks and plucking sites only occur in close proximity to the nest, which is the most actively defended area. The ‘ownership hypothesis’ for mammalian scent marking (Lewis 2006) predicts that some markers are designed to claim ownership or exclusive use of focal resources or sites. Under this hypothesis, and as observed in the present study of Eagle Owls, markers are not necessarily expected to occur on the boundaries of the home range, because some portions of the home range may overlap with neighbouring breeding places. The shift from a peripheral to a central position of marks within the home range has been attributed to the size of the territory/home range or the energetic and time constraints limiting border patrolling behaviour (Robert & Lowen 1977, Gosling & Roberts 2001). However, although some of these factors may be true for Eagle Owls, the concentration of faecal marks exclusively around active nests may indicate a stronger territoriality in this area (as we have previously stated, home range overlaps never occur in the 50% core area). The effective transmission of a signal requires the signaller to assess and react to changes in the signal. The recorded patterns show that Eagle Owls rapidly detect a change in faecal marks and compensate by re-marking the same location with fresh faeces, suggesting that owls are capable of controlling their displays through behavioural compensation.

Visual communication and owl vision The features of Eagle Owl vision represent a crucial aspect of visual communication, yet this latter should depend on the way owls are able to see, i.e. the characteristics of their eyes. In particular, based on the information of the characteristics and type of visual signals used by Eagle Owls, it would be very important to understand whether they can see colours, and if they are able to see in the UV range of reflectance (320–400nm). Unfortunately, to our knowledge, we still do not have detailed information on the visual sense of this species. The little information we have on other owl species does not help us much to understand how visual signals may have evolved as a consequence of the visual sense of Eagle Owls. For example, we know that the eyes of the Great Horned Owl Bubo virginianus (Fite 1973) are highly adapted for nocturnal viewing (rods are more abundant than cones), with a retina dominated by rods (cells able to capture a very small amount of light), which comprise about 90% of the receptor population. It also appears that they may have a weak colour vision capacity (Jacobs et al. 1987). On the other hand, the Tawny Owl has a visual acuity comparable with that of diurnal birds (Martin & Gordon 1974a) and a functional photopic system capable of mediating colour vision (Martin 1974, Martin & Gordon 1974b, 331

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Bowmaker & Martin 1978). The higher absolute sensitivity exhibited by the Tawny Owl over that of diurnal birds should be the result of a greater area of the retina covered by rods, rather than the presence of exceptionally long rod outer segments (Bowmaker & Martin 1978). However, it does not seem that the Tawny Owl is able to see in the UV range of reflectance because its photopic sensitivity starts at >500nm. Generally, owl eyes have different cone photoreceptors associated with specific oil droplets (Bowmaker 1991), which have the potential to allow UV vision. However, the presence of an eye structure potentially allowing for UV vision does not necessarily mean that the species can see in this range (Cuthill et al. 2000, Hart 2001), and thus behavioural tests are necessary to confirm this possibility. To date, the only study to focus on the eye structure of the Eagle Owl has been published by Lledó De Villar (2010), who affirms that: (a) like other owls, the number of rods is much higher than that of cones (the contrary occurs in diurnal birds), confirming that the relationship between rods and cones depends on the habits (crepuscular and nocturnal vs. diurnal) of the different species; (b) in the retina photoreceptors there are green and yellow oil droplets, which are not expected to allow UV vision; (c) there are no transparent oil droplets, which are generally associated with UV vision; and (d) Eagle Owls may have a dichromatic colour vision, more similar to mammals than diurnal birds. Dichromatic colour vision could explain the main features of the used visual signals, which rely more on brightness and contrast than on colours.

The function of head ornamentation in owls Eagle Owls, and more generally owls, show quite amazing, but frequently overlooked, head/ face plumage ornamentation. In many owl species, the most widely known ornaments are eartufts and/or the brightly coloured iris, although there are other regions of the head that may display evident white and black achromatic patches/stripes. Some of these ornamentations such as ear-tufts, eyebrows, bristles and chin may be modified in shape and conspicuousness depending on the emotional state of individuals. Galeotti & Rubolini (2007), following the logic of previous works on visual communication in owls (and, more generally, on crepuscular and nocturnal species), suggest that head ornaments might have evolved to communicate with conspecifics, i.e. partners, offspring and rivals, and/or with members of other species. Under the hypothesis of intraspecific communication, head cues might represent: (1) an adaptation for social conspicuousness, i.e. they help to localise individuals under generally low light conditions (e.g. at night and in closed habitats like forests); (2) condition-dependent, sexually selected signals of status or other qualities; (3) individual recognition signals between mates, family members and/or rivals; (4) signals of a given emotional state (e.g. ear-tufts, front and chin change their appearance depending on the feelings and intentions of individuals); and (5) a way to enhance a specific behavioural display, by making it more obvious and less ambiguous. On the other hand, under the hypothesis of interspecific communication, head ornamentations may be directed towards individuals of other species in order to: (1) signal unprofitability or unpalatability (i.e. aposematic patterns) or (2) confuse or surprise as a result of the abrupt change in facial appearance, which may startle/intimidate potential predators and mobbers. 332

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These authors mainly found support for the possibility that head ornaments may serve a communication function, since interspecific variation in the degree of ornamentation is associated with differences in habitat use (highly ornamented owl species live in open habitats) and activity rhythm (highly ornamented species show some kind of diurnal and crepuscular activity). Indeed, conspicuous ornaments and contrasting white badges may be detectable from a greater distance in open habitats or in the limited light conditions typical of dusk and dawn, but they are less effective in closed habitats and in total darkness, because they may be detected only at short distances (Martin 1990). Although white patches may have principally evolved for long-distance signalling, head ornamentation seems to have predominantly evolved to signal intentions or emotions at short distances, e.g. between mates and family members. Finally, since ornamentation does not vary according to the size of owl species, the hypothesis that head cues may represent components of an antipredator device seems weak. However, because of the need to remain concealed from daytime predators and mobbers, as well as to startle them when discovered, a startling mechanism might still be useful.

Summary Our contribution to this line of research has clearly demonstrated that Eagle Owls communicate by visual signals, challenging the common belief that social communication in this nocturnal species is limited to vocal signalling. Visual signalling is in fact very important for these nocturnal birds, especially in the context of being able to convey information on their quality by the brightness of their feathers. Territorial and courtship displays by the Eagle Owl peak at sunrise and sunset, when the light conditions are optimal for the visibility of the white throat patch that individuals flash to deter intruders (as well as to impress females). To date, research into the wonderful and amazing world of visual communication in Eagle Owls has offered many surprises, and shows us that there is still much to learn about this astonishing species.

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1. Nest site in a chalk plateau in the Atyrau region, western Kazakhstan (Photo: Mark Pestov).

2. Nesting biotopes in the high mountains of Altai, Altai-Sayan region (Photos: Igor Karyakin).

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3. Examples of nesting places in boreal forests, southwest Finland: (A) cliff in a patch of forest opened by clear-cutting; (B) a rocky nest close to a clear-cutting; (C) a nest on a small cliff, under the protection of a tree trunk; (D) a nest under the root of a cut tree in the middle of a clear-cutting; (E) a nest is present under the floor of a cabin in the woods; (F) a nesting cliff (on the left) adjoining a golf course (Photos: Vincenzo Penteriani).

4. A female and her three young in the nest in downtown Helsinki, Finland. Inset: the nest in the summer after the first unsuccessful breeding, when the female was disturbed while incubating by maintenance works on the roof (Photos: Vincenzo Penteriani).

5. Post-fledging dependence period in downtown Helsinki, Finland (Photo: Vincenzo Penteriani).

6. A breeding area in Lurøy, a municipality in Nordland county, Norway (Photo: Karl-Otto Jacobsen).

7. The nestling of an Eagle Owl pair that bred in a wooden nest box installed for the Saker Falcon in a lowland floodplain forest in South Moravia, Czech Republic (Photo: David Horal).

8. A nest inside a dead branch of an oak near the branch fork, Schleswig-Holstein, Germany (Photo: Uwe Robitzky).

9. A young Eagle Owl in a Grey Heron nest in a colony in the district of Pinneberg, in SchleswigHolstein, Germany. One of its siblings was already on the ground and the nest had lost approximately two-thirds of its original size (Photo: Anke Brandt).

10. An Eagle Owl nest in the fortifications of Luxembourg City, Luxembourg (Photos: Patric Lorgé).

11. Nesting site in Luberon Massif, Provence, southern France (Photo: Vincenzo Penteriani).

12. A view of the nesting habitat in the Sierra Morena massif, southwest Spain (Photo: María del Mar Delgado).

13. A nest at the base of a tree trunk in a eucalyptus forest, southwest Spain (Photo: Vincenzo Penteriani).

14. A small patch of eucalyptus trees used as a breeding site. Inset: the nest at the base of a trunk. Guadiamar River area, southwest Spain (Photos: Vincenzo Penteriani).

15. An abandoned nest of Spanish Imperial Eagle allowed an Eagle Owl pair to breed in a small and isolated patch of eucalyptus trees in Doñana National Park, southwest Spain (Photo: Vincenzo Penteriani).

16. A Spanish Imperial Eagle nest occupied by a breeding female Eagle Owl in Doñana National Park, southwest Spain (Photo: Vincenzo Penteriani).

17. An old stand of pines in Doñana National Park, southwest Spain, where Eagle Owls bred in a Buzzard nest, visible in the top right of the picture (Photo: Vincenzo Penteriani).

18. Settlement areas of dispersing juveniles may differ from breeding sites, as in the case of this ‘dehesa’, an agro-sylvo-pastoral system mainly consisting of pastureland and oaks, in southwest Spain (Photo: Vincenzo Penteriani).

19. Another typical settlement area frequented by juveniles during natal dispersal in southwest Spain. Such landscapes are generally rich in prey and the small patches of trees are used as roosts during the day (Photo: María del Mar Delgado).

20. A four-year-old radiotagged female bringing an adult Rabbit to her three one-month-old young just after sunset in the Sierra Morena massif, southwest Spain. Rabbits are the main prey in several areas of southern Spain, where Eagle Owl breeding pairs show some of the highest densities recorded in the distribution range of the species (Photo: Vincenzo Penteriani).

21. It has been suggested that the laying of additional eggs later on in the incubation period might be the reason for the remarkable age difference between chicks of the same brood (Photos: Igor Karyakin).

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22. Estimated spatial variation of the percentage of given prey groups in the Eagle Owl diet, as illustrated by kriging spatial interpolations. This procedure allows a continuous spatial pattern of diet variations to be visualised, including those areas for which specific data on the Eagle Owl diet are not available. The gradient of variation ranges from red (high consumption of a given prey group) to blue (low

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consumption of a given prey group). (A) mammals; (B) birds; (C) insectivores; (D) rodents; (E) lagomorphs; (F) carnivores; (G) Galliformes; (H) birds of prey; (I) owls; (J) corvids; (K) reptiles; (L) amphibians; (M) fish; (N) invertebrates; (O) mammal biomass; (P) bird biomass; (Q) rodent biomass; (R) lagomorph biomass; (S) Shannon index; (T) mean weight of prey (Maps: Luísa Crisóstomo).

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F

23. Eagle Owls leave their marks in strategic positions to give long-lasting visual signals that are easily detectable from afar. Faecal marks are often positioned on the most prominent rock near the nest (A, B), or at the entrance to the valley in which the nest is located (C). Plucking sites also occur in conspicuous positions, such as at the highest point of a valley slope (D). In experiments where faecal marks are deliberately covered by spray paint, Eagle Owls have been found to return and re-mark the same site within two nights, often in exactly the same position (E, F) (Photos: Vincenzo Penteriani).

B

A

24. When calling, Eagle Owls show a patch of white feathers on the throat (A) that represents an important visual signal for intraspecific communication during territorial and sexual displays (B) (Photos: Vincenzo Penteriani).

25. A male Eagle Owl cleaning its feathers while perched on one of its calling posts at sunset. The white patch on the breast is clearly visible (Photo: Vincenzo Penteriani).

First year (August)

Second year (August)

Second year (November)

Sixth year

26. Some examples of Eagle Owl moult patterns. In the August of the first year, all the primaries, primary coverts and secondary coverts are of the same generation time and easily distinguishable from adult feathers because the last dark stripe of each feather is close to the feather tip. Generally, by November of the second year, Eagle Owls have only moulted tertials, although some individuals may also start to moult primaries. All the feathers are moulted by the sixth year (Photos: Raúl Alonso).

Acknowledgements This book The Eagle Owl is the result of not only our research over the course of more than three decades, but also the collaboration with many people who helped us in different ways during the four years that the book took to prepare. Some of these people shared with us their knowledge, observations and experiences, others sent us data to be included in the analyses that we ran specifically for the book or gave us their pictures, whereas some others visited us during fieldwork to share remarkable moments. We want to thank all of you for your kind help, support and companionship. This book would not have been possible without you. We would also like to thank Chiara Bettega, Luísa Crisóstomo, Karl-Otto Jacobsen, Rui Lourenço and Mike Toms for the time they devoted to help improve this book by revising some chapters, and helping us with figures and copyrights. David Aragonés of the LAST (Laboratory of GIS and Remote Sensing) at the Estación Biológica de Doñana helped us to transform data on Eagle Owl distribution into beautiful maps. The chapter on Eagle Owl distribution and population estimates is the result of the collaboration with many Eagle Owl experts living in Europe and Russia, who revised both the text and the distribution maps. We made a special effort to present in this chapter the current knowledge of the status of the species across its distribution, and the help of the following people was crucial to improving the quality of the reported information. Our sincerest thanks to (by country in alphabetical order): H. Frey, M. Görner and C. Leditznig (Austria); D. Vangeluwe (Belgium); I. Hristov (Bulgaria); D. Horal (Czech Republic); P. Sunde (Denmark); A. Lõhmus and R. Nellis (Estonia); P. Saurola and J. Valkama (Finland); P. Balluet, G. Cochet, P. Demarque, Y. Martin and R. Nadal (France); L. Dalbeck, K.-H. Reiser and U. Robitzky (Germany); S. Dudley, T. Melling, M. Toms and T. Warburton (Great Britain); I. Beraudo and B. Caula (Italy); J. Lipsbergs (Latvia); P. Lorgé (Luxembourg); G. Wassink (the Netherlands); K.-O. Jacobsen (Norway); R. Mikusek and P. Mirski, (Poland); R. Lourenço (Portugal); I.V. Karyakin, V.V. Ryabtsev and S. Volkov (Russia); A. Vrezec and T. Mihelic (Slovenia); P. Hellström (Sweden); A. Aebischer and R. Arlettaz (Switzerland); and A.-T.V. Bashta (Ukraine). As can be seen in the central section of the book, R. Alonso, A. Brandt, D. Horal, K.-O. Jacobsen, I. Karyakin, P. Lorgé, M. Pestov and U. Robitzky provided photos, which helped us to illustrate some peculiar breeding habitats. Additional data for some specific analyses we ran for the book were kindly provided by S. Aftyka, E. Bassi, P. Bayle, C. Bearzatto, P. Demarque, S.V. Domashevsky, M.N. Gavrilyuk, A. Leduc, M. León-Ortega, R. Lourenço, J.A. Martínez, Yu.V Milobog, R. Nadal, M. Noga, C. Riols, B. Stien, V.V. Vetrov, V.I. Voronetskiy and T. Warburton. For their help with logistics as well as during fieldwork and the different stages of book preparation, we are grateful to (in alphabetical order): A. Aebischer, S. Allavena, R. Alonso, C. Alonso-Álvarez, A. Andreychev, D. Aragonés, R. Arlettaz, J. Ayala, J. Balbontín, P. Barbieri, J. Barreiro, A. Basanta, P. Bayle, C. Bearzatto, J. Bejarano, I. Beraudo, H.M. Berg, W. Bergerhausen, M. Bernoni, R. Bionda, Z. Bochenski, P. Cabot, E. Casado, R. Casalini, B. Caula, M. Cerasoli, N. Cillo, G. Cochet, A. Conejo, P. Cortone, J.M. Cugnasse, S. De Champsavin, M. De La Riva, P. Delgado, P. Demarque, R. Díaz, G. Di Croce, J.A. Donázar, El Largo and Pablo, C. Escot, E. Faraglia, E. Fernández, N. Fernández, M. Ferrer, M. Forero, Francisco de Borbollón, Francisco de Burguillos, Francisco de la Lapa, H. Frey, R&G Gillette Sirente forestguard, M.E. Galka, I. Galván, L. García, M. Garfagnini, J.L. Garrido, L.-H. Gee, A. Gómez, F. González, M. González, F. Goytre, M. Grande, F. Hiraldo, Huerto de los Arroyos, D.H. Johnson, R. Jovani, M. Juillard, H. Knobloch, O. Krischer, A. Kuparinen, P. Laiolo, A. Leduc, E. Luque, A. Manzi, G. Marangoni, L. Marchesi, A. Mariotti, P. Marotto, Y. Martin, A. Martina, M. Méndez, H. Mikkola, R. Mikusek, I. Molina, P. Montero, M. Montes Turatti, F. Moreno, S. Moreno, P. Morini, P. Mosimann, E. Muscianese, R. Nadal, J.F. Navarro, J.J. Negro, M. Noga, R. Larsen, V. Olsson, L. Ornero, Paco Merino de la Cantina de la Ruta del Agua, L. Palazzi, M. Pandolfi, P. Pedrini, F. Perco, M. Pettavino, H. Pietiäinen, M. Pineda, M.C. Quintero, C. Quirós, J.E. Rabaça, E. Ramanujam, L. Rigacci, C. Rodríguez, F. Rodríguez, R. Rodríguez, G. Salvatori, G. Sammuri, C. Sanchez, E. Sauli, J. Saunder, J. Savolainen, B. and H. Schaefer, W. Scherzinger, J. Sériot, D. Serrano, J. Shergalin, A. Sieradzki, A. Sigismondi, B. Stien, S. Sulkava, P. Tella, M. Thiollay, S. Tribuzi, M. Vazquez, S. Velasco, Vilches, N. Viqueira, M. Visentín, A. Vrezec and J. Wiss. Funding for our studies was provided, from the 1980s to 2012, by the Parco Nazionale d'Abruzzo, Lazio e Molise (Abruzzo, Italy), Parco Nazionale della Majella (Abruzzo, Italy), Parco Naturale Regionale Sirente Velino (Abruzzo, Italy), Parc Naturel Régional du Luberon (Provence, France), Consejo Superior de Investigation Científica (CSIC, Spanish Council of Scientific Research, Spain), Proyectos Intramurales Especiales (DG-2606-PC 8 CSIC), LICOR43 of Diego Zamora S.A., EMASESA, Junta de Andalucía grants (Consejería de Educación y Ciencia; Spain), the program ‘Incorporación de Investigadores al Sistema Español de Ciencia y Tecnología’ (CCAA de Andalucía), Junta de Andalucía Excellence Project RNM-5090, Spanish Secretaría General de Universidades, Ministry of Education (Salvador de Madariaga Program), a postdoctoral grant (nº 140367) from the KONE FOUNDATION (Finland) and a postdoctoral grant from the Academy of Finland, a Spanish ‘Ramón y Cajal’ grant (RYC-2014-16263), as well as research projects Nos. CGL2004-02780/BOS, CGL2008-02871/BOS, CGL2012–33240 of

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Acknowledgements different Spanish Ministries of Science, with FEDER co-financing. We trapped and marked owls under Junta de Andalucía–Consejería de Medio Ambiente authorisations Nos. SCFFS-AFR/GGG RS-260/02 and SCFFS-AFR/CMM RS-1904/02. P.J. Macaluso (https://sites.google.com/site/pjamesmacalusojr/home/english-proofreading-service) edited the English of the whole book, and J. and D. Shergalin translated the Russian bibliography. Tom Björklund (http://www.tombjorklund.fi/) did a great job illustrating Eagle Owls with his beautiful drawings; thank you for your patience with our continuous and ever-changing requests. Giulia Bombieri gave permission to use some of her beautiful paintings. Working with our publisher has been as smooth as one could hope for. We would therefore like to thank editors Jim Martin and Katy Roper for their support during the writing of the book and their patience each time we asked for a new deadline! Their help has been invaluable through the entire process. We are also very grateful to Brad Scott (copy-editor) and Rod Teasdale (designer) for all their hard work. For permission to reproduce copyrighted material, we would like to thank Airo, University of Chicago Press, Elsevier, Kohlhammer, Netherlands Ornithologists' Union, Oxford University Press, Raptor Research Foundation, Springer, Sveriges Ornitologiska Förening, Taylor & Francis, Vogelkundliche Nachrichten aus Oberösterreich (www.landesmuseum.at/en/), Wiley and William Collins. More specifically: (a) Animal Behaviour, Vol. 74, Sergio, F., Marchesi, L., Pedrini, P. & Penteriani, V., Coexistence of a generalist owl with its intraguild predator: distance-sensitive or habitat-mediated avoidance?, pp. 1607–1616, 2007, with permission from Elsevier; (b) The Journal of Raptor Research, Vol. 43, Penteriani, V. & Delgado, M.M., Thoughts on natal dispersal, pp. 90–98, 2009, with permission from The Raptor Research Foundation; (c) The Journal of Raptor Research, Vol. 47, Bettega, C., Campioni, L., Delgado, M.M., Lourenço, R. & Penteriani, V., Brightness features of visual signaling traits in young and adult Eurasian Eagle Owls, pp. 197–207, 2013, with permission from The Raptor Research Foundation; (d) Behavioral Ecology and Sociobiology, Bright moonlight triggers natal dispersal departures, 68, 2014, 743–747, Penteriani, V., Delgado, M.M., Kuparinen, A., Saurola, P., Valkama, J., Salo, E., Toivola, E., Aebischer, A. & Arlettaz, R., with permission of Springer; (e) Behavioral Ecology and Sociobiology, There is a limbo under the moon: what social interactions tell us about the floaters’ underworld, 66, 2012, 317–327, Penteriani, V. & Delgado, M.M., with permission of Springer; (f ) Oecologia, Individual and spatio-temporal variations in the home range behaviour of a long-lived, territorial species, 172, 2013, 371–385, Campioni, L., Delgado, M.M., Lourenço, R., Bastianelly, G. & Penteriani, V., with permission of Springer; (g) Population Ecology, Evaluating the influence of diet-related variables on breeding performance and home range behaviour of a top predator, 57, 2015, 625–636, Lourenço, R., Delgado, M.M., Campioni, L., Korpimäki, E. & Penteriani, V., with permission of Springer; (h) Population Ecology, Superpredation patterns in four large European raptors, 53, 2011, 175–185, Lourenço, R., Santos, S. M., Rabaça, J.E. & Penteriani, V., with permission of Springer; (i) Science of Nature (Naturwissenchaften), Quantifying space use of breeders and floaters of a long-lived species using individual movement data, 102, 2015, 21, Penteriani, V., Delgado, M.M. & Campioni, L., with permission of Springer; (j) Variation in the function of Eagle Owl vocal behaviour: territorial defence and intra-pair communication?, Penteriani, V., Ethology, Ecology & Evolution, copyright © Dipartimento di Biologia, Universitá di Firenze, Italia, reprinted by permission of Taylor & Francis Ltd, www.tandfonline.com, on behalf of the Dipartimento di Biologia, Universitá di Firenze, Italia; (k) The effect of phenotypic traits and external cues on natal dispersal movements, Delgado, M.M., Penteriani, V., Revilla, E. & Nams, V.O., Journal of Animal Ecology, 69, 620–632, 2010, permission conveyed through Copyright Clearance Center, Inc., John Wiley and Sons; (l) Spatial refugia and the coexistence of a diurnal raptor with its intraguild owl predator, Sergio, F., Marchesi, L. & Pedrini, P., Journal of Animal Ecology, 72, 232–245, 2003, permission conveyed through Copyright Clearance Center, Inc., John Wiley and Sons; (m) Tawny owl vocal activity is constrained by predation risk, Lourenço, R., Goytre, F., Delgado, M.M., Thornton, M., Rabaça, J.E. & Penteriani, V., Journal of Avian Biology, 44, 461–468, 2013, permission conveyed through Copyright Clearance Center, Inc., John Wiley and Sons; (n) Conspecific density biases passive auditory surveys, Penteriani, V., Gallardo, M. & Cazassus, H., Journal of Field Ornithology, 73, 387–391, 2002, permission conveyed through Copyright Clearance Center, Inc., John Wiley and Sons; (o) Development of chicks and predispersal behaviour of young in the Eagle Owl Bubo bubo, Penteriani, V., Delgado, M.M., Maggio, C., Aradis, A. & Sergio, F., Ibis, 147, 155–168, 2005, permission conveyed through Copyright Clearance Center, Inc., John Wiley and Sons; (p) Breeding density affects the honesty of bird vocal displays as possible indicators of male/territory quality, Penteriani, V., Ibis, 145, E127–E135, 2003, permission conveyed through Copyright Clearance Center, Inc., John Wiley and Sons; (q) Individual acoustic monitoring of the European Eagle Owl Bubo bubo, Grava, T., Mathevon, N., Place, E. & Balluet, P., Ibis, 150, 279–287, 2008, permission conveyed through Copyright Clearance Center, Inc., John Wiley and Sons; (r) Brightness variability in the white badge of the eagle owl Bubo bubo, Penteriani, V., Alonso-Álvarez, C., Delgado, M.M., Sergio, F. & Ferrer, M., Journal of Avian Biology, 37, 110–116, 2006, permission conveyed through Copyright Clearance Center, Inc., John Wiley and Sons; and (s) Vocal behaviour and neighbour spatial arrangement during vocal displays in eagle owls (Bubo bubo), Delgado, M.M. & Penteriani, V., Journal of Zoology, 271, 3–10, 2007, permission conveyed through Copyright Clearance Center, Inc., John Wiley and Sons.

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Appendix 1 List of prey species found in the diet of the Eagle Owl VERTEBRATES Mammalia

Chiroptera Basbastella basbastella Eptesicus serotinus Myotis bechsteinii Myotis blythii Myotis daubentonii Myotis myotis Myotis mystacinus Myotis nattereri Nyctalus noctula Pipistrellus pipistrellus Pipistrellus savii Plecotus auritus Rhinolophus ferrumequinum Tadarida teniotis Vespertilio murinus Eulipotyphla Atelerix (=Erinaceus) algirus Crocidura lasiura Crocidura leucodon (=lasia) Crocidura russula Crocidura sibirica Crocidura suaveolens (=mimula) Desmana moschata Erinaceus amurensis Erinaceus concolor Erinaceus europaeus E. europaeus italicus Erinaceus roumanicus Hemiechinus auritus Mesechinus dauricus Mogera robusta Mogera wogura Neomys anomalus Neomys fodiens Sorex alpinus Sorex araneus Sorex caecutiens Sorex coronatus Sorex isodon Sorex minutus Suncus etruscus Talpa altaica Talpa caucasica/coeca Talpa europaea Talpa occidentalis

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Carnivora Canis familiaris Felis catus Genetta genetta Herpestes ichneumon Martes foina Martes martes Martes zibellina Meles meles Mustela erminea Mustela lutreola Mustela nivalis Mustela putorius Mustela sibirica Mustela vison Putorius (=Mustela) eversmanni Vormela peregusna Vulpes vulpes Artiodactyla Capreolus capreolus Cervus elaphus Ovis aries Rupicapra rupicapra Sus scrofa Rodentia Allactaga bullata Allactaga elater Allactaga euphratica Allactaga major (=jaculus) Allactaga sibirica Allactaga williamsi Allocricetulus curtatus Alticola argentatus Alticola barakshin Alticola roylei Alticola strelzowi Apodemus (=Sylvaemus) microps Apodemus agrarius Apodemus flavicollis (=tauricus) Apodemus mystacinus Apodemus peninsulae Apodemus sylvaticus Arvicola sapidus Arvicola terrestris Cardiocranius paradoxus Cavia porcellus Chionomys (=Microtus) nivalis

Citellus (=Spermophilus) major Citellus (=Spermophilus) pygmaeus Citellus (=Spermophilus) undulatus Citellus citellus Clethrionomys glareolus Clethrionomys rufocanus Clethrionomys rutilus Cricetulus (=Tscherskia) triton Cricetulus barabensis Cricetulus longicaudatus Cricetulus migratorius Cricetus cricetus Dipus sagitta Dryomys nitedula Eliomys quercinus Ellobius fuscocapillus Ellobius talpinus Eolagurus luteus Eolagurus przewalskii Eutamias sibiricus Glis glis Jaculus jaculus Lagurus lagurus Lemmus lemmus Marmota bobak Marmota caudata Marmota marmota Marmota sibirica Meriones libycus Meriones meridianus Meriones tamariscinus Meriones tristrami Meriones unguiculatus Meriones vinogradovi Mesocricetus auratus Mesocricetus brandti Mesocricetus newtoni Micromys minutus Microtus (=Pitymys) pyrenaicus Microtus agrestis Microtus arvalis Microtus brandtii Microtus cabrerae Microtus carruthersi Microtus duodecimcostatus Microtus fortis Microtus gregalis

Microtus guentheri (=socialis) Microtus incertus Microtus irani Microtus kirgisorum Microtus liechtensteini Microtus limnophilus Microtus multiplex Microtus nivalis Microtus oeconomus (=ratticeps) Microtus rossiaemeridionalis Microtus savii Microtus subterraneus (=majori) Microtus tatricus Microtus thomasi Mus domesticus Mus macedonicus Mus musculus Mus spicilegus Mus spretus Muscardinus avellanarius Myocastor coypus Myomimus setzeri Myopus schisticolor Myospalax baileyi Myospalax psilurus Ondatra zibethicus Phodopus campbelli Phodopus sungorus Pitymys majori Pitymys pyrenaicus Pitymys subterraneus Psammomys obesus Pteromys volans Pygeretmus pumilio Rattus norvegicus Rattus rattus Rattus turkestanicus Rhombomys opimus Salpingotus crassicauda Sciurus anomalus Sciurus vulgaris Sicista betulina Spalax (=Nanospalax) leucodon Spalax microphtalmus Spermophilus pygmaeus Spermophilus undulatus Stylodipus telum

Appendix 1 Lagomorpha Caprolagus brachyurus (=Lepus brachyurus) Lepus capensis Lepus coreanus Lepus europaeus Lepus granatensis Lepus oiostolus Lepus timidus Lepus tolai Ochotona alpina Ochotona cansus Ochotona curzoniae Ochotona dauurica Ochotona pallasii Ochotona pusilla Ochotona rutila Oryctolagus cuniculus

Aves

Podicipediformes Podiceps auritus Podiceps cristatus Podiceps grisegena Podiceps nigricollis Tachybaptus ruficollis

Anas querquedula Anas strepera Anas zonorhyncha Anser albifrons Anser anser Anser domesticus Aythya ferina Aythya fuligula Aythya marila Aythya nyroca Bucephala clangula Clangula hyemalis Cygnus olor Histrionicus histrionicus Melanitta fusca Melanitta nigra Mergus albellus Mergus merganser Mergus serrator Netta rufina Oxyura leucocephala Somateria mollissima Tadorna ferruginea Tadorna tadorna

Ciconiiformes Ardea cinerea Ardea purpurea Ardeola ralloides Botaurus stellaris Bubulcus ibis Ciconia ciconia Ciconia nigra Egretta garzetta Ixobrychus minutus Nycticorax nycticorax Platalea leucorodia Plegadis falcinellus

Accipitriformes Accipiter gentilis Accipiter gularis Accipiter nisus Buteo buteo Buteo hemilasius Buteo lagopus Circaetus gallicus Circus aeruginosus Circus cyaneus Circus pygargus Elanus caeruleus Falco biarmicus Falco cherrug Falco columbarius Falco naumanni Falco peregrinus Falco rusticolus Falco subbuteo Falco tinnunculus Falco vespertinus Hieraaetus pennatus Milvus migrans Milvus milvus Pandion haliaetus Pernis apivorus

Anseriformes Aix galericulata Anas acuta Anas bahamensis Anas clypeata Anas crecca Anas domesticus Anas falcata Anas penelope Anas platyrhynchos Anas poecilorhyncha

Galliformes Alectoris chukar Alectoris graeca Alectoris rufa Ammoperdix griseogularis Bonasa bonasia Coturnix coturnix Gallus gallus domesticus Lagopus lagopus Lagopus mutus

Pelecaniformes Butorides striata Phalacrocorax (=Microcarbo) pygmeus Phalacrocorax aristotelis Phalacrocorax carbo Phalacrocorax filamentosus (=capillatus) Phalacrocorax pelagicus

Meleagris gallopavo domestica Perdix daurica Perdix perdix Phasianus colchicus Tetrao tetrix Tetrao urogallus Tetrao (=Lyrurus) mlokosiewiczi Tetraogallus altaica Tetraogallus himalayensis Tetrax tetrax Gruiformes Crex crex Fulica atra Gallinula chloropus Grus grus Porphyrio porphyrio Porzana parva Porzana porzana Porzana pusilla Rallidae Rallus aquaticus Tetrax tetrax Charadriiformes Actitis hypoleucos Aethia cristatella Alca torda Alle alle Arenaria interpres Burhinus oedicnemus Calidris alba Calidris alpina Calidris canutus Calidris maritima Calidris minuta Calidris ruficollis Cephus grylle Cepphus carbo Charadrius alexandrinus Charadrius dubius Charadrius morinellus Chlidonias leucopterus Chlidonias niger Ciceronia (=Aethia) pusilla Fratercula arctica Gallinago gallinago Haematopus ostralegus Himantopus himantopus Hydrocoloeus minutus Larus (=Chroicocephalus) ridibundus Larus argentatus Larus audouinii Larus cachinnans Larus canus Larus crassirostris Larus fuscus Larus genei

Larus marinus Larus michahellis Limosa limosa Lymnocryptes minimus Numenius arquata Numenius phaeopus Philomachus pugnax Pluvialis apricaria Pluvialis squatarola Puffinus puffinus Recurvirostra avosetta Rissa tridactyla Scolopax rusticola Stercorarius parasiticus Sterna hirundo Sterna macrura Sterna sandvicensis Sternula albifrons Synthliboramphus antiquus Tringa brevipes Tringa erythropus Tringa glareola Tringa nebularia Tringa ochropus Tringa totanus Uria aalge Vanellus vanellus Columbiformes Columba livia domestica Columba oenas Columba palumbus Columba rupestris Streptopelia decaocto Streptopelia orientalis Streptopelia senegalensis Streptopelia turtur Strigiformes Aegolius funereus Asio flammeus Asio otus Athene noctua Bubo bubo Bubo scandiacus (=Nyctea scandiaca) Glaucidium passerinum Ninox scutulata Nyctea scandiaca Otus bakkamoena Otus scops Otus sunia Strix aluco Strix nebulosa Strix uralensis Surnia ulula Tyto alba Caprimulgiformes Caprimulgus aegyptius Caprimulgus europaeus Caprimulgus ruficollis

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The Eagle Owl Cuculiformes Clamator glandarius Cuculus canorus Apodiformes Apus apus Apus melba Apus pacificus Apus pallidus Coraciiformes Alcedo atthis Coracias garrulus Merops apiaster Upupa epops Piciformes Dendrocopos major Dendrocopos minor Dendrocopos syriacus Dryocopus martius Jynx torquilla Picoides medius Picoides tridactylus Picus canus Picus viridis Psittaciformes Melopsittacus undulatus Platycercus eximius Passeriformes Corvidae Corvus corax Corvus cornix Corvus corone Corvus dauuricus Corvus frugilegus Corvus monedula Cyanopica cooki Cyanopica cyanus (=cooki) Garrulus glandarius Nucifraga caryocatactes Perisoreus infaustus Pica pica Pyrrhocorax graculus Pyrrhocorax pyrrhocorax Other Passeriformes Acanthis (=Carduelis) flavirostris Acridotheres tristis Acrocephalus arundinaceus Acrocephalus schoenobaenus Acrocephalus stentoreus Alauda arvensis Anthus campestris Anthus pratensis Anthus richardi Anthus spinoletta Anthus trivialis

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Bombycilla garrulus Carduelis cannabina Carduelis carduelis Carduelis chloris Carduelis spinus Cettia cetti Cinclus cinclus Coccothraustes coccothraustes Cyanistes caeruleus Delichon urbica Emberiza cia Emberiza cirlus Emberiza citrinella Emberiza hortulana Emberiza schoeniclus Eremophila alpestris Erithacus rubecula Ficedula hypoleuca Ficedula parva Fringilla coelebs Galerida cristata Galerida theklae Hippolais polyglotta Hirundo rustica Hypsipetes amaurotis (=Ixos amaurotis; =Microscelis amaurotis) Lanius collurio Lanius cristatus Lanius excubitor Lanius meridionalis Lanius minor Lanius senator Leucosticte arctoa Loxia curvirostra Lullula arborea Luscinia luscinia Luscinia megarhynchos Luscinia svecica Melanocorypha calandra Miliaria calandra Monticola saxatilis Monticola solitarius Montifringilla nivalis Motacilla alba Motacilla flava Muscicapa striata Oenanthe isabellina Oenanthe leucura Oenanthe oenanthe Oenanthe pleschanka Oriolus oriolus Parus ater Parus major Parus palustris (=montanus) Passer domesticus Passer montanus Petronia petronia Phoenicurus ochruros Phoenicurus phoenicurus

Phylloscopus sibilatrix Phylloscopus trochilus Phyloscopus bonelli Phyloscopus collybita Prunella collaris Prunella modularis Ptyonoprogne rupestris Pyrrhula pyrrhula Regulus ignicapillus Regulus regulus Riparia riparia Saxicola torquata Serinus citrinella Serinus serinus Sitta europaea Sitta tephronota Sturnus roseus Sturnus unicolor Sturnus vulgaris Sylvia atricapilla Sylvia borin Sylvia cantillans Sylvia hortensis Sylvia melanocephala Sylvia nisoria Sylvia undata Troglodytes troglodytes Turdus hortulorum Turdus iliacus Turdus merula Turdus obscurus Turdus pallidus Turdus philomelos Turdus pilaris Turdus torquatus Turdus viscivorus Zoothera dauma Pteroclidiformes Syrrhaptes paradoxus

Reptilia

Anguis fragilis Clemmys caspica Coluber (=Dolichophis) caspius Coluber (=Dolichophis) jugularis Coluber viridiflavus Coronella austriaca Elaphe longissima Elaphe scalaris Lacerta agilis Lacerta hispanica Lacerta lepida Lacerta muralis Lacerta saxicola Lacerta viridis Lacerta vivipara Malpolon monspessulanus Natrix maura Natrix natrix Natrix tessellata

Psammodromus algirus Tarentola mauritanica Testudo graeca Timon lepidus Vipera ammodytes Zamenis longissimus

Amphibia

Alytes obstetricans Bufo (=Pseudepidalea) raddei Bufo bufo Bufo viridis Hyla arborea Pelobates cultripes Pelobates fuscus Pelobates syriacus Rana arvalis Rana camerani (=macrocnemis) Rana dalmatina (=agilis) Rana esculenta Rana perezi Rana ridibunda Rana temporaria Salamandra salamandra

Osteichthyes

Abramis brama Anguilla anguilla Barbus barbus Barbus bocagei Barbus meridionalis Barbus tauricus Blicca bjoerkna Brosmius brosme Carassius auratus Chondrostoma nasus Chondrostoma polilepis Chondrostoma toxostoma Cyclopterus lumpus Cyprinus carpio Esox lucius Gadus morhua Gobio gobio Labrus berggylta Leuciscus cephalus Leuciscus idus Leuciscus rutilus Lophius piscatorius Lota lota Lota vulgaris Lucioperca lucioperca Oncorhynchus mykiss Perca fluviatilis Pollachius pollachius Pollachius virens Salmo trutta Salmonidae ni Scardinius erythrophthalmus Thymallus thymallus Vimba vimba tenella

Appendix 2

INVERTEBRATA Arachnida

Buthus occitanus Euscorpius carpathicus Scorpionidae ni Solifugae ni

Bivalvia

Dreissena polymorpha

Chilopoda

Scolopendra cingulata Scolopendra spp.

Gastropoda

Buccinum spp. Cepaea hortensis Cepaea nemoralis Limacidae ni Littorina obtusata Thais spp.

Insecta

Abax parallelus Acrididae ni Aegosoma scabricorne Akis spp. (Tenebrionidae) Amphimallon solstitialis Anacridium aegyptium Anechura bipunctata Blaps spp. Buprestidae ni Buprestis rustica

Calosima violaceus Calosoma maderae Calosoma sycophanta Capnodis tenebrionis Carabus auratus Carabus auronitens Carabus cancellatus Carabus coriaceus Carabus granulatus Carabus hortensis Carabus intricatus Carabus melancholicus Carabus monilis Carabus nemoralis Carabus purpurascens Carabus violaceus Cerambyx cerdo (Cerambicidae) Cerambyx scopolii Ceratophyus hoffmannseggi Cetonia aurata Chrysotribax hispanicus Colymbetes fuscus Copris hispanus (Scarabaeidae) Copris lunaris Cybister lateralimarginalis Decticinae spp. Decticus albifrons Dorcus parallelipipedus Dytiscidae ni Elateridae ni Ephippiger ephippiger

Ergates faber Euscorpius flavicaudis Geocoridae ni Geotrupes stercorosus Geotrupes vernalis (Scarabaeidae) Gryllus campestris Grylotalpa grylotalpa Haplotropis brunneriana Holotrichia diomphalia Hydrous piceus Hylobius abietis Locusta migratoria Lucanus cervus (Lucanidae) Lyristes plebejus Mantis religiosa Melolontha melolontha (Coleoptera) Netocia morio Nicrophorus spp. Odonata spp. Oedipoda germanica Ognevia sergii Orthoptera ni Oryctes nasicornis (Scarabaeidae) Oryctes spp. Pentodon bidens Pieridae ni Pimela spp. Pimelia costata Platycleis albopunctata Polysarcus denticauda Prismognathus subaneus

Procrustes coriaceus Rhizotrogus spp. Saga pedo Scarabeus sacer Scarites spp. Silpha carinata Silphidae ni Staphylinidae ni Tentyria spp. Tettigonia viridissima Tettigonidae spp. Tineidae ni Tomocarabus convexus Typhaeus momus Typhaeus spp. Typhaeus typhaeus (Coleoptera) Malacostraca Astacus fluviatilis Austropotamobius pallipes italicus Cambarellus montezumae Cambarus affinis Cancer pagurus Carcinus aestuari Carcinus maenas Decapoda ni Orconectes limosus Potamon potamius Procambarus clarkii

Appendix 2 List of abbreviations a.s.l. above sea level DDT dichlor-diphenyl-trichlor-ethane HPAI highly pathogenic avian influenza HRS home range size

MCP NND PCB RSD

Minimum Convex Polygon nearest-neighbour distance polychlorinated biphenyl reversed sexual size dimorphism

339

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Index Accipiter gentilis 156 Aegolius funereus 23, 132, 171, 182 age, and moulting 27–9, 311 agriculture, changes in 50, 121, 269 Albania 36, 84, 96 Alectoris rufa 125 alloparental care 200, 202 altitude, effect on clutch size 210 nesting sites 106–8 number of chicks 215, 216 prey species diversity 142, 153 timing of egg laying 205–6 amphibians 122, 134, 135, 139, 153 see also individual species Anas platyrhynchos 131 Apodemus spp. 125 Aquila adalberti 113 chrysaetos 10, 101, 156 fasciata 156, 180 pennata 160 pomarina 101 Ardea cinerea 101 lignitum 32 Arvicola amphibius 123, 134 terrestris 50 Asio flammeus 330 otus 9, 156, 299 Athene noctua 156 Austria 35, 77–9, 96, 118, 133, 208, 209, 211, 214, 215, 216, 225, 226, 244, 245 avian influenza (HPAI) 277 Azure-winged Magpie 330 AZWU (Initiative for Reintroduction of the Eagle Owl) 69, 70, 72, 73, 74 Badger 161 Barn Owl 132, 155, 156, 159, 187, 188, 315, 330 behaviour chicks see chicks fledging 195–6 foraging 123–31 home range see home range behaviour hunting 124, 127–31 movement during dispersal 247–50 nest-switching 200–2 parental 16, 194–5, 200 post-fledging 199–202, 224, 235–40 scavenging 131 see also visual communication; vocal communication Belarus 35, 62, 96, 101, 102, 117, 176, 187, 207, 209, 211 Belgium 35, 52, 68, 69–70, 73, 92, 95, 96, 101, 104, 118, 208, 209, 214 birds, in diet 134, 135, 136, 140, 142, 144, 146, 150

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birds of prey 134, 135, 146, 152, 156, 200, 270 see also individual species Black Grouse 10 Black Kite 101, 160, 165 Black Rat 126 Black Stork 80, 101 Blakiston’s Fish Owl 8 blood chemistry 10 Bonelli’s Eagle 101, 156, 164 Booted Eagle 113, 160, 164 Boreal Owl 22, 23, 25, 132, 171, 175, 182 Bosnia-Herzegovina 36, 83, 96 breeding brooding 177–8, 183–5, 194, 216 cooperative breeding 175 copulation 174, 283 density 115–21 divorce rates 175, 262 egg laying see eggs/egg laying genetic diversity 29–32 home range behaviour 224–35 number of breeding pairs 35 pair bonds 175–6, 226, 262 parental behaviour 16, 23, 194–5, 200 parental fitness 178, 179 proximity to hunting habitat 103–4 renesting 178, 180, 181 sexual maturity 174 see also breeding performance; eggs/egg laying; nesting sites breeding dispersal 241, 262–4 breeding performance breeding failures 205, 329–30 clutch size 176–7, 185, 210–11 diet-related variables 217–20 fecundity 212–20, 304 and landscape 220–2, 229–31 number of chicks 214–16 timing of egg laying 203–9 and weather 222 Britain 35, 64–8, 96 brooding 177–8, 183–5, 194, 216 Brown Rat 47, 78, 110, 122, 123, 134 Bubo ascalaphus 11, 15, 18, 19, 20, 21, 22, 177 Bubo bengalensis 11, 15, 22, 66, 194 Bubo blakistoni 8 Bubo bubo 8–10, 16 b. armeniacus 18 b. auspicabilis 15, 19 b. bashkiricus 18 b. borissowi 15, 19 b. bubo 11, 12, 17, 18, 20, 21, 24, 25, 58, 73, 81, 133 b. dauricus 19 b. davidi 33 b. desertorum 22 b. eversmanni 18 b. gladkovi 14, 17, 19 b. hemachalana 14, 17, 19, 20, 21, 133, 264 b. hispanus 12, 18, 19, 20, 21, 22, 24, 25, 30, 133 b. inexpectatus 19

Index b. interpositus 12, 13, 17, 18, 19, 20, 21, 22, 58, 133 b. jakutensis 14, 19, 20, 21 b. jarlandi 19 b. kiautschensis 14, 19, 21, 133 b. meridionalis 18 b. nikolskii 13, 18, 20, 21 b. omissus 14, 15, 17, 19, 20, 21, 264 b. ruthenus 13, 17, 18, 20, 21, 58, 133 b. setschuanus 19 b. sibiricus 13, 17, 18, 20, 21, 54, 58, 81, 133, 263 b. swinhoei 19 b. tarimensis 13, 15, 18 b. tenuipes 19 b. tibetanus 14, 15, 19 b. turcomanus 13, 15, 17, 18, 19, 20, 21, 22, 58, 133, 264 b. ussuriensis 14, 19, 20, 21, 133 b. yamashinai 19 b. yenisseensis 13, 17, 18, 19, 20, 21, 54, 264 b. zaissanensis 19 Bubo incertus 32 Bubo lignitum 32 Bubo scandiacus 8, 21, 187, 188 Bubo virginianus 9–10, 21, 66, 187, 188, 325, 331 Bulgaria 36, 83–4, 96, 104, 106, 108, 109, 124, 133, 177, 207, 209 Buteo buteo 10, 156 Buzzard 10, 101, 113, 156, 159, 160

climate, effects 50, 133, 142, 152, 222 clutch size 176–7, 185, 210–11 collision, as cause of death 266, 268, 270–3, 279 colour patterning 8, 9, 21, 315 colour perception 310 Columba palumbus 330 Common Scops Owl 182 communication see visual communication; vocal communication competitor-removal hypothesis 155, 158, 161 cooperative breeding 175 copulation 174, 283 Cormorant 101 corvids 55, 133, 134, 135, 138, 146, 152 see also individual species Corvus corax 101, 161 cornix 101 frugilegus 101 crepuscular activity 16, 310–11 Cricetidae spp. 152 Cricetus cricetus 78 Crimea 18, 35, 58, 94 Croatia 36, 82, 96, 277 Cyanopica cyanus 330 Czech Republic 15, 24, 36, 72, 80, 81, 96, 101, 102, 104, 105, 109, 117, 133, 176, 207, 211, 214, 215, 216, 244, 245

call posts 293–6, 297 cannibalism 193 Capra 131 Capreolus capreolus 131 Caprimulgus ruficollis 311 carnivores 61, 122, 134, 135, 137, 142, 156, 157, 158, 217 see also individual species Carnus haemapterus 278 carrion consumption 131 Central Asia 17, 19, 20, 21, 35, 52–60, 95, 104, 117, 135, 207, 210, 211, 214, 263 Chicken 131 chicks climbing 195–6 development 186–93 fledging 195–202 in nest 188–93 nest-switching 200–2 nestling mortality 213 numbers 214–16 post-fledging 196–202, 224, 235–40 prey delivery to 195 sex allocation 182–5 visual communication 315–17, 326–8 vocal communication 280–2, 285–9 weight 186–8 China 17, 18, 19, 133 Chlamydia psittaci 277 Ciconia ciconia 101 nigra 80, 101 Circus cyaneus 64 Clanga pomarina 164

dawn choruses 309, 320, 321 death see mortality Denmark 15, 24, 35, 47, 95, 96, 101, 104, 133, 225, 226, 231 departure 242–4 dichromatic colour vision 332 diet breeding performance 217–20 diversity 133, 135, 141, 151, 153, 301 general patterns 152–3 global patterns 133–53 and home range behaviour 232 see also food availability diseases 276–7 dispersal age 244, 257 breeding 241, 262–4 departure 242–4 direction 250–2 evolutionary traps 260–1 habitat preference 234, 254 and home range behaviour 235–40 and mortality risk 247, 260 natal see natal dispersal seasonal migration 263–4 settlement 242, 257–62 sex-biased 258–9 temporary settlements 254–7 wandering 242, 245–54 distribution 34–96 Europe 37 global 37 overview 34–7, 94–6 by subspecies 17–22

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The Eagle Owl diurnal raptors 10, 22, 132, 138, 147, 157, 160, 161, 162, 168, 174, 217 see also individual species divorce rates 175, 262 Dove 156 drainage, at nesting sites 100 dusk choruses 298, 306–9, 321, 322 Edible Dormouse 107, 111 eggs/egg laying brooding 177–8, 183–5, 194, 216 clutch size 176–7, 185, 210–11 collectors 269 cooperative breeding 175 and disease 277 hatching asynchrony 183–5 incubation 176–8, 181–2 multiple broodings 177–8 replacement clutches 178–81 sex allocation 182–5 size of eggs 176 timing 176, 179–80, 203–9 see also chicks Egretta garzetta 330 Egyptian Vulture 164 electrocution 40, 50, 80, 266, 268, 270–3 Eliomys quercinus 132 Erinaceus europaeus 75, 134 Escherichia 276 Estonia 35, 61, 96, 115, 117, 133, 207, 214, 215, 246 evolutionary traps 260–1 eye colour 9 faeces, as communication 328–31 Falco cherrug 80, 102 peregrinus 10, 162 tinnunculus 102, 156 falcon herpesvirus (FHV) 277 farmland habitats 50, 121, 269 fecundity 212–17, 304 feeding ecology 122–53 food stores 132–3, 161 foraging behaviour 123–31 global diet patterns 133–53 temporal changes in 124–7 see also food availability feeding time hypothesis 132–3 females breeding dispersal 262 home range behaviour 224, 226, 227, 232, 234, 236, 237, 238, 294 mortality 276 as parent 194, 195 role of 173, 174, 181 temporary settlements 254 territorial contests 319 visual communication 312–15, 317, 319, 321 vocal communication 282, 283–4, 289–96, 297, 298, 303 Finland 15, 24, 29, 35, 47–52, 61, 73, 95, 96, 104, 109, 115, 116, 117, 120, 121, 123, 133, 161, 163, 164, 175, 203, 207, 209, 211, 214, 215, 216, 232, 243, 244, 245,

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246, 250, 257, 258, 259, 261, 262, 266, 268, 269, 270, 278, 292 fish 112, 122, 123, 124, 129, 134, 135, 139 fledglings 195–202 visual communication 315–17, 326–8 vocal communication 280–2, 285–9 flying abilities 9–10 food availability and breeding density 120, 300 and breeding performance 217–20 food stores 132–3, 161 food-stress 155, 159 and home range behaviour 232 and mortality risk 155, 278, 279 and sex allocation 182–3 temporal changes in 124–7 and timing of egg laying 204 see also diet; feeding ecology food-stress 155, 159 foraging behaviour 123–31 France 15, 16, 24, 32, 36, 73, 88–92, 95, 96, 101, 102, 104, 108, 109, 112, 115, 116, 118, 120, 123, 124, 129, 131, 133, 135, 158, 162, 164, 175, 176, 204, 205, 208, 209, 211, 212, 214, 215, 216, 221, 259, 266, 268, 270, 290, 298, 302, 303 Galliformes 134, 135, 138, 146, 152 Gallus domesticus 131 Garden Dormouse 132 Garrulus glandarius 283 genetic diversity 29–32 geographical variations characteristics 8 subspecies 17–19 geomorphology, and nesting sites 103–4 Germany 15, 24, 30, 35, 47, 68, 71–6, 90, 95, 96, 101, 102, 104, 108, 109, 113, 115, 118, 124, 133, 159, 162, 163, 175, 176, 205, 207, 209, 211, 214, 215, 216, 222, 225, 226, 231, 235, 245, 254, 266, 268, 270, 277, 291, 302 Golden Eagle 10, 101, 156, 160, 164 Goshawk 101, 113, 156, 162–3, 172 Great Grey Owl 8 Great Horned Owl 9–10, 21, 66, 187, 188, 195, 325, 331 Greece 15, 24, 36, 84, 96, 109, 133, 135 Grey Heron 101 Grouse 10, 156 guilds 154–5 Gull 131, 164, 232, 279 habitat destruction 40, 50, 121, 269 habitat heterogeneity 111, 127, 168, 234 habitat preference breeder/disperser differences 234, 254 and breeding performance 220–2, 229–31 haematology 10 Haliaeetus albicilla 42, 101, 160, 164, 294 Hamster 78 Hare 40, 78, 97, 122, 156, 158, 217, 221 hatching asynchrony 183–5 Hawk Owl 27 head ornamentation 9, 332–3

Index hearing sensitivity 9, 124 Hedgehog 75, 107, 110, 122, 123, 134, 153, 158, 221 Hen Harrier 64 Herpesvirus strigis 276–7 heterozygosity 30 Hieraaetus fasciatus 101 pennatus 113, 164 home range behaviour 223–4 and biological cycle 233–4 breeder sizes 225, 226 in breeders 224–35 call posts 293–4 and diet-related variables 232 in dispersers 235–40 and habitat 229–31 and nocturnal activity 227–9, 230, 231, 232–3 homozygosity 30 Hooded Crow 101 House Mouse 178 human disturbance 40, 42, 50, 121, 164, 194, 269 human habitat 104–5, 112, 164, 269, 279 Hungary 36, 81, 96, 101, 133, 207, 216 hunting behaviour 124, 127–31 habitat 103–4, 110–11 incubation 176–8, 181–2 Indian Eagle Owl 11, 15, 20, 22, 66, 194 infectious diseases 276–7 insectivores 134, 135, 137, 142, 144, 152 see also individual species interspecific interactions competitor-removal hypothesis 155 lethal 154, 155–6 at nesting sites 161–72 non-lethal 154 predator-removal hypothesis 159, 160–1, 162 superpredation 156–9 intraguild predation 155, 156, 158, 159, 165–6 predator-avoidance 165, 166, 167, 168, 169, 170 predator-removal hypothesis 160–1, 162 invertebrates 122, 134, 135, 139 see also individual species isothermality and prey species biomass 148, 149, 150 and prey species diversity 143, 144, 145, 147 Italy 15, 24, 36, 93–4, 96, 104, 106, 107, 108, 109, 110, 112, 118, 119, 121, 133, 165, 208, 209, 211, 212, 214, 215, 259, 266, 268, 269, 270, 276, 298 Jay 283 Kazakhstan 17, 19, 58, 133, 176, 207, 210, 211, 214, 215, 216, 263 Kestrel 102, 156, 159 Kosovo 36, 84, 96 Kyrgyzstan 58, 133 lagomorphs 127, 128, 133, 134, 135, 137, 140, 142, 145, 147, 148, 152 see also individual species

Lagopus mutus 129 Lagurus lagurus 134 landscape breeder/disperser differences 234, 254 and breeding performance 220–2, 229–31 Larus 330 canus 131, 164 latitude, effect on clutch size 210 nesting sites 108 number of chicks 215, 216 prey species biomass 148 prey species diversity 142, 144, 145, 147, 152 timing of egg laying 205–6 Latvia 35, 61, 96, 102, 104, 207, 214, 215 lead poisoning 275 Lepus europaeus 78 timidus 40 Lesser Spotted Eagle 101, 164 lethal interactions 154, 155–6 Leucocytozoon ziemanni 278 Liechtenstein 35, 77, 96 life expectancy 174 Lithuania 35, 62, 96 Little Bustard 310 Little Egret 330 Little Owl 156 Long-eared Owl 9, 156, 159, 299 longitude, effect on clutch size 210 number of chicks 215, 216 prey species biomass 148, 149, 150 prey species diversity 142, 143, 144, 145, 146, 147, 152–3 timing of egg laying 205–6 Lucilia cuprina 277 sericata 277 Luxembourg 35, 70, 73, 95, 96, 104, 118, 209, 214 Lynx lynx 278 Macedonia 36, 84, 96 males breeding dispersal 262 home range behaviour 224, 226, 227, 232, 234, 236, 237, 238, 293–4 mortality 276 as parent 194, 195 role of 173, 174 temporary settlements 254 territorial contests 319 visual communication 312–15, 317, 318–19, 321 vocal communication 282, 283, 284, 289–96, 297–8, 299–301, 303, 307–8 Mallard 131 mammals 22, 25, 32, 61, 64, 110, 129, 133, 134, 135, 136, 140, 142–5, 147–9, 152, 157, 258, 284, 301, 328, 330–1, 332 see also individual species Martes martes 161 Meles meles 161 mercury poisoning 42–3, 273–4

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The Eagle Owl mesopredators 154, 155, 156–7, 158, 159, 165–72, 217, 218, 278 see also individual species metal contamination 274–5 Mew Gull 164 Mexican Spotted Owl 325 Microtinae spp. 152 Microtus 125 agrestis 50 arvalis 134 duodecimcostatus 134 gregalis 134 rossiaemeridionalis 50 Milvus migrans 101, 160 milvus 160 Moldova 16, 18, 35, 63, 96, 178 Mongolia 18, 133 Montenegro 36, 84, 96 moon, effect on home range behaviour 232–3 natal dispersal 244 visual communication 323–6 mortality age-dependent 275–6, 278 causes 38, 39–40, 41, 42, 267–75 collision 266, 268, 270–3, 279 diseases 276–7 and dispersal 247, 260 electrocution 40, 50, 80, 266, 268, 270–3 and genetic diversity 30, 31 human-related 265, 269 natural 265, 268 parasitisms 277–8 persecution 266, 268–9 pesticides 273–5 pollutants 273–5 power lines 265, 266, 270–3 predation 278 sex-dependent 276, 278 starvation 278, 279 weather 278 moult patterning 27–9 Mountain Hare 40 multiple broodings 177–8 Muridae spp. 152 Mus 125 musculus 178 Mustela erminea 156 nivalis 156 Myodes glareolus 134 Myoxus glis 107 natal dispersal age 244, 257 departure 242–4 direction 250–2 evolutionary traps 260–1 habitat preference 254 and home range behaviour 235–40 moon phases 244 and mortality risk 247, 260

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research 241, 261–2 settlement 242, 254–62 sex-biased 258–9 temporary settlements 254–7 transience 242 wandering 242, 245–54 nearest neighbour distance (mean NND) 116–19 Neophron percnopterus 164 nest-switching 200–2 nesting sites and altitude 106–8 conspecific density 298–300 and fecundity 212 fidelity and occupancy 113–14 and fledging age 195–6 and geomorphology 100, 103–4 habitat 110–12, 220–2 interactions at 161–72 nest spacing 115–21, 178 and vocal communication 298–301, 306–9 see also nests nestlings visual communication 315–17, 326–8 vocal communication 280–2, 285–9 nests abandoning 178, 179, 180, 182 artificial 102 characteristics 98–102 orientation 108–10 preparation 173–4 spacing 115–21, 178 see also nesting sites Netherlands 24, 35, 68–9, 96, 131, 133, 207, 209, 211, 215, 244, 245, 246 New World Great Horned Owl 9–10, 21, 66, 187, 188, 195, 325, 331 Newcastle disease virus (NDV) 277 Nightjar 99, 311 Norway 15, 24, 32, 35, 38–42, 45, 65, 66, 73, 95, 96, 123, 124, 126, 133, 225, 226, 231, 245, 257, 261, 262, 269, 270, 271, 274, 279, 292, 294 Nyctea scandiaca 21 Ogygoptynx wetmorei 32 Omsk haemorrhagic fever with renal syndrome (OHFRS) 277 optimal foraging theory 128, 129, 217, 219 orientation of nests 108–10 origins 32–3 Oryctolagus cuniculus 75, 134 Otus asio 187 scops 182 Ovis 131 owl herpesvirus (OHV) 277 pair bonds 175–6 parasitisms 277–8 Partridge 156, 275 Peregrine Falcon 10, 162 persecution 34, 116, 121, 268–9 and nesting sites 97–8, 105–6 as percentage of deaths 265, 266, 267

Index pesticides, as cause of death 273–5 Phalacrocorax carbo 101 Pharaoh Eagle Owl 11, 15, 18, 19, 20, 21, 22, 177 Pigeon 52, 156, 158, 217, 221, 275 Pine Marten 161 plumage achromatic 311–20, 332 adults 315–17 ageing 27–9, 311 chicks 188–93, 315–17, 326–8 colour patterning 8, 9, 21, 315 head ornamentation 9, 332–3 moult patterning 27–9 prey feathers 328–9, 330, 331 and silent flight 10 white throat badge 315–21 poisonings 42–3, 273–5 Poland 35, 79, 95, 96, 101, 102, 104, 107, 115, 117, 118, 133, 207, 211, 214, 215, 231, 279 pollutants, as cause of death 273–5 populations by country 38–96 densities 116 genetic diversity 29–32 overview 34–7, 94–6 Portugal 36, 85–6, 96, 102, 104, 109, 119, 126, 131, 133, 135, 208, 209, 211, 214 post-fledging dependence 196–202, 224, 235–40 power lines 265, 266, 270–3 predation, of Eagle Owls 278 predator-avoidance 165, 166, 167, 168, 169, 170 predator-removal hypothesis 159, 160–1, 162 prey species biomass 135, 140, 148, 149, 150 contamination 274–5 diversity 142, 143, 144, 145, 146, 147, 152–3 main groups 134 mean weight 141, 151, 153 spatial patterns of distribution 135–41 quarries, and nesting sites 104 Rabbit 52, 75, 87, 105, 110, 111, 116, 120, 122, 125, 126, 127, 128–30, 131, 132, 134, 152, 153, 155, 156, 157, 158, 217, 218, 219–20, 232, 250, 272, 275 rainfall, effect on breeding performance 222 mortality risk 278 prey species biomass 148, 149, 150 prey species diversity 142, 143, 145 Rana temporaria 134 Raptor Grid Project 47 Rat 40, 47, 78, 105, 110, 122, 123, 126, 132, 134, 152, 153, 217, 218 Rattus 110, 152, 217 norvegicus 48, 134 rattus 126 Raven 101, 161 Red Fox 156, 161, 278 Red Kite 160 Red-legged Partridge 125, 330 reintroduction projects 29–32, 39, 44, 46, 47, 73–4, 81 replacement clutches 178–81

reptiles 134, 135, 139, 153 see also individual species reversed sexual size dimorphism (RSD) 22, 23, 25, 182, 183, 315 Rhipicephalus spp. 278 river habitat 112 Rock Eagle Owl 11, 15, 20, 22, 66, 194 Rock Ptarmigans 129 rodenticides 44, 45, 69, 274 rodents 125, 134, 135, 137, 140, 142, 143, 148, 149, 152, 153, 156, 274 see also individual species Roe Deer 131 Romania 36, 81–2, 96, 133, 211 Rook 101 RSD (reversed sexual size dimorphism) 22, 23, 25, 182, 183, 315 Russia 15, 16, 17, 18, 19, 21, 24, 25, 35, 52–60, 63, 66, 95, 97, 101, 104, 108, 115, 117, 133, 135, 176, 200, 207, 211, 214, 215, 216, 266, 268, 269, 273, 274, 275, 277 Saker Falcon 80, 102 salmonellosis 277 scavenging behaviour 131 Scops Owl 182, 292 seasonal migration 263–4 Serbia 36, 83, 96 settlement 242, 257–62 sex allocation 182–5 sex-biased dispersal 258–9 sexual dichromatism 311, 312, 313, 314 sexual dimorphism 16, 22–6, 182, 184, 315 sexual maturity 174 Shannon index 133, 135, 141, 151, 152, 301 Short-eared Owl 330 siblings post-fledging distance 199 siblicide 183, 184, 185 silent flight 10 size dimorphism 22–6, 184, 314, 315 variations in 9, 11–15, 16 Slovakia 36, 73, 78, 80–1, 96, 101, 133, 207, 211, 216, 244 Slovenia 36, 82, 96, 133 Snowy Owl 8, 21, 187, 188 soil nutrients, at nesting sites 100 South Korea 133, 135, 225, 226, 277 Spain 15, 16, 24, 25, 26, 30, 31, 32, 36, 85, 86–8, 95, 96, 101, 104, 106, 109, 111, 112, 114, 115, 116, 119, 120, 125, 126, 129, 131, 132, 133, 135, 161, 164, 174, 175, 177, 179, 184, 185, 187, 193, 197, 204, 205, 208–9, 211, 214, 215, 216, 218, 219, 225, 226, 229, 232, 243, 244, 245, 246, 247, 250, 252, 254, 255, 257, 258, 261, 262, 263, 266, 267, 268, 269, 270, 272, 274, 275, 276, 277, 278, 292, 298, 300, 302, 303, 304, 307, 313 Spanish Imperial Eagle 113 starvation 278, 279 Stoat 156 Stork 80, 101, 113 Strigidae 21 Strigiphilus striges 277

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The Eagle Owl Strix aluco 9, 156 nebulosa 8 occidentalis lucida 325 uralensis 171, 284 superpredation 156–9, 217, 218 Surnia ulula 27 Sus scrofa 131 Sweden 15, 24, 32, 35, 39, 40, 42–6, 96, 100, 114, 116, 117, 133, 207, 209, 214, 216, 244, 245, 246, 259, 266, 268, 270, 273, 274, 279 Switzerland 15, 24, 35, 73, 76–7, 90, 96, 108, 116, 118, 124, 129, 131, 133, 208, 210, 214, 215, 216, 225, 226, 229, 243, 244, 245, 246, 254, 257, 259, 261, 266, 267, 268, 270, 271, 278 Syria 18, 22, 133 Tawny Owl 9, 156, 159, 166–70, 175, 182, 299, 302, 331–2 temperature, effect on breeding performance 222 prey species biomass 148, 149, 150 prey species diversity 143, 144, 145, 146, 147, 153 temporary settlement areas 254–7 Tengmalm’s Owl 23, 25, 171, 175, 182 Tetrao tetrix 10 Tetrax tetrax 310 Toxoplasma gondii 277–8 transience 242 Turkey 36, 84, 133, 135, 277 Tyto alba 132, 155, 187, 188, 315, 330 Tytonidae 21 Ukraine 18, 35, 63, 96, 133 Ural Owl 171, 175, 182, 284 valleys, and nesting sites 103 Viper 279 Vipera berus 279 vision 9, 124, 310, 331–2 visual communication achromatic plumage 311–20 adults 315–21 colour perception 310

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faeces 328–31 females 312–15, 317, 319, 321 head ornamentation 332–3 males 312–15, 317, 318–19, 321 and moonlight 323–6 prey feathers 328–9, 330, 331 role of white throat badge 315–21 and vision 310, 331–2 and vocalisation 320–3 and young 326–8 vocal communication adults 282–5, 289–96 alarms 284, 292 call posts 293–6, 297 conspecific density 298–301, 305 daily/seasonal 285–92, 296, 298 dawn choruses 309, 320, 321 dusk choruses 298, 306–9, 321, 322 females 282, 283–4, 289–96, 297, 298, 303 January to mid-February period 296, 297, 298 males 282, 283, 284, 289–96, 297–8, 299–301, 303, 307–8 and moonlight 323 October–December period 296, 297, 298 population characteristics and call identity 302–6 and white throat badge contrast 320–3 and young 280–2, 285–9 Vole 48, 50, 97, 120, 125, 127, 131, 134, 217, 232, 250 vulnerability levels 16 Vulpes vulpes 156, 161 wandering 242, 245–54 Water Vole 40, 50, 123, 126, 127, 134, 274, 279 Weasel 156 weather, effect on breeding performance 222 mortality risk 278 vocal communication 282 White Stork 101, 113 White-tailed Eagle 101, 160, 164, 294 white throat badge, role 315–21 Wild Boar 131 wings 9, 11–15, 20, 24, 27–9 Wood Pigeon 330