Behavioural Ecology of Western Palearctic Falcons [1st ed.] 9783030605407, 9783030605414

Table of contents :
Front Matter ....Pages i-xvi
Western Palearctic Falcons (Giovanni Leonardi)....Pages 1-33
Reproductive Strategies (Giovanni Leonardi)....Pages 35-93
Competition and Defence (Giovanni Leonardi)....Pages 95-109
Exploitation of Resources (Giovanni Leonardi)....Pages 111-139
Dispersal Patterns (Giovanni Leonardi)....Pages 141-168
Communication (Giovanni Leonardi)....Pages 169-185
Living in Groups (Giovanni Leonardi)....Pages 187-200
Back Matter ....Pages 201-206

Citation preview

Giovanni Leonardi

Behavioural Ecology of Western Palearctic Falcons

Behavioural Ecology of Western Palearctic Falcons

Giovanni Leonardi

Behavioural Ecology of Western Palearctic Falcons

Giovanni Leonardi Hierofalcon Research Group Catania, Italy

ISBN 978-3-030-60540-7    ISBN 978-3-030-60541-4 (eBook) https://doi.org/10.1007/978-3-030-60541-4 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

To my family Giovanni Leonardi

Preface

This book, in many ways, represents a real challenge. First, it could be considered as the ideal continuation of very important monographs such as Population ecology of raptors (Newton 1979), Falcons of the World (Cade 1982), and many others entirely devoted to some falcons (i.e. Ratcliffe 1980; Village 1990). Second, there is a huge number of publications about falcons, including several comprehensive studies. Third, it has proved particularly difficult to find old papers and those published in local journals, the so-called grey literature. At the end of this bibliographic survey in order to write this book, more than 3,300 publications entirely dedicated to falcons have been consulted, of which about 30% belong to the grey literature. The historical analysis of this long series of publications clearly reveals the birth and development of a new approach in the methodology used in studies on falcon behaviour. One of the first papers devoted on falcons with this modern point of view was produced by A. J. Cavé on common kestrels (Cavé 1968). This is a seminal work where he built up the main research design used in future papers. In the same way, the study conducted by Tom Cade on the behavioural ecology of gyrfalcons and peregrine falcons in Alaska can be considered innovative (Cade 1960). This new and different approach came from previous studies on other birds and has been well delineated by famous David Lack’s books (Lack 1954, 1966), for example new approach and terminology on breeding biology (parental investment, clutch and brood size, etc.) derived from Perrins (1965), as well as the main study by Hinde (1956) on territoriality and stimulated papers on the same matter by Cade (1960) and Ratcliffe (1962). Before that, papers concerned mainly with the analysis of behavioural patterns such as causation and ontogeny; a very fine example is a study by L. Tinbergen (1940) on common kestrels’ nesting behaviour. The great majority of bibliographic references concern a small number of falcons that are the best subjects for both field and laboratory research. The common kestrel is an appropriate study subject, since this bird easily accepts nesting boxes and breeds in dense populations where nesting sites are available and the area is otherwise suitable (Fig. 1; Cavé 1968), as well as captive American kestrels, which are suitable for various experimental sessions for testing behavioural patterns from predation to breeding (Bird 1982). The lesser kestrel is also an appropriate model for vii

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Fig. 1  The common kestrel is one of the most studied falcon both for field and laboratory research. (Credit: Arno van Zon)

the study of species–habitat relationships because of its role as biological indicator for the monitoring of population dynamics (Bustamante 1997). Although, this book is devoted to falcons of the Western Palearctic, it is quite impossible not to include data from other ecozones primarily because falcons are quite widespread in several ecozones. Over the past 30 years, studies on falcons have separated through two main routes which are unfortunately moving away from each other. The first route has broadened the horizons of research on falcons by considering them an excellent group for studying the aspects of ecology on a large spatial and temporal scale. Accordingly, a growing number of papers are published in wide-ranging ecology journals. On the other hand, a large number of papers are published on non-­academic journals but they represent a key source of local information. The main aim of this book is to provide an updated picture on the main aspects of behavioural ecology of falcons that is accessible to all categories of readers. Catania, Italy  Giovanni Leonardi

References Bird DM (1982) The American kestrel as a laboratory research animal. Nature 299:300–301 Bustamante J (1997) Predictive models for lesser kestrel Falco naumanni distribution, abundance and extinction in southern Spain. Biol Conserv 80:153–160 Cade T J (1960) Ecology of the peregrine and gyrfalcon populations in Alaska. Univ Calif Publ Zool 63:151–290 Cade TJ (1982) The Falcons of the World. Cornell University Press, Ithaca

Preface

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Cavé AJ (1968) The breeding of the kestrel, Falco tinnunculus L., in the reclaimed area Oostelijk Flevoland. Neth Jour Zool 18(3):313-40. https://doi.org/10.1163/002829668X00027 Hinde RA (1956) The biological significance of territories of birds. Ibis 98:340–369 Lack D (1954) The Natural Regulation of Animal Numbers. Oxford University Press, Oxford Lack D (1966) Population studies of birds. Oxford University Press, Oxford Newton I (1979) Population Ecology of Raptors. Poyser, Berkhamsted Perrins CM (1965) Population fluctuations and clutch-size in the Great Tit, Parus major L. J Anim Ecol 34:601–647 Ratcliffe DA (1962) Breeding density in the Peregrine Falco peregrinus and Raven Corvus corax. Ibis 104:13–39 Ratcliffe DA (1980) The Peregrine Falcon. Poyser, Calton Tinbergen L (1940) Beobachtungen über die Arbeitsteilung des Turmfalken (Falco tinnunculus) während der Fortpfanzungszeit. Ardea 29:63–98 Village A (1990) The Kestrel. Poyser, Calton 14 July 2020

Acknowledgements

This book could not have been prepared without the kind assistance of many people and institutions. This project was born some 7  years ago and many people have kindly contributed their personal field observations, publications, suggestions and comments. I would like to offer special thanks to all of the photographers who permitted the use of their stunning images that greatly improved the quality of this book. I am particularly grateful to Marco Preziosi and Gaia Sorrentino for their amazing artwork for this book.

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Contents

1 Western Palearctic Falcons ��������������������������������������������������������������������    1 1.1 The Falconidae Family����������������������������������������������������������������������    1 1.1.1 The Genus Falco ������������������������������������������������������������������    3 1.1.2 Subspecies����������������������������������������������������������������������������    8 1.1.3 Endemic Subspecies�������������������������������������������������������������   10 1.1.4 Hybrids����������������������������������������������������������������������������������   11 1.2 Plumage and Bare Parts��������������������������������������������������������������������   12 1.2.1 Pigmentary Colours��������������������������������������������������������������   13 1.2.2 Polymorphism����������������������������������������������������������������������   14 1.2.3 Effects of Melanin- and Carotenoid-Based Colouration������   15 1.3 Reversed Size Dimorphism��������������������������������������������������������������   18 1.4 Flight Performances��������������������������������������������������������������������������   20 1.5 Vision������������������������������������������������������������������������������������������������   21 1.6 Behavioural Repertoire of Falcons ��������������������������������������������������   23 References��������������������������������������������������������������������������������������������������   25 2 Reproductive Strategies��������������������������������������������������������������������������   35 2.1 The Breeding Life History Stage������������������������������������������������������   35 2.2 Developmental Phase������������������������������������������������������������������������   36 2.2.1 Photoperiod ��������������������������������������������������������������������������   36 2.2.2 Moult������������������������������������������������������������������������������������   38 2.2.3 Food Availability During the Developmental Phase ������������   39 2.2.4 Weather Effects ��������������������������������������������������������������������   39 2.3 The Breeding Population������������������������������������������������������������������   41 2.3.1 First Year and Second Calendar Year Breeders��������������������   41 2.3.2 Inbreeding ����������������������������������������������������������������������������   43 2.3.3 Polyandry, Polygyny, and Cooperative Breeding ����������������   43 2.3.4 Full Adults Non-breeders������������������������������������������������������   44 2.4 Mate Choice��������������������������������������������������������������������������������������   44 2.4.1 Copulation and Extra-Pair Fertilization��������������������������������   46 2.4.2 Assortative Mating: Heterozygosity ������������������������������������   47 xiii

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2.4.3 Early Mate Replacement������������������������������������������������������   47 2.5 Nest-Site Choice ������������������������������������������������������������������������������   48 2.5.1 Height and Slope������������������������������������������������������������������   49 2.5.2 Aspect ����������������������������������������������������������������������������������   51 2.5.3 Nest Structure�����������������������������������������������������������������������   51 2.5.4 Public Information����������������������������������������������������������������   51 2.5.5 Area Surrounding the Nest���������������������������������������������������   52 2.5.6 Occupancy and Alternative Nest Sites����������������������������������   53 2.6 Egg-Laying ��������������������������������������������������������������������������������������   54 2.6.1 Early and Late Breeding ������������������������������������������������������   54 2.6.2 Maternal Resources��������������������������������������������������������������   55 2.6.3 Eggs��������������������������������������������������������������������������������������   56 2.6.4 Re-laying������������������������������������������������������������������������������   58 2.7 Clutch Size����������������������������������������������������������������������������������������   59 2.7.1 Extra Eggs����������������������������������������������������������������������������   60 2.8 Incubation ����������������������������������������������������������������������������������������   60 2.8.1 Desertion ������������������������������������������������������������������������������   61 2.9 Hatching��������������������������������������������������������������������������������������������   61 2.9.1 Hatching Failures������������������������������������������������������������������   62 2.9.2 Hatching Asynchrony������������������������������������������������������������   62 2.10 Nestlings ������������������������������������������������������������������������������������������   63 2.10.1 Growth of Nestlings��������������������������������������������������������������   64 2.10.2 Relationships Between Survival and Environmental Conditions����������������������������������������������������������������������������   65 2.10.3 Sex Ratio������������������������������������������������������������������������������   66 2.10.4 Siblicide, Cannibalism, Infanticide, and Conspecific Nest Predation����������������������������������������������������������������������   66 2.10.5 Adoptions of Nestlings ��������������������������������������������������������   67 2.10.6 Parasites and Infectious Diseases������������������������������������������   67 2.11 Parental Care and Investment������������������������������������������������������������   68 2.11.1 Role Specialization ��������������������������������������������������������������   68 2.11.2 Prey Provisioning������������������������������������������������������������������   70 2.11.3 Parental Investment��������������������������������������������������������������   72 2.11.4 Parental Favouritism ������������������������������������������������������������   73 2.11.5 Helpers����������������������������������������������������������������������������������   73 2.12 Fledglings������������������������������������������������������������������������������������������   73 2.12.1 Post-fledging Dependence Period ����������������������������������������   74 2.13 Turnover and Recruitment����������������������������������������������������������������   74 2.14 Total Failures������������������������������������������������������������������������������������   75 References��������������������������������������������������������������������������������������������������   77 3 Competition and Defence������������������������������������������������������������������������   95 3.1 Introduction��������������������������������������������������������������������������������������   95 3.2 Abundance and Population Dynamics����������������������������������������������   96

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3.3 Breeding Dispersal and Territoriality������������������������������������������������   98 3.3.1 The Nearest Neighbour Distance (NDD)������������������������������  100 3.4 Nest Defence by Parents ������������������������������������������������������������������  100 3.5 Interspecific Interactions������������������������������������������������������������������  101 3.5.1 Predator–Prey Association����������������������������������������������������  101 3.5.2 Sympatric Falcons����������������������������������������������������������������  101 3.5.3 Intra-guild Kleptoparasitism ������������������������������������������������  102 3.5.4 Proximity with Other Competitors ��������������������������������������  102 3.5.5 Intra-guild Predation ������������������������������������������������������������  104 References��������������������������������������������������������������������������������������������������  104 4 Exploitation of Resources������������������������������������������������������������������������  111 4.1 Introduction��������������������������������������������������������������������������������������  111 4.2 Anatomical and Physiological Adaptations��������������������������������������  111 4.2.1 Shape of Beaks, Prey Handling, and Prey Consumption����������������������������������������������������������  113 4.2.2 Pellets������������������������������������������������������������������������������������  114 4.2.3 Energetics������������������������������������������������������������������������������  115 4.3 Diet����������������������������������������������������������������������������������������������������  116 4.3.1 Seasonal Variation in Diet����������������������������������������������������  118 4.4 Foraging Area������������������������������������������������������������������������������������  119 4.5 Hunting Techniques��������������������������������������������������������������������������  120 4.5.1 Aerial Attacks�����������������������������������������������������������������������  121 4.5.2 Crepuscular and Nocturnal Hunting Activities ��������������������  122 4.5.3 Cooperative Hunting ������������������������������������������������������������  123 4.5.4 Surprise Attack����������������������������������������������������������������������  124 4.5.5 Kleptoparasitism ������������������������������������������������������������������  125 4.5.6 Scavenging����������������������������������������������������������������������������  125 4.6 Prey ��������������������������������������������������������������������������������������������������  125 4.6.1 Prey Choice��������������������������������������������������������������������������  125 4.6.2 Predator/Prey Relationship ��������������������������������������������������  127 References��������������������������������������������������������������������������������������������������  130 5 Dispersal Patterns������������������������������������������������������������������������������������  141 5.1 Introduction��������������������������������������������������������������������������������������  141 5.2 Individual Conditions������������������������������������������������������������������������  143 5.3 Individual Decisions ������������������������������������������������������������������������  145 5.4 Juvenile Dispersal ����������������������������������������������������������������������������  145 5.5 Philopatry������������������������������������������������������������������������������������������  147 5.6 Seasonal Movements������������������������������������������������������������������������  147 5.7 Pre-migration������������������������������������������������������������������������������������  148 5.8 Constraints����������������������������������������������������������������������������������������  150 5.8.1 Barriers����������������������������������������������������������������������������������  150 5.8.2 Weather Conditions��������������������������������������������������������������  151

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5.9 Long Movements������������������������������������������������������������������������������  153 5.9.1 Nocturnal Flights������������������������������������������������������������������  155 5.9.2 Feeding Habits during Migration������������������������������������������  155 5.10 Wintering������������������������������������������������������������������������������������������  156 References��������������������������������������������������������������������������������������������������  159 6 Communication����������������������������������������������������������������������������������������  169 6.1 Signals����������������������������������������������������������������������������������������������  169 6.2 Plumage Colouration������������������������������������������������������������������������  170 6.2.1 Juvenile Plumage and Delayed Plumage Maturation ����������  170 6.2.2 Sexual Dichromatism������������������������������������������������������������  172 6.2.3 Sexual Monochromatism������������������������������������������������������  173 6.2.4 Polymorphism����������������������������������������������������������������������  175 6.2.5 Sexual Ornaments ����������������������������������������������������������������  176 6.3 Skin Colouration ������������������������������������������������������������������������������  177 6.4 Parental–Offspring Communication ������������������������������������������������  177 6.5 Calls��������������������������������������������������������������������������������������������������  178 6.6 Courtship Displays and Pair Bonding����������������������������������������������  180 References��������������������������������������������������������������������������������������������������  181 7 Living in Groups��������������������������������������������������������������������������������������  187 7.1 Introduction��������������������������������������������������������������������������������������  187 7.2 Coloniality����������������������������������������������������������������������������������������  188 7.2.1 Occupation����������������������������������������������������������������������������  188 7.2.2 Costs and Benefits����������������������������������������������������������������  189 7.2.3 Effects of Climate and Land Use������������������������������������������  190 7.2.4 Colony Dynamics������������������������������������������������������������������  191 7.2.5 Inter-Colony Movements������������������������������������������������������  192 7.2.6 Prey Depletion Around the Colony��������������������������������������  192 7.2.7 Paternal Assurance and Extra-Pair Fertilization Inside a Colony ��������������������������������������������������������������������  194 7.3 Artificial Colonies����������������������������������������������������������������������������  194 7.4 Mixed Species Colonies��������������������������������������������������������������������  195 7.5 Nonreproductive Stage����������������������������������������������������������������������  196 References��������������������������������������������������������������������������������������������������  196 Index������������������������������������������������������������������������������������������������������������������  201

Chapter 1

Western Palearctic Falcons

1.1  The Falconidae Family After a very long time, members of the Falconidae family were separated from the traditional large group of diurnal birds of prey that is comprised of eagles, hawks, the osprey, the secretary bird, and vultures (Accipitridae, Cathartidae, Sagittariidae, and Pandionidae families; Hackett et  al. 2008). Indeed, falcons share a common ancestor with parrots (Psittaciformes), seriemas (Cariamiformes), and passerine birds (Passeriformes; Pyle 2013; Jarvis et al. 2014). In fact, both morphological and molecular studies suggested that apparent similarities among ‘raptors’ should be due to a convergent evolution (Hackett et al. 2008; McCormack et al. 2013; Yuri et al. 2013). Thus, many analogous features including strong beaks and sharp talons evolved independently in different lineages (cf. Australaves and Afroaves) or presumably from an even more ancient landbird ancestor (Telluraves) with a raptorial lifestyle (McClure et al. 2019). Overall, the Falconidae is a well-supported monophyletic group that rapidly radiated from South America during the early Miocene, the first geological epoch of the Neogene period (ca. 23 to 16 million years ago; Fig.  1.1; Fuchs et  al. 2015; Cenizo et al. 2016). The global change from sub-tropical forested environments to cooler climates favoured new habitat types such as temperate forests, grasslands, and steppes (Blondel and Mourer-Chauviré 1998). Then, falcons dispersed into Europe via North America in the mid to late Cenozoic Era. Nevertheless, after the initial burst, the diversification process slowed down. This plausible scenario, based both on bone characteristics and genetics, also suggests an origin of Falconidae from primitive Neotropical forms (Fuchs et al. 2015; Cenizo et al. 2016). In fact, extant species are concentrated in the Neotropics except the Afro-Asian falconets (Microhierax and Polihierax genus) and the worldwide distributed genus Falco (Ericson 2012). Accordingly, an early fossil from the Lower Eocene (56 to 33.9 million years ago) found in Antarctica reinforces this hypothesis about a Neotropical or Austral origin of this group (Cenizo et al. 2016). © Springer Nature Switzerland AG 2020 G. Leonardi, Behavioural Ecology of Western Palearctic Falcons, https://doi.org/10.1007/978-3-030-60541-4_1

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Fig. 1.1  The diversification of the family Falconidae into three subfamilies (Herpetotherinae, Polyborinae, and Falconinae). The Polyborinae were sisters to the Falconinae. Among the Falconinae group, Microhierax and Polihierax genera split earlier than the genus Falco (Fuchs et al. 2015; Cenizo et al. 2016). From left to right: Polihierax semitorquatus (credit: Derek Keats/ Wikipedia); Microhierax caerulescens (credit: Raju Kasambe/Wikipedia); Falco rusticolus (credit: Ólafur Larsen/Wikipedia); Caracara plancus (credit: Dario Sanches/Wikipedia); Herpetotheres cachinnans (credit: Bernard Dupont/Wikipedia)

This family, divided into 3 subfamilies (Herpetotherinae, Polyborinae, and Falconinae), includes 11 genera and 64 species with 72% of them belonging to the Falconinae (Fig.  1.1; Ferguson-Lees and Christie 2001; Dickinson 2003; Fuchs et al. 2015). Morphological differences of the three subfamilies seem to be related to their feeding habits. In Polyborinae, hindlimb great muscle mass permits an efficient walk on the ground combined with a light wing musculature (Mosto 2017; Mosto et al. 2019). Inversely, falcons have higher mass in muscles of wings and tail and those that flex the digits related to fast flights, to high manoeuvrability, and to grip prey, respectively (Mosto 2017; Mosto et al. 2019). Among Falconinae, the body size of species vary greatly from sparrow-sized Microhierax falconets to largest true falcons (i.e. F. rusticolus; Fig. 1.1; Ferguson-­ Lees and Christie 2001). Accordingly, they show a wide range of habitat preferences, from forests to true deserts, and also feeding habits spanned from generalist to specialist species (Ferguson-Lees and Christie 2001). Intrinsic traits (e.g. adaptation

1.1  The Falconidae Family

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to a new diet or habitat) and also extrinsic events (e.g. climatic or tectonic changes) may explain variation in diversification rates through time (Fuchs et al. 2015). The Falconinae started to diversify between 12.6 and 19.3 Mya, and the divergence of the genus Falco was favoured by the spread of savannahs during the late Miocene (Fig. 1.2; Fuchs et al. 2015). Furthermore, an endemic species such as the white-rumped falcon (P. insignis) is sister to the widely distributed genus Falco separated about 10.1 Mya and 15.3 Mya by increasing aridity after the mid-Miocene Climatic Optimum. Interestingly, the appearance of migration patterns is a relatively recent phenomenon (last 5 Mya) and evolved independently multiple times mainly in Old World hobbies, merlins, and red-footed/Amur falcons (Fuchs et al. 2015).

1.1.1  The Genus Falco This species-rich genus started to diversify in the Late Miocene, between 5.0 Mya and 7.7 Mya, as an adaptation to the extension of open environments such as grasslands xeric habitats (Fig. 1.2; Blondel and Mourer-Chauviré 1998; Finlayson 2011; Fuchs et  al. 2015). This differentiation coincided with a series of strong, short-term, wet–dry and cool–warm climatic fluctuations (Blondel and Mourer-­ Chauviré 1998). Overall, members of this genus show very low genetic divergence in spite of geographical segregation and current systematic (Nittinger et al. 2005, 2007; Fuchs et al. 2015; Yang et al. 2018). In fact, several species shared different alleles among them such as the lanner with red-necked and peregrine falcons or the bat falcon with the Eurasian hobby and the Eleonora’s falcon (Table  1.1; Fuchs et al. 2015). The diversification of the primary lineage occurred during a short period of time with the earliest stem represented by Old World kestrels (Groombridge et al. 2002; Nittinger et al. 2005, 2007; Zhan et al. 2013; Fuchs et al. 2015; Yang et al. 2018). Accordingly, the common kestrel had the ancestral karyotype of Falconinae with the highest number of chromosomes (2n = 52; Nishida et al. 2008). Unfortunately, there is a scarcity of well-preserved fossils from pre-Pleistocene and none in Africa where kestrels should have originated (Fig. 1.2; Groombridge et al. 2002; Li et al. 2014). In addition, a new falconid fossil from late Miocene of northeastern China (F. hezhengensis) should support an alternative Eurasian origin of kestrels (Fig. 1.2; Li et al. 2014). Interestingly, remains of a jerboa (Dipodidae ssp.), preserved in the abdominal region of this specimen, suggest an open steppe environment and great availability of small mammals as prey (Li et al. 2014). From the late Miocene to early and middle Pliocene, kestrels, hobbies, and other larger true falcons radiated in the Western Palearctic as confirmed by fossils (Fig.  1.2). Overall, globally climatically tolerant falcons such as the common kestrel, the Eurasian hobby, and the peregrine falcon become widespread; another group including lanner, sooty, barbary, and saker falcons occupied driest areas and few ones cold climates (gyrfalcon and merlins; Finlayson 2011). At least 11 groups

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Fig. 1.2  Fossils belonging to the genus Falco from mid-Miocene to late Pleistocene. Most fossils are within the present geographical range. Notably, the gyrfalcon presence in southern countries covered by tundra and the westward extension of red-footed falcon and saker falcon along steppe environments. F. tinnunculus atavus was the larger kestrels and coexisted with the recent form F. tinnunculus. During the Weichselian glaciation, both gyr- and saker falcons lived in Europe as parapatric species. Thus, the separation occurred during the last interglacial period where population become isolated by the taiga belt. A, extinct species (Hol = Holocene); B1, extant and extinct subspecies (LP  =  Late Pleistocene); B2, glaciations (LGM  =  Last Glacial Maximum; WLG = Weichselian late glacial). (Data from Umanskaya 1981; Becker 1987; Tyrberg 1991; Boev 1999; Mlíkovský 2002; Tyrberg 2008; Finlayson 2011; Tomek et al. 2012; Bedetti and Pavia 2013; Holm and Svenning 2014; Li et al. 2014; Carrera et al. 2018)

were recognized by genetics including 12 resident and breeding species of the Western Palearctic ecozone (Table 1.1; Plates 1.1 and 1.2; Box 1.1). Overall, highly supported monophyletic groups include the Old World kestrels and their Australian and Indian sister groups and the Old World hobby clade including the widely distributed Eurasian hobby (very close to the African

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Table 1.1  Main clades including all species of the genus Falco as resulting from a recent phylogenetic study carried out by Fuchs et al. (2015). Species marked in red are resident in the Western Palearctic ecozone Clades Old World kestrels American kestrel Red-footed/Amur falcons Grey/Dickinson’s kestrels Merlin Brown falcon Aplomado/New Zealand falcons Old World hobby falcons New World hobby falcons Prairie/grey/ peregrine-like falcons Hierofalcons/ red-necked falcon

Species F. naumanni, F. tinnunculus, F. cenchroides, F. moluccensis, F. rupicolus, F. newtoni, F. araeus, F. punctatus, F. rupicoloides, F. alopex, F. zoniventris F. sparverius F. vespertinus, F. amurensis F. ardosiaceus/F. dickinsoni F. c. aesalon/F. c. Columbarius F. berigora F. femoralis/F. novaezeelandiae F. concolor, F. cuvierii, F. eleonorae, F. longipennis, F. severus, F. subbuteo F. rufigularis/F. deiroleucus F. mexicanus, F. hypoleucos, F. pelegrinoides, F. peregrinus, F. fasciinucha F. biarmicus, F. cherrug, F. jugger, F. rusticolus, F. subniger, F. chicquera

counterpart F. cuvierii) and the patchily distributed Eleonora’s and sooty falcons (Table 1.1; Groombridge et al. 2002; Fuchs et al. 2015). Additionally, the Hierofalco complex is an assemblage of morphospecies not yet differentiated with a close counterpart in Australia: the black falcon (F. subniger; Nittinger et al. 2005, 2007; Fuchs et  al. 2015). In the past, eco-morphological similarities confounding taxonomic identification include, for instance, the American prairie falcon (F. mexicanus) in the red-brownish hierofalcons and not with peregrine-like falcons (Table 1.1; Nittinger et al. 2005). The lanner falcon, the saker falcon, and the gyrfalcon are not monophyletic with the saker falcon paraphyletic with both lanner and gyrfalcons (Nittinger et al. 2005, 2007; Dawnay et al. 2008). This pattern could be explained either due to stochastic lineage sorting of ancestral polymorphism or interspecific gene flow through hybridization (Nittinger et al. 2005, 2007; Johnson et al. 2007; Fuchs et al. 2015). The highest haplotype diversity in the sub-Saharan nominate race of the lanner falcon suggests an ancestral African origin of this group (Nittinger et al. 2005). In fact, hierofalcons have similar genome organization with kestrels (2n  =  52–54; Joseph et  al. 2018). Saker falcon populations occupy a very ample distributional range in Europe and Asia and show two main genetic clusters (eastern and western) with an intermediate third in Central Eurasia clustered between the two main groups (Zhan et  al. 2015). The split between the saker and peregrine falcon has been estimated to have occurred about 2.1 Mya (Zhan et al. 2013). As an adaptation to these cold open habitats, the saker falcon shows genes involved in hypoxia and immune response (Zhan et al. 2013; Pan et al. 2017).

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Plate 1.1  Small-sized falcons of the Western Palearctic. Only males depicted. (original artwork by Marco Preziosi)

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Plate 1.2  Large-sized falcons of the Western Palearctic. Only males depicted. (original artwork by Marco Preziosi)

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Box 1.1: The Palearctic Ecozone A traditional biogeographic approach divides the Earth’s land surface by gross biophysical features such as rainfall, temperature, and soil or by vegetation structure (Olson et al. 2001). On the contrary, ecozones are large units based mainly on distributional patterns of assemblage of natural communities and species (Schultz 2005). Thus, each ecozone is a geographical region having a distinct biodiversity of flora and fauna and corresponds to the biogeographic realms early described by Udvardy (1975). Overall, the terrestrial world is subdivided in to eight ecozones including Antarctica, Oceania, Australasia, Neotropic, Afrotropic, Indo-Malay, Nearctic, and the Palearctic (Olson et al. 2001). Although the Palearctic is the largest region, it is not the richest ornithologically with 10% of species and 14% of genera only (Newton and Dale 2001). Accordingly, Palearctic islands hold 60%) of barbary falcons in the Canaries have the species-specific appearance, but there is much overlap with peregrine falcon subspecies brookei (Rodríguez et al. 2011). Hybrids may also be bred in genetic mixture of 50–50, 62.5–37.5, 75–25, or in combination of more than two species. They varied in an unpredictable way due a mixture of characteristics, and this mix is not consistent (i.e. individuals are dissimilar in spite of the same parental species; Eastham and Nicholls 2005).

1.2  Plumage and Bare Parts Plumage patterns and the colouration of falcons are frequent topics of debate for identification purposes. Spots, stripes, and several other plumage features together with size and structure were used also for the systematics of this group. The relatively huge importance given to plumages as diagnostic traits raises some basic questions about how much they are genetically determined or sensitive to the environment.

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Avian genes involved in colour production are multi-allelic. These variant forms of a gene, precisely the alleles, can produce different observable phenotypic traits. Thus, different degrees of genetic variation among inter- and intraspecific levels are due to variable allele frequencies between populations (Roulin and Ducrest 2013). This is the case of melanin pigments, whereas the expression of carotenoid-based colouration depends mainly on the environment and slightly on the genetic component. Among polymorphic falcons, the presence of a few discrete colour morphs, regardless of age and gender, usually follows a Mendelian mode of segregation. Colour morphs may confer advantages in some specific local habitats.

1.2.1  Pigmentary Colours 1.2.1.1  Melanin Primitive plumage colouration mechanisms of falcons are based on the presence/ absence of melanin pigmentation (eu- and pheomelanin) that produced black to white colours (Hill 2007; Galván and Jorge 2015). The melanin-based colouration is qualitatively controlled by a gene called MC1R that regulates melanin synthesis for deposition in melanocytes, the specialized pigment cells (Johnson et al. 2012; Zhan et al. 2012). Overall, the eumelanin produces bold black patches as well as black spots, bars, bands, and stripes (Hill 2007; Galván et  al. 2010; Galván and Jorge 2015). Conversely, the pheomelanin is responsible for lighter colours such as brown, red, and orange (Toral et  al. 2008). Indeed, melanogenesis produces a mixture of pheo- and eumelanin polymers. 1.2.1.2  Carotenoids Yellow to red colourations are usually generated by carotenoid pigments. Carotenoids are obtained by ingesting specific dietary carotenoid pigments as macromolecules such as lutein, zeaxanthin, and beta-carotene involving a low genetic component (Bortolotti et al. 2000; Hill 2007). Variations in expression skin and bare parts colouration can be in the form of continuous variation both in hue, chroma, and brightness and/or extent of colour displays (Casagrande et al. 2007; Hill 2007). Supplementation of carotenoids in the diet produces visible effects in colour but also antioxidant and immunostimulatory effects (Sumasgutner et  al. 2018). Nevertheless, above a certain physiological threshold, carotenoids can cause detrimental effects (Costantini et al. 2007a).

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1.2.2  Polymorphism Predator–prey relationships are important in the evolution and maintenance of colour polymorphism in predator species that prey upon animals with a strong ability to detect predators such as mammals (Roulin and Wink 2004). In fact, the maintenance of colour polymorphisms may be due to differential hunting success of colour morphs depending on varying environmental light conditions (Galeotti et al. 2003; Johnson et al. 2012). Overall, five falcon species show plumage polymorphism including two Australian species (the black F. subniger and the brown F. berigora falcons) and the American kestrel (F. sparverius; Roulin and Wink 2004). The other two with this trait are the gyrfalcon and the Eleonora’s falcon (Table 1.2; Gangoso et al. 2011; Johnson et al. 2012). In these species, individuals of the same age and sex display one of several colour variants that are genetically inherited but neither sensitive to the environment nor condition-dependent in their expression (Roulin and Wink 2004). Overall for the Eleonora’s falcon, the frequency of the dark morph is lower than that of the pale one (dark ca. 30% and pale ca. 70%), and both morphs co-occur at temporally and geographically stable frequencies in sympatric populations (Mayol 1977; Ristow et al. 1998). General colouration follows a complex scheme based on dominance/recessive mechanisms of genes that produced, in turn, differential pigment deposition. For instance, three discrete colour forms (morphs) were formerly recognized for the gyrfalcon from white to darker forms passing through an intermediate ‘silver’ pattern (Table 1.2; Fig. 1.5; Flann 2003; Chang et al. 2010; Gangoso et  al. 2011). In northern Greenland, where white gyrfalcons predominate, a single MC1R allele was observed at high frequency (98%), whereas in Iceland where only grey gyrfalcons are known to breed, seven alleles were observed (Johnson et al. 2012). Indeed, birds collected in various locations of their range show great plumage colour and pattern polymorphism (Johnson et al. 2007; Chang et  al. 2010). In addition, melanistic nestlings should be produced from white parents and vice versa (Chang et al. 2010). Accordingly in Eleonora’s falcon offspring, the dark allele D dominates over the pale one but most dark individuals are heterozygous for the MC1R allelic variant (Wink et  al. 1978; Ristow et  al. 1998; Gangoso et al. 2015a). Thus, the gene involved in pigment production follows Mendelian dominance inheritance, and in the homozygous recessive situation, the pigment production would not occur (white/pale phase; Chang et  al. 2010; Johnson and Burnham 2011). Nevertheless, for the gyrfalcon, only multiple possible combinations using two genes and three alleles should explain the near continuous colour variation (phases) among distant population (Flann 2003; Chang et al. 2010). In Eleonora’s falcons, the homozygous dark morph DD is rare with a frequency of 2% (Ristow et al. 2000).

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1.2.3  Effects of Melanin- and Carotenoid-Based Colouration Genes responsible for the expression of melanin-based colour traits are also involved in other important physiological functions such as immunity and metabolism by pleiotropy, suggesting the possibility of multivariate evolution (Kim et al. 2013). In fact, melanin-based colouration co-varies with other phenotypic traits such as size, body mass, morphology, physiology, and behaviour (López-Rull et al. 2016). For example, most colour polymorphisms are due to differences in the type or amount of melanin present in each morph, which also differ in several behavioural, morphometric, and physiological attributes (Galván et al. 2010). Effects of carotenoid levels should be revealed on fledglings from genetic or maternal origins (Laaksonen et al. 2008).

Fig. 1.5  A perched dark morph male Eleonora’s falcon. Adult dark individuals have lower immune responses due a greater activity of MC1R gene. Nevertheless, dark males produced more fledglings than pale males with humoral responses which produce specific antibodies (credit: Paolo Griva and Michele Santona)

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Fig. 1.6  Marking spots of white gyrfalcons are due to a second co-dominant gene with a pleiotropic effect on feathers durability and parasite resistance. (Original artwork by Gaia Sorrentino)

1.2.3.1  Melanin Melanin-based plumage colouration is affected by the levels of glutathione (GSH), an intracellular antioxidant, involved in the melanogenesis (Galván et  al. 2010; Galván and Jorge 2015). In fact, melanic morph individuals have low GSH level due a greater activity of MC1R gene (Galván et  al. 2010). Thus, the expression of discrete melanic morphs should have pleiotropic effects such as the immune capacity (Gangoso et  al. 2011). For instance, ‘white’ gyrfalcons should have different numbers marking spots, particularly on the tips of primaries, due to a second co-dominant gene (Fig. 1.6; Chang et al. 2010; Johnson and Burnham 2011; Johnson et  al. 2012). As pleiotropic effect from this gene, dark tips on white primaries should increase structural support and durability or antimicrobial and parasite resistance (Johnson et  al. 2012). The ability to mount an inflammatory response was morph and sex specific in Eleonora’s falcon nestlings where dark eumelanic males had lower immune responses than pale ones (Galván et al. 2010; Gangoso et  al. 2011, 2015a). Accordingly, adult dark Eleonora’s falcons have a higher prevalence of Plasmodium blood parasite due to lower resistance to parasites rather than exposure to vectors (Gangoso et  al. 2016). Nevertheless, dark males produced more fledglings than pale males, and nestlings raised by dark fathers had higher humoral responses which produce specific antibodies than those raised by pale fathers (Gangoso et al. 2015b). Although mixed morph pairs occur, alternative morphs were segregated over suitable habitats by forming permanent same colour clusters (Fig.  1.7; Gangoso et al. 2015a). In fact, the pale morph, which was less dominant but more aggressive than the dark morph, is highly colonial, whereas the dark morph is more territorial (Gangoso et al. 2015a).

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Fig. 1.7  A mixed morph pair of Eleonora’s falcon bonding through mate-feeding. (credit: Paolo Griva and Michele Santona)

Differences in breeding performances among morphs were also observed in gyrfalcons inhabiting Greenland (Johnson and Burnham 2013). It seems that territory occupancy and timing of breeding may regulate reproductive success differently between colour variants, with directional selection favouring light-­coloured gyrfalcons and resulting in earlier lay dates and a high frequency of white plumage colour variants in this population (Johnson and Burnham 2013). 1.2.3.2  Carotenoids Carotenoids are important for health as they are precursors of vitamin A and also have a role to play as immunostimulants (Hill 2007). Among falcons, the concentrations of vitamin A in blood plasma are species-specific (and also vary between wild and captive states) due to different nutritional strategies (Schink et al. 2008; Müller et al. 2012). Similarly, vitamin E protects lipids from peroxidation and in combination with vitamin C has a more beneficial antioxidant effect (Marri and Richner 2014). Nevertheless, the sensitivity of birds to carotenoid deprivation appears to vary substantially among species (Karu et al. 2008). In fact, carotenoid-­ based colouration can be correlated with significantly increased growth rate of nestlings when the chicks received supplemental vitamins (Marri and Richner 2014). Accordingly, younger kestrel nestlings cope with oxidative stress, but this pattern decreased with age without influence of carotenoid availability (Costantini et  al. 2007b). In addition, within a breeding season, earlier nesting kestrels had higher carotenoid concentrations than later nesting kestrels, a pattern that is coincidental with seasonal trends in local fitness (Sassani et al. 2016). Consequently, fledglings in enlarged broods had higher plasma carotenoid concentrations (Laaksonen et al. 2008).

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Fig. 1.8 Carotenoid-­ dependent yellow cere and beak of an adult peregrine falcon. (credit: Iñigo Zuberogoitia)

American kestrels are more brightly coloured during the mating period than in winter, and plasma carotenoid concentrations declined from the time of mating to the rearing of young (Negro et al. 1998). Carotenoid-dependent colours can thus signal individual quality despite annual variations in carotenoid availability (Bostrom and Ritchinson 2006; Laaksonen et al. 2008). In fact, skin colour appears to be an honest indicator of quality for male American kestrels and may serve both intersexual (territory acquisition) and intersexual (mate choice) functions during the breeding and non-breeding seasons (Fig.  1.8; Bostrom and Ritchinson 2006). In addition, colouration of males significantly predicted their response to future parasitism (Dawson and Bortolotti 2006). In common kestrels, female parents had twice the plasma carotenoid levels of males (Laaksonen et  al. 2008). Inversely, plasma carotenoid concentrations of American kestrel males were significantly higher than females, but this depended on year (Sassani et al. 2016). Probably, pair and individual identity explained variation in carotenoid concentrations (Sassani et al. 2016). In fact, plasma carotenoids levels of adult American kestrels being correlated within mated pairs, and having a significant association with the abundance of voles, the primary prey species, per territory (Bortolotti et al. 2000). Common kestrel males work physically harder than females, and they might thus also use more carotenoids against oxidative stress than females. Alternatively, females could be gaining back the carotenoid stores they depleted during egg-laying by eating primarily carotenoid-rich food items during the early nestling stage (Laaksonen et al. 2008).

1.3  Reversed Size Dimorphism All falcons, more or less, show a strong reversed size dimorphism (RSD), where females are larger than males both in structure and flight apparatus (Table  1.2; Fig.  1.9; Krüger 2005). Cade (1960) points out that sexual dimorphism is more pronounced in the larger falcons than in the smaller. A reliable measure of RSD

1.3  Reversed Size Dimorphism

19

Fig. 1.9  A smaller male peregrine falcon offers prey to the larger female during a courtship display. (credit: Marc Templier)

could be calculated by dividing the wing length of males by the wing length of females, with this ratio subsequently cubed to estimate differences in bulk and flight performance (Krüger 2005). Overall, Krüger (2005) suggests that hunting method was negatively correlated with RSD, whereas sexual plumage dimorphism was positively correlated with RSD, indicating that the species showed smaller plumage differences between the sexes. The intrasexual competition hypothesis states that RSD is maintained by the higher competitive ability of larger females to secure higher-quality territories, to increase mortality of larger immature males, and hence to obtain greater breeding success (McDonald et al. 2005). As nestling in the same brood, larger female chicks become more competitive than males when parents brings prey at nest (Anderson et al. 2003b). McDonald et al. (2005) found inside a breeding population of brown falcon (F. berigora) a strong relationship between body size and recruitment rate in adult females but also a severe decrease in survival in larger immature males. Accordingly in merlin populations, body size strongly influences fitness, with larger individuals benefiting in terms of both lifetime reproductive success and survivorship (Warkentin et  al. 2016). Body size also drives parental roles, with females responsible for incubation, brooding, and feeding of young, whereas males provide much of the food for the pair and young until late in the nestling period (Newton 1979; Andersson and Norberg 1981). This presumed enhanced foraging efficiency of males is based on wing length and especially on tail length, characteristics that allow for more manoeuvrability during hunting flights (Wiklund 1996). Accordingly, males probably delay moult to avoid the loss of manoeuvrability and flight

20

1  Western Palearctic Falcons

performance when there is a need to provide prey to incubating mates (Steenhof and McKinley 2006). Falcons that exhibit strong RSD tend to hunt larger and more agile prey (e.g. birds), and this specialization might have evolved in order to allow more efficient foraging (Table  1.2; Andersson and Norberg 1981; Krüger 2005). Inversely, insectivorous falcons are less dimorphic (Newton 1979). Nevertheless, Eleonora’s and sooty falcons do not show a pronounced RSD, but they are exclusively bird-­ eating during the breeding season (Table 1.2). Indeed, a higher abundance of arthropods are a primary source of food for these falcons on their wintering grounds characterized by hot and wet weather (Zefania 2001; Ristow 2004). Thus, there is a conflict between the benefit of being small during breeding and the benefit to be large outside the breeding season (Slagsvold and Sonerud 2007), a big challenge mainly for the smaller males. A recent comprehensive study confirmed the prey type, hunting habits, and territoriality as the main selective pressures for the evolution of the RSD (Pérez-Camacho et al. 2018). In fact, the RSD increase with increasing size of breeding territory which in turn depends on the proportion of avian species in the diet (Peery 2000; Pérez-Camacho et al. 2018).

1.4  Flight Performances Aerodynamic differences are apparent between species, races, sexes, and age classes. Both tail and wing parameters should predict energetic costs of flight for hunting and migratory purposes. Ultimately, flight styles influence the feather morphology from the hovering common kestrel with its primaries that withstand large mechanical forces to the peregrine falcon that needs a high stiffness of the tail feathers during breaking after diving (Schmitz et al. 2015). Amadon (1943) suggested that tail/wing ratios are directly related to longer wings that, in turn, greatly improve the flight performances of migratory birds. In fact, when migratory distances increase, morphological shift becomes evident which results in a larger forward component in flight due to a more prominent distal part of the wing (see Sect. 5.1; Norberg 1990). As a consequence of this adaptation, wings are more slender and pointed and along with a shorter tail length relative to the wing length reduce the induced drag of the wings and produce greater uplift and thrust (Norberg 1990). This is true for full or partial migratory small falcons such as common and lesser kestrels, hobby, red-footed, and Eleonora’s falcons (Spaar 1997). These species show the highest proportion of flapping-gliding flight and had the least gliding performance with low air speeds and large gliding angles (Spaar 1997). Juvenile falcons appeared to have longer flight feathers and lighter wing loads that increase their survival chances through increased manoeuvrability and reduce energetic costs of flight (Ward and Laybourne 1985; Jenkins 1995). For example, immature peregrine falcons have great lift, manoeuvrability at low speeds, and low-­ energy flight, whereas adults have great acceleration, fast flight, but high-energy expenditure and little manoeuvrability (Ward and Laybourne 1985). Thus, adults

1.5 Vision

21

have a greater impact than immatures when striking prey. Inversely from soaring flight, young birds can drop below the prey and grasp it from below and behind (Yosef 1991). Interestingly, rump colouration of feral pigeons seems to distract the attention of attacking peregrine falcons helping them to escape (Palleroni et al. 2005). Among adult large falcons, lower wing loads enable lanner falcons to soar well and facilitate slower flight when coursing low over the ground searching for prey such as grasshoppers (Jenkins 1995). This enables the species to make use of several different types of hunting method, and they are therefore versatile in their hunting ability. In addition, falcons try to reduce energy used for hunting by the use of vertical winds for soaring when available (Rosén and Hedenström 2002). To increase its air speed, the falcon adjusts its wing span in flight to achieve nearly the maximum possible lift to drag ratio value over its range of gliding speeds (Tucker and Parrott 1970). The top speeds reached during a dive depend on the mass of the bird and the angle and duration of the dive. Falcons can control their speeds in a dive by changing their drag and by choosing the length of the dive and fly at a speed of over 100 km/h and dive about 400  km/h (Tucker 1998; Tucker et  al. 1998). Ponitz et  al. (2014) found experimentally that during some dives peregrine falcons can reach velocities of more than 320  km/h using a V-type structure with the open end between the shoulders and the tip at the tail of the body. Also, high-resolution pictures of diving peregrine falcons indicate that small feathers pop up in the back which prevents local flow separation (Fig.  1.10; Ponitz et  al. 2014). In fact, flow separation can often result in increased air resistance. Furthermore, feathers pop up if flow separation starts to develop on the upper side of their wing resulting in higher lift at lower flight speeds (Carruthers et al. 2007). Falcons kept their heads oriented close to the forward direction and flew relatively directly towards the prey along an inclined glide path that levelled out near impact. Thus, flight velocity is reduced by increasing the angle of attack which results in increased drag (Tucker et al. 1998; Ponitz et al. 2014). In fact, by adopting a moderate stooping speed, falcons may gain hunting precision (Alerstam 1987).

1.5  Vision Falcons have a high visual spatial resolution that defines the detail that can be resolved in a visual scene (Land and Nilsson 2012). In fact, kestrels and other falcons have maximal spatial acuities comparable to those of humans (Bringmann 2019). A large eye with a dense photoreceptor array in the retina permits long focal length that results in a correspondingly large retinal image and more acute vision (Fig. 1.11; Reymond 1987; Kiltie 2000). Thus, falcons have a relatively large blind area above and behind the head correlated to binocular field vision and high visual acuity to facilitate prey detection and capture. For instance, the saker falcon and the American kestrel have similar maximum binocular overlap reflecting hunting attitude towards small rodents (Potier et al. 2018). Nevertheless, small falcons have smaller eyes (focal length of 9–12 mm) than humans (Hirsch 1982).

22 Fig. 1.10  V-type structure of a diving peregrine falcon. Small feathers which are popped up to prevent local flow separation. (original artwork by Gaia Sorrentino)

Fig. 1.11  A large eye and a relatively large blind area characterized by small mammals eaters such as the kestrel. (credit: Wietze Janse)

1  Western Palearctic Falcons

1.6  Behavioural Repertoire of Falcons

23

The fovea is the region in the retina where photoreceptor density is highest, and therefore it is the place of the highest spatial resolution (Land and Nilsson 2012). In fact, the high acuity of kestrels and falcons should be explained by the higher receptor density and especially the image magnification provided by the fovea (ca. 4.77-fold; Fox et  al. 1976; Hirsch 1982; Bringmann 2019). Falcons such as the saker falcon possess a deep central fovea that views the lateral visual field and a shallower temporal fovea that views the frontal visual field (Potier et al. 2018). The presence of a temporal fovea may be important to improve fixation on an object ahead of the bird and perhaps enhance sensitivity to a moving object, two functions that may be particularly important when capturing prey on the wing at high speed (Tucker et al. 2000). Thus, it is necessary that the falcon holds its head straight when flying along a logarithmic spiral path that keeps the line of sight of the deep fovea pointed sideways at the prey (Tucker et al. 2000). Likewise, falcons are obliged to continuously move their head due to the large size of eyes in relation to the head the presence of bi-foveal vision (Tucker 2000). Nevertheless, recent experiments on gyrfalcons, peregrines, and hybrids wearing miniature video cameras demonstrated that these birds maintain their prey in the temporal fovea in the dives (Kane and Zamani 2014). Accordingly, the temporal resolution is high (fast vision) in falcons such as the peregrine and the saker allowing them to track fast and highly manoeuvrable prey (Potier et al. 2020). The central fovea of falcons also allows high-resolution tetrachromatic vision. In fact, they possess cone-dominant retinas, which confer sharp visual acuity and colour perception in relatively bright-light environment (Jones et al. 2007). Viitala et al. (1995) suggested that kestrels were capable of perceiving ultraviolet reflectance produced by the urine of voles and rats. Indeed, the optical equipment of falcons exhibits lower transmittance to ultraviolet light rejecting this hypothesis (Lind et al. 2013). Accordingly, gene expression coding for opsins (UV-sensitive proteins) in falcons indicates that this group is sensitive to violet (V-type sensitivity) more than ultraviolet (Wu et al. 2016). Thus, falcons underwent a parallel adaptive evolution to dim-light conditions such as crepuscularity (see Sect. 4.5.3; Jones et al. 2007; Wu et al. 2016).

1.6  Behavioural Repertoire of Falcons In ethological studies, discrete behavioural events and states are the measuring units which represent an arbitrary selection of available information (Hinde 1966; Drummond 1981). Thus, a behavioural repertoire contains an inventory of well-­ defined units defined to reflect their structural, functional, and causal attributes (Martin and Bateson 2007). For example, Wrege and Cade (1977) described 13 displays that were common to the courtship of large falcons and named 6 vocalizations used by gyrfalcons. Descriptions of postures, movements, and vocalizations are parts of a structural definition of a behavioural unit, whereas a functional definition focuses on environmental effects on behavioural patterns

24

1  Western Palearctic Falcons

Table 1.3  Main behavioural classes as reviewed by Jones (2001) Behavioural class Innate behaviour

Imprinted behaviour

Learned behaviour

Behavioural patterns Foraging Courtship behaviours Nesting behaviours Parents and siblings imprinting Imprinting on future mates Environmental imprinting Conditioned learning Experience

Sections 4.2.1 6.6 2.8 2.11; 2.11.4 2.3.2; 2.4.2 2.6 – 1.6

(Hinde 1966; Martin and Bateson 2007). Overall, physiological and/or environmental factors should act as triggers for behaviours (Hinde 1966). Unfortunately, several constraints must be overcome when gathering behavioural data from falcons in the wild. In fact, many of our current knowledge on falcon behavioural repertoire come from captive birds (Wrege and Cade 1977). In particular, captive small falcons such as American and common kestrels are relatively easy manageable species for these purposes (Bird 1982; Csermely et al. 1989). It must however be considered that falcons confined to captivity are susceptible to behavioural disorders (Jones 2001). In fact, captivity may exaggerate or eliminate some types of behaviour (Wrege and Cade 1977; Jones 2001). The behavioural repertoire is very similar in genus Falco with more than 75% of displays common to all species (Wrege and Cade 1977). The complexity and frequencies of nonaggressive behaviours is strongly related to the female dominance, and it becomes necessary for successful reproduction (Wrege and Cade 1977; Carlier and Gallo 1989; Gallo et al. 1991). Overall, innate behaviours are essentially unchangeable, imprinted behaviours are related mainly to the recognition of parents and siblings, and learned behaviours can be modified throughout the life of the falcon (Table 1.3; Jones 2001). Nevertheless, it is not easy to define a precise border among these classes. For example, the stimulus which releases prey capture behaviour is a combination of innate patterns and prey status, which is killed with a neck bite (see also Sect. 4.2.1; Brosset 1973). Young falcons will imprint on future breeding partners with or without the presence of siblings in their natal nest; hence this recognition depends on instinctual behaviours such as the courtship (see also Sect. 6.6; Fox 1995; Jones 2001). The imprinting process occurs around 10–14 days (such as in American kestrel) but continues throughout the permanence of chicks at the nest (Jones 2001). After hatching, chicks begin to imprint on the sounds produced by parents and on parents themselves when they improved their visual acuity (Brosset 1973; Fox 1995). At the same time, they recognized its siblings as competitor for food (Fox 1995). Environmental cues and the nest-site characteristics strongly influenced future nest location by imprinting of fledglings (see also Sect. 2.5; Fox 1995; Jones 2001).

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Learned behaviour is poorly understood and includes conditioned learning, used mainly in falconry practice, and voluntary learning such as experience (Jones 2001). Young falcons devote a relatively fixed amount of time to manipulate objects resembling natural prey for the maturation of hunting skills (Negro et  al. 1996). Nevertheless, innovative behaviours should be adopted by adults. For example, some pairs of wild common kestrels for catching elusive prey (bats, swifts) improved the breeding success of such pairs (Mikula et al. 2013). Presumably, prey capture rates provided direct evidence of profitability to the predator that employs a novel technique (Leonardi and Bird 2011; Mikula et al. 2013; Bijlsma 2016).

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Chapter 2

Reproductive Strategies

2.1  The Breeding Life History Stage Reproduction is costly, and individuals that invest too much in a given reproductive bout pay with reduced reproductive output in the future (Sockman et  al. 2006). Thus, the precise timing of reproduction is an important determinant of fitness (Davies and Deviches 2014). Overall, the reproduction of a falcon is not only dependent on food supply during the breeding period but also on conditions under which the partners lived before breeding (Cavé 1968). As well other vertebrates, falcons show an associated breeding life history stage where gonadal (see Box 2.1), territorial (see Chap. 3), mating (see Sect. 2.4), and parental (see Sect. 2.11) periods compose a long breeding season (Wingfield 2008). A developmental phase (see Sect. 2.2), when the reproductive system matures, preceded the mate choice (see Sect. 2.4), the nest choice (see Sect. 2.5), and the production (see Sect. 2.6) and incubation (see Sect. 2.8) of the eggs (Cade 1960; Wingfield 2008). The time lag between the start of reproductive development and the chick rearing phase forced parents to begin the developmental phase well before environmental conditions are optimal for feeding young (Thomson 1950; Davies and Deviches 2014). On the other hand against basic requirements for survival, a female must be in a suitable energetic state to initiate reproductive development (see Sect. 2.2) and egg formation (see Sect. 2.6.2; (Siivonen 1957; Drent 2006). Again, food availability plays a crucial role for the female state (Davies and Deviches 2014).

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2.2  Developmental Phase 2.2.1  Photoperiod Photoperiod changes predictably and invariably between years. Breeding birds use day length as cue to broadly time their seasonal reproduction, especially in temperate zones where there is a clear distinction of day length in different seasons (Wingfield 2008). In fact, increasing day length stimulates several neuroendocrine and endocrine secretions (see Box 2.1) and triggers gonadal development in the anticipation of the breeding season (Lofts and Murton 1968; Wingfield 2008). Thus, hormones are secreted into the general circulation and stimulate gametogenesis and production of gonadal steroids (e.g., oestradiol and testosterone). As consequent, falcons excreted higher concentrations of faecal E (oestrogens), P (progestogens), and T (testosterone) during the breeding season than the non-breeding season (Staley 2003). Photoperiod is also the major regulator of haematocrit changes in falcons, but its control decreases 2 months before the onset of egg-laying (Rehder and Bird 1983). The timing of egg-laying of captive pairs was similar to the dates when eggs were found in the nests of wild falcons, but, under advanced photoperiod, these latter ones were induced to lay early (Nelson 1972; Olsen and Olsen 1980; Meijer et al. 1992). For example, male falcons primarily respond to photoperiod cues, and, experimentally, their gonads can usually be brought to a state of complete spermatogenesis by light stimulation (Lofts and Murton 1968; Boyd et al. 1977). Likewise, Cavé (1968) showed that in autumn, when day length decreases from 12 to 8 h, resident female common kestrels developed the ovaries slowly. Thus, females display a gonadotropic response to photostimulation, but they may adjust reproductive timing using more supplemental cues such as climate conditions (see Sect. 2.2.4), resource availability (see Sect. 2.2.3), or behavioural cues (see Sect. 2.4; Lofts and Murton 1968). Ultimately, the photoperiod synchronized the endogenous system to lay within an annual reproductive window (Meijer 1989; Meijer et al. 1992). The length of the day was inversely correlated with the time to onset of laying and with the period during which first clutches were started (Fig.  2.1; Bird et  al. 1980; Meijer et  al. 1992). In other words, increased photoperiod corresponds to earlier laying dates in Western Palearctic falcon populations (Meijer et al. 1992). Thus, southern populations lay earlier than those located northernmost that required longer photoperiods in order to be stimulated sufficiently to breed (Fig. 2.1; Cade 1960; Nelson 1972). Willoughby and Cade (1964), manipulating the amount of daylight, were able to induce egg-laying of American kestrels regardless of other environmental factors such as the temperature. Although the photoperiod should be considered as the main start factor, food resources and local weather conditions (e.g. temperatures and rainfalls) were clearly involved too (Lofts and Murton 1968; Daan et al. 1990; Carrillo and González-Dávila 2010a).

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Box 2.1 Hormonal Patterns and Pre-laying Physiological State Increasing day length in the spring stimulates the hypothalamic production and secretion of GnRH hormone which stimulates gonadotropins, follicle-­ stimulating hormone (FSH), and luteinizing hormone (LH). For example, luteinizing hormone levels in breeding male kestrels were elevated from courtship through incubation, and androgen concentration peaked during courtship and laying (Meijer and Schwabl 1989). In both breeding and non-­ breeding females hormones increased during pair formation and courtship, although maximum levels were lower in non-breeders (Meijer and Schwabl 1989). Androgen levels in breeding males were higher as a result of intermale aggression during courtship with a difference of body weight averaged 19% (Rehder et al. 1988; Meijer and Schwabl 1989). In male kestrels, the testes remained fully mature, and in females ovaries remained partially mature throughout the breeding season (Young et al. 2009). Egg-laying was characterized by marked increases in both female body mass and plasma concentrations of luteinizing hormone (LH) and corticosterone (Rehder et al. 1986; Meijer and Schwabl 1989). The increase in corticosterone occurred concomitantly with the onset of rapid follicle growth and probably compensates for the energetic costs of egg production (Lamarre et al. 2017). However, high levels of corticosterone may be associated with low individual fitness and may affect balance between pro-oxidants and antioxidants (Costantini et al. 2008). Androgens declined sharply after laying in breeding males slightly less for non-breeding males (Rehder et  al. 1988; Meijer and Schwabl 1989). Nevertheless, experimental evidences show that breed early, late, or not at all is primarily an effect of food availability and is not due to hormonal modulation of the reproductive cycle (Meijer and Schwabl 1989). In fact, during early courtship and before breeding, females and males have similar concentrations of luteinizing hormone and for males only of androgens (Meijer and Schwabl 1989). In addition, measured plasma concentration of β-hydroxybutyric acid (BUTY) and triglyceride (TRIG), two metabolites related to short-term changes in fasting and fattening rate, lower rates of pre-laying fattening and/ or lower mobilization rate of lipoproteins to ovarian follicles delayed laying (Lamarre et al. 2017). Overall, hormonal patterns, reproductive behaviour, and body mass suggest that female falcons have a wider annual reproductive window than males (Meijer 1989).

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Fig. 2.1  Variation in laying dates of Western Palearctic common kestrel populations is related to photoperiod. Accordingly, common kestrel laying date was also affected by latitude, with a delay of about 6 days for every 10° towards the north. (data from two comprehensive studies: Carrillo and González-Dávila 2009, 2010a; the laying date is the mean laying date of a given population over all study years and was recorded in Julian dates)

2.2.2  Moult Most falconids start moulting P4 and S5 during the breeding season and finish moulting P10 and S1  in the autumn (Zuberogoitia et  al. 2018b). Overall, female falcons begin to moult before males (Dementiev and Gortchakovskaya 1945; Wayre and Jolly 1958; Beebe 1960). For instance, female gyrfalcons moult during incubation, and they are about half-way through the moult when young aged 1 month (Dementiev and Gortchakovskaya 1945; Wayre and Jolly 1958). Overall, the metabolizable energy requirement for maintenance is higher during moult than during non-moult period and is higher in males than in females (Dietz et al. 1992). Presumably, these differences between the sexes may be related to their roles in reproduction (Espie et al. 1996). In breeding merlins, nearly one-half of all males examined had an arrested moult, while less than 30% of females had arrested moult (Espie et al. 1996). Males that provide most of the food for the family necessitate to slow down and/or arrest moult, whereas females do the same with enlarged broods only (Espie et al. 1996). Interestingly, males that occupy higher-quality territories are able to moult at a faster rate than those of lower quality (Espie et al. 1996).

2.2 Developmental Phase

39

2.2.3  Food Availability During the Developmental Phase Although the physiological links between food supply and reproductive timing are enigmatic, food availability during the developmental phase advances lay date (Davies and Deviches 2014). Experimentally, food supplementation to free-ranging common kestrels advance lay date and produce larger clutches with no effects of habitat selection (Daan et al. 1989; Aparicio 1994a, 1998; Aparicio and Bonal 2002; Davies and Deviches 2014). Inversely, gonad size in raptors that had starved tended to be small (Young et al. 2009). Thus, low food availability limits the initiation of breeding because birds do not have sufficient energy stores to develop their reproductive system at the maximum rate (Lofts and Murton 1968; Davies and Deviches 2014). This is crucial, for example, for migrant falcons where acquisition and allocation of energy after arrival on the breeding grounds largely determine reproductive decisions (Cavé 1968; Lamarre et al. 2017). An extensive review made by Meijer and Drent (1999) demonstrates that the energy requirements of forming eggs are considerable comparing the mass of the average clutch of a species against the average mass of the laying female. In common kestrels during the last 2 weeks before laying, the female increased its mass by 70 g (10% development of oviduct, 30% eggs, and 60% body reserves; Meijer et al. 1989). In fact during the courtship period, females consume more energetically rich prey than males (Catry et al. 2016). During early incubation, the female loss a negligible percentage of body reserves, whereas energy and nutrients for egg-laying come directly by prey delivered by the male (Meijer et al. 1989; see also Sect. 2.4). Thus, the female starts egg formation after the achievement of body reserves that loss later during the early nesting period (Dijkstra et al. 1988). In fact in supplemented female common kestrels, the advance in low prey years is effective (ca. 3 weeks) but not in good prey years (Meijer et al. 1988; Wiebe and Bortolotti 1995). Ultimately, female kestrels in poor condition cannot lay as early as females in better condition nor large eggs (Wiebe and Bortolotti 1995; Aparicio 1998).

2.2.4  Weather Effects The advance of laying time of kestrels depends on the food supply in winter and spring, the amount of precipitation at that time of the year, and the spring temperatures (Cade 1960; Cavé 1968; Potters 1998; Zellweger-Fischer et al. 2011). In harsh and less predictable environments, breeding success is affected by the number of days with extreme weather and extremely low temperature (Bradley et  al. 1997;

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Fig. 2.2  Winter rains may hamper hunting activities of falcons. A common kestrel in the rain grabs an earthworm, easy to capture. (credit: Gaspare Indelicato).

Carlzon et al. 2018). In Arctic environments, the nest choice by peregrine falcons depends on the date of snowmelt that constrains prey availability (Bruggeman et al. 2016). In addition, the mean date of clutch initiation of peregrines and gyrfalcons should be significantly delayed in spring with lower temperatures and high precipitation (Cade 1960; Court 1986; Burke et  al. 2015). On the other hand, in arid ­environments, rainfall may encourage breeding because summer rainfall triggers seeding and hence population increase of seed-eating prey (Sutton 2011). In temperate zone, rain has a delaying effect probably because it hampers hunting activity and prey movements (Fig. 2.2). Accordingly, feeding frequency and daily growth rate of nestlings correlated negatively with the daily rainfall (Kuusela and Solonen 1984). Horváth (1975) reported that common kestrel males only bring prey at nest during windy and rainy days. Unfortunately, daily energy expenditure of kestrels exposed to rain may increase markedly, and they would have to increase their food intake to compensate for the higher energetic demands (Wilson et al. 2004). Indeed, weather effects should be uneven in relation to time when they occur during the breeding season. For instance, high rainfall in April had a negative effect on productivity of peregrines in Northern Spain and sakers in Slovakia (Chavko et al. 2014; Zuberogoitia et al. 2015). Kestrels benefit of warmer and rainier springs but have

2.3 The Breeding Population

41

detriment from rainier springs and warmer and rainier winters (Costantini et  al. 2010a). Dry years result in lower vegetation growth and lower grasshopper population that in turn affect lesser kestrel breeding success (Rodríguez and Bustamante 2003). Nevertheless, weather was not the only important influence on breeding success (Bradley et al. 1997). The breeding success of the common kestrel inhabiting xerophytic scrub on Tenerife Island coincided with stable prey availability (arthropods, lizards, mice, and birds) regardless of wet and dry years (Carrillo and Gonzáles-Dávila 2010b). In fact, it seems unlikely that temperatures could have a direct effect on the process of ripening of the oocytes in the ovary but for homoeothermic maintenance (Cavé 1968). Plasticity in response to these very variable conditions is effectively changing the settlement dates (Mihoub et al. 2012).

2.3  The Breeding Population Among a common kestrel breeding population, Cavé (1968) distinguish (1) breeding individuals of previous years, (2) first year individuals from the same area, and (3) floaters. Thus, options for female choice differed considerably in years of different kestrel density (Palokangas et al. 1992). Floaters have a fundamental role on population growth, especially in small size populations (Hunt 1998; Altwegg et al. 2014). These 1–3-year-old non-breeding individuals vary considerably from year to year (Hunt 1998). Overall, the proportion of fledglings remaining from the previous breeding season is very low compared to adults. Clearly, a very high number of fledglings die or emigrate before the next breeding season (Cavé 1968). For example in Eleonora’s falcons, the mortality of young individuals from fledging and first breeding is ca. 78%, whereas the adult mortality is ca. 13% (Ristow et al. 1989). Favourable conditions allowed females to breed for a second year as well as the female nestlings to remain as breeding birds in their first year (Cavé 1968). Nevertheless, the cost of reproduction reduces subsequent survival and reproduction of parents that raise large number of offspring (Daan et  al. 1996). In fact ­experimentally, 60% of the kestrel parents raising two extra nestlings died before the end of the first winter compared to 29% of those raising reduced broods (Daan et al. 1996).

2.3.1  First Year and Second Calendar Year Breeders Age at first breeding for peregrines is usually 2 years although some females and a few males begin breeding at 1 year of age (Ratcliffe 1962; Tordoff and Redig 1997). The Eleonora’s falcon presents deferred sexual maturity where the age at first breeding is usually 2 (3rd calendar year) for females and 3 (4th calendar year) for males (Ristow et al. 1989). However, a longer delay in the age of first breeding of males

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compared with females has been reported for many falcons including gyrfalcons, peregrines, and merlins (Mearns and Newton 1984; Nielsen 1991; Heyne and Wegner 1991; Lieske et al. 1997). In fact, male juveniles seem to be inefficient foragers or are confined to areas where preys are scarce (Wendt and Septon 1991; Lieske et al. 1997). Thus, it seems that experience of an individual is a key ­component when a male can breed successfully for the first time (Cade 1960; Lieske et  al. 1997). Hickey (1942) reported immature peregrines that fail to lay eggs or incubate unusually small clutches. Nevertheless in a long-monitored peregrine population in Spain, the breeding performance of inexperienced adults did not differ from that of experienced adults (Zabala and Zuberogoitia 2015). As well, the onset of laying of 2cy female common kestrels can be later than adult females, but reproductive outputs among these age categories did not vary significantly (van Dijk 2005). Probably, these females exploit alternative profitable food sources, available later in the breeding season but crucial especially in years with a poor vole abundances (van Dijk 2005). Overall, the percentage of first year breeders is variable but usually low (Cade 1960; Beebe 1960). For instance, among merlin populations in Scotland, the proportion of first year male breeders was 2% overall equivalent to 4% rust year female breeders (Rebecca et al. 1992). The gonads were smaller than in adults throughout spring and started to mature later (Young et al. 2009). In addition, immatures seem to be more prone in prevalences of infections than older falcons (Dawson and Bortolotti 1999). Thus, when yearlings or 2cy individuals attempt to breed, many fail to produce eggs or lay a smaller than normal clutch (Wendt and Septon 1991; Warkentin et al. 1992; Ivanovski 2003). Interestingly, Negro et al. (1992a) recorded two main peaks of copulation before laying by lesser kestrel pairs, but those with yearlings had a single maximum ca. 15 days before laying. At the end of the breeding season, the productivity of pairs which included a yearling member was significantly lower than that of pairs with two adult birds (Warkentin et al. 1992). On the contrary, among adult pairs there was no significant assortment by age (Warkentin et al. 1992). Immature birds (≥ 2 cy) ranged from 13% to 30% of a breeding population (Ristow et al. 1983; Cugnasse et al. 2003). Interannual variability in clutch size and productivity can be attributable to this fraction of the breeding population (mainly females; Cugnasse et al. 2003; Corso and Gustin 2012). In fact, they occupy suboptimal nesting sites, and consequently their breeding success is lower (< 50%) in comparison to ca. 90% of older females (Ristow et al. 1983). Anyway, they attempted to breed paired with adult individuals participating to the saturation of the breeding sites initiated by adult males which prioritize the access to a breeding territory (Cugnasse et al. 2003; Corso and Gustin 2012).

2.3 The Breeding Population

43

2.3.2  Inbreeding Close inbreeding increases the probability of expression of deleterious recessive genes as well causes a detrimental effect that reduces genetic diversity of breeding populations (Tordoff and Redig 1999; Ortego et al. 2008b). Inbreeding is generally considered rare, especially among siblings. Tordoff and Redig (1999) reported inbreeding in 4% of cases among peregrine falcons, including pairs of full siblings from the same brood. Overall, this low ratio limited breeding depression as observed in merlin populations (5% of polygynous males in a kestrel breeding population in Finland. Generally, two females use separate nest close together, but in Israel a case study included two females and one male participated in the raising of young in only one nest (Bond 1946; Korpimäki 1988; Sodhi 1989; Potters 2001; Charter et  al. 2008). Polygyny occurred precisely at laying time, and it is more apparent when food conditions were favourable ­ (Korpimäki 1988; Tella et al. 1996). In fact in those years, unpaired males should be successful adopting polygyny even though the attempt started late in the season (Tella et  al. 1996). Food availability influences also polyandry. For instance, although the male kestrel copulated with two females, eggs failed to hatch because food provisioning should be insufficient (Hugense 2009).

2.3.4  Full Adults Non-breeders The number of non-breeders may vary from 10 to 15% of a breeding population (Cade 1960; Ratcliffe 1962; 1984). Several possible causes were evoked: (1) a higher mortality rate for males, (2) a general excess of females, and (3) more complete desertion by males after being disturbed or after unsuccessfully attempting to breed (see also Sect. 2.14; Olendorff 1971). Unlike breeding females, non-breeding females maintained a relatively low body mass throughout the reproductive season (Dijkstra et al. 1988).

2.4  Mate Choice The arrangement of individuals into mated pairs through intersexual selection is based on the quality of the potential mate (Bortolotti and Iko 1992). For instance, this choice becomes crucial for kestrels that migrate towards the breeding areas after winter or for gyrfalcons that face unpredictable food availability (Cavé 1968; Barichello 2012). A female can choose between several free males with already established nesting territories. In fact such as kestrels in Finland, males arrived a few days before females, but late males arrived much later than early females (Palokangas et al. 1992). As well in peregrines, mate fidelity appears to be a by-­ product of territorial fidelity (Hickey 1942; Tordoff and Redig 1997). For instance, high mate fidelity and nest-site fidelity characterized an urban peregrine population

2.4 Mate Choice

45

with only 9.8% mate changes and 4.9% nest-site changes occurred (Caballero et al. 2016). Nevertheless in merlins, males showed significantly higher levels of site fidelity from year to year than females, 61% and 28%, respectively (Warkentin et al. 1991). Accordingly, mate fidelity was low with only 20% of 60 pairings contained the same birds for 2 successive years (Warkentin et al. 1991). In addition during second year, >68% of individuals change its mate despite it being alive (Warkentin et al. 1991). Thus, the number of male territory holders is one of the main factors limiting the number of breeding pairs and nest-site switching depends mainly on females (Lieske et al. 1997; Zuberogoitia et al. 2015). The breeding season is short with rapidly declining prospects of successful breeding, suggesting that females should make their mating decision quickly. However, she formed short-term pairs until she found the male with the most ­favourable nesting territory (Mihtieva et al. 2017). Thus, prior experience with the area may determine a male’s success in obtaining a territory (Bortolotti and Iko 1992). Females significantly chose the most active and effective hunter male, and its ‘genetic quality’ may be less important in influencing mate choice than factors which directly affect the survival of the females’ offspring (Duncan and Bird 1989; Hakkarainen et al. 1996). Inversely, parental investment by males is high before, during, and after fertilization. In fact, males may vary considerably in their ability to provide and defend resources due to factors such as age or experience. Thus, the benefits to a discriminating female would be large under these circumstances. In general, as male parental investment increases, both the benefits and costs to choosy females increase (Duncan and Bird 1989). In Finland, pairs were usually formed within 2 days of male arrival, and only early males were available to early females (Palokangas et al. 1992). Although falcons are not sensitive to ultraviolets (see Sect. 1.5), Zampiga et al. (2008) reported that female kestrels prefer males with UV-reflecting plumage used as cue of quality. The early peak of copulation would be almost certainly outside the females’ fertile period and would be related to pair bonding and sexual stimulation of the pair members (Negro et al. 1992a). Aborting the reproductive effort early when failure likely may improve the ability of pairs to overwinter and retain nesting territories and so contribute to long-term reproductive success (Barichello 2012). Male kestrels perform courtship feedings during the pairing period that continues during nearly the whole breeding season, whereas females spend less time hunting for herself. Overall, female choice and the pair formation were not significantly correlated with rate of courtship feeding by males (Donázar et al. 1992; Palokangas et al. 1992). Nevertheless in lesser kestrels and gyrfalcons, the mate-feeding seems to increase the female’s body mass, possibly to allow the laying of earlier and larger clutches (Donázar et  al. 1992; Barichello 2012). In fact, the start of the mate-­ feedings was correlated with laying date, and the earlier the laying, the larger the clutch (Donázar et  al. 1992; Barichello 2012). Courtship feeding of gyrfalcons seems to be associated with the decline in the number of the main prey, the ptarmigan (Lagopus muta; Barichello 2012). Nest visits, copulations, and aerial aerobatic displays were more frequent and extended into the post-laying period when ptarmigan were scarce and were more evident of failed and delayed nests (Barichello

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Fig. 2.3  Copulations occur more often than necessary for fertilising eggs because they are adaptive for males as useful mechanisms that reduce cuckoldry. The male lands on female back depressing his tail to one side and maintaining his balance by fluttering his wings. (credit: Paolo Griva and Michele Santona)

2012). However, these behavioural displays appeared to be no substitute for food provisioning, as eventual failures were common where advertising was frequent and provisioning rates were low (Barichello 2012).

2.4.1  Copulation and Extra-Pair Fertilization Copulations last 10–15 seconds and occur more often than necessary for fertilising eggs (i.e. 60 copulations for the whole breeding season of merlins or 0.72 copulation per h in common kestrels; Fig. 2.3; Cade 1960; Sodhi 1991; Korpimäki et al. 1996). Indeed, males having larger clutches showed shorter copulas (Vergara and Fargallo 2008a). In fact, fertile eggs can appear 48 h after insemination indicates that onset of fertility is at most 1 day, allowing for 1 day of egg formation (Bird and Buckland 1976). Thus, frequent or long copulations along with mate guarding are adaptive for males as useful mechanisms that reduce cuckoldry (Sodhi 1991; Korpimäki et al. 1996; Vergara and Fargallo 2008a). Within-pair copulations were not timed during the fertile period, and mate attendance did not increase as the fertile period approached (Villaroel et al. 1998). Nevertheless, in wild lesser kestrels most extra-pair copulation attempts occurred mainly 5 days before the laying of the first egg (Cade 1960; Negro et al. 1992a). It seems that copulation attempts depended on the females, which is the larger sex, and their pairing status (Negro et al. 1992a). Thus, some hypotheses suggesting benefits for females such as food or improved

2.4 Mate Choice

47

mate guarding are improbable. In fact, copulations most often occurred without food transfers, especially outside the fertile period, and female solicitations do not differ in time and frequency (Villaroel et al. 1998). Overall, there were few extra-pair fertilization attempts inside breeding populations, ca. 7% in lesser kestrel colonies and merlins, 50% of young produced by the breeding population (Amato et al. 2014). As well among peregrine falcons, the average number of young fledged from territories with a choice of nest was about 50% greater than that from territories with only one nest (Olsen and Olsen 1989). The access and choice of high-quality sites depend also on intrinsic factors related to the pair or to each partner, respectively (Wright 2003; Zuberogoitia et al. 2015). In lesser kestrels, 78% of all breeding attempts recorded were in previously occupied nests and were more successful than attempts in holes used only once (Negro and Hiraldo 1993). Indeed, reoccupied nests registered earlier laying dates determined by the different times of arrival of each individual at the colony after the winter (see Sect. 2.4; Negro and Hiraldo 1993). Likewise, pairs and single males of gyrfalcons overwintering in the nest cliff in order to secure it (Bente 1981). Overall, cliff occupation by gyrfalcons occurred 3–6 weeks prior to the initiation of egg-­ laying (enough time for the 14-day follicle development; see Sect. 2.2.1 and Box 2.1; Bente 1981). Gyrfalcons exhibited low nest-site fidelity where only 22% of birds returned to the same nest site the following year (Booms et al. 2011). Within territories, peregrines used alternate eyries following 58% of nesting attempts, but females are more flexible in choice of nest site than males (Ratcliffe 1962; Tordoff et al. 1998; Barnes et al. 2015). In fact, in males only 19% chose a different site (Mearns and Newton 1984; Tordoff et  al. 1998; Zuberogoitia et  al. 2015). In addition, newly established females showed a higher tendency to switch (59%) than older territorial females (38%; Zuberogoitia et al. 2015). Nevertheless, both sexes switching nest sites tended to move to ones of higher quality (Espie et  al. 2004). Interestingly among male merlins, mate and site fidelity was statistically independent, whereas females who changed nesting area were unlikely to have the same mate (Warkentin et al. 1991; Wright 2003). Inversely, those females that remained on the same site

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were more likely to have the same mate (Warkentin et al. 1991). As well for merlins, change in eyrie location by peregrines did not influence breeding success and was not predicted by breeding productivity (Espie et al. 2004; Zuberogoitia et al. 2015). Merlins that had been paired for 2 or more years did not have significantly higher productivity than those pairs that remained together for only 1 year (Warkentin et al. 1991). Accordingly, previous experience on a site did not significantly improve an individuals’ productivity when both birds were the same or only one of the pair was the same, compared with that of pairs where both birds were new to the site (Warkentin et  al. 1991). Nevertheless, Zuberogoitia et  al. (2015) found that the number of fledglings in the previous season was the main factor explaining the eyrie switching decision, with successful pairs being more prone to move. In particular, peregrines changed eyries after successfully raising large broods, and eyrie switching increased the breeding success of females but not of males (Zuberogoitia et al. 2015). Perhaps, this strategy should overcome the reduced survival and reproduction chances of parents that raise large number of offspring (Daan et al. 1996).

2.6  Egg-Laying Mean laying dates varied significantly between years (Aparicio and Bonal 2002). In fact, falcons maximize their fitness by two major decisions concerning the start of laying eggs and when to stop laying eggs (Daan et al. 1990). The end of this period is very less variable. Thus, there is a long laying period when the first eggs are early and a short laying period when the first eggs are late (Daan et al. 1990). Among common kestrels, the eggs being laid every other day and a clutch of five eggs take about 10 days (Daan et al. 1990; Wassink 2008). In Eleonora’s falcons and merlins, laying intervals are 2 and 3 days between first and second and second and third egg, respectively (Wink et  al. 1985; Ivanovski 2003). Overall, the egg-laying interval appeared to be greater between the first two and last two eggs of the clutch than between intervening eggs (Porter and Wiemayer 1972). During egg-laying the female’s presence in the nest increased by hours per day till the clutch was completed (Wassink 2008). Exposure to other pairs may have had a negative effect on onset of breeding. For instance, captive paired female American kestrels came into egg production more frequently and more quickly in isolated quarters (Bird et al. 1976). Females that fail to lay are generally unresponsive, and courtship by the male lasts a month longer than normal (Hickey 1942).

2.6.1  Early and Late Breeding An adaptive decline in average fertility, clutch size, and hatchability with progressive date of laying is characteristic of most bird species with a single clutch of variable size per year such as falcons (Cavé 1968; Bird and Laguë 1982b; Meijer 1989;

2.6 Egg-Laying

55

Daan et al. 1990). For instance, common kestrels and Eleonora’s falcons showed the same seasonal decrease in clutch size in different areas and in different years (Wink et al. 1985; Meijer et al. 1988). It seems profitable for the falcon to be early, since early broods are larger and more young fledge (Olsen and Cockburn 1991; Aparicio and Bonal 2002; Zellweger-Fischer et al. 2011). In addition after fledging, the early young seem to have a better chance of survival (Cavé 1968; Daan et  al. 1989; Aparicio and Bonal 2002; Zellweger-Fischer et al. 2011). Casagrande et al. (2006) documented the existence of genetic differentiation between chicks from early (larger) clutches and chicks from late (smaller) clutches. Nevertheless, body condition of fledglings, measured as their pectoral muscle thickness, did not depend on their hatching dates (Aparicio and Cordero 2001; Aparicio and Bonal 2002). During a field experiment using common kestrels, control clutches were also more productive than experimentally delayed clutches despite young being of similar quality. Thus, optimal laying date depends on parental quality, and phenotypic consequences seem to be minor compared with those of hatching early in the season (Aparicio 1998). Indeed, early hatched individuals benefit of a longer growth period s­ uggesting a selection that favours females with high fecundity early in the season. Accordingly, female peregrines on nesting territories that for the first time appeared to lay later had significantly smaller clutches, but did not show a difference in the number of young fledged when compared to females with greater experience on nesting territories (Court 1986).

2.6.2  Maternal Resources Falcons have a predetermined clutch size, and nutrients from the female, deposited in the egg prior to laying, provide all the necessary nutrition for the embryo to develop and for the chick to survive for a few days after hatching (Cavé 1968; Barton et al. 2002). Females rapidly increased in mass 2 weeks before laying, and a significant decrease in size occurred when females laid a second and a third clutches in the same year (Burnham et al. 1984; Masman et al. 1988). Food supplementation experiments with American kestrels showed that laying females probably depend on both stored energy reserves and on daily energy surpluses to form eggs (Wiebe and Bortolotti 1996). Thus maternal resources are limited; the female should adjust resource allocation to different eggs in the clutch influencing the survival expectancies of particular nestlings or entire broods (Wiebe and Bortolotti 1992; Blanco et al. 2003a; De Neve et al. 2008). Maternally derived androgen hormones concentrate in avian egg yolks as the yolks grow on the female’s ovary. Male American kestrel nestlings were more susceptible than female nestlings to growth inhibition by yolk androgen elevation but no bias in sex ratio appeared with respect to laying order (Sockman and Schwabl 2000; Sockman et al. 2008). Production of carotenoid-rich eggs coincided with low levels of circulating carotenoids in females, indicating that carotenoids might be a limited resource for laying female falcons (Dierenfeld 1989; Casagrande et al. 2011). In fact in the yolk, vitamin E and carotenoids are lipid-soluble antioxidants which promote the func-

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tion of the immune system of the developing embryo and chick (Dierenfeld 1989; De Neve et al. 2008). These are stored in the maternal liver and mobilized during the laying cycle. In addition, nestlings of carotenoid-supplemented females were infested by less intestinal parasite groups, had higher lymphocyte concentrations in blood plasma, and were less stressed (De Neve et al. 2008). Interestingly, different globulin concentrations suggest that males benefited more than females from an increase in maternal carotenoid investment (De Neve et al. 2008). Such ability to resist pathogens may account for the probability of offspring returning to the nesting area in subsequent years (Tella et al. 2000a). These results suggest that maternal effects on offspring immunocompetence should have a great role on optimal clutch size rather than a trade-off between the number and quality of the offspring (Tella et al. 2000a).

2.6.3  Eggs Egg characteristics are influenced by different factors in different taxa but primarily by incubation period and efficiency, clutch size, and the incubation site (Bárta and Székely 1997; Birkhead et al. 2019; Nagy et al. 2019). As well as the majority of bird species, the eggs of the larger falcon species are a smaller proportion of the adult female’s body mass (Fig. 2.5; Lack 1968 but see contra Porter and Wiemayer 1972). In addition, falcon’s eggs, laid on bare rock surfaces, were significantly pointed, elongated, and asymmetrical than those incubated in cups controlling for egg volume, clutch size, and developmental mode (Birkhead et al. 2019). Overall, egg formation is proximately constrained by the food supply and, experimentally, falcons laid significantly larger eggs in response to extra food (Wiebe and Bortolotti 1994a, 1995; Aparicio 1998). Daily costs of egg formation were similar for large and small clutches but were extended at the maximum level for 2 more days for every extra egg laid (Meijer et al. 1989). In American kestrels, seasonal and laying-associated increases in plasma prolactin concentrations elevate yolk testosterone concentrations. Thus, food availability and other factors may interact with date to regulate the effects of prolactin on yolk testosterone deposition (Sockman et  al. 2001). Wink et  al. (1985) and Horváth (1955) found a laying sequence in Eleonora’s and red-footed falcons, respectively. In fact, third eggs are significantly smaller than first and second eggs (Wink et al. 1985). Interestingly, initial eggs of common kestrels bearing male embryos were heavier than initial eggs bearing female embryos (Blanco et al. 2003a). In particular, in clutches started with a male egg, egg mass declined in subsequent eggs and vice versa in clutches started by a female egg (Blanco et  al. 2003a). Thus, female embryos had a short embryonic period; hence they befitted to hatch earlier and to obtain large residual reserves and thus larger mass (Olsen and Cockburn 1991; Blanco et al. 2003b).

2.6 Egg-Laying

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Fig. 2.5  Negative relationship between the relative egg weight and female body weight in WP falcons. The relative egg weight is egg weight as a percentage of female weight. (data from Schönwetter 1960 and Newton 1979).

Egg length and breadth of peregrines and kestrels were significantly related to chick mass and sex (Burnham et al. 2003; Blanco et al. 2003a, b). In addition, egg breadth slightly increased and then decreased over the years a captive falcon laid (Burnham et  al. 1984, 2003). Females produce clutches strictly limited by their physiological characteristics, such as endogenous protein stores, oviduct mass, and the rate of protein uptake by ovarian follicles that ultimately determines egg size (Wink et al. 1985). In fact, when comparing between females with a similar investment in eggs, it would only be possible to increase clutch size by laying smaller eggs. In fact, slight intra-clutch variations in egg size occur in response to short-­ term food shortages during laying (Wink et al. 1985; Wiebe and Bortolotti 1995, 1996). Accordingly during years with low prey abundance, mean egg size of clutches of common kestrel in Finland declined with laying date, whereas during increase phases mean egg size remained stable within the season (Valkama et al. 2002). Overall, productivity is dependent on good quality eggs, and their size can be an important determinant of offspring survival in birds (Wiebe and Bortolotti 1995, 1996; Barton et al. 2002). Nevertheless, field studies over multiple years demonstrated that egg volume poorly predicted the probabilities of hatching and fledging as in the case of the common kestrels (Costantini et al. 2010b). As well, egg mass and volume did not significantly vary in relation to weather conditions and did not predict hatching or fledging mass and success (Costantini et al. 2010b; Martínez-­ Padilla et al. 2017).

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2.6.3.1  Eggshells Eggshell colour carries information on intrinsic properties of the female that laid the egg in terms of concentrations of two key eggshell pigments, protoporphyrin IX (related to calcium), and biliverdin (Cassey et al. 2012). The protoporphyrin pigment concentrations were also consistently associated with species that lay maculated eggs and nest in cavities, such as falcons (Cassey et al. 2012). Overall, darker pigmentation has a thermoregulatory effect (Wisocki et al. 2020), but also eggshell pigment deposition has been considered as a mechanism to resist pathogen penetration into the egg (Fargallo et al. 2014). A recent experimental study demonstrated a lack of detrimental effects of the reduction of eggshell pigments on egg hatchability, mortality of kestrel chicks during the nesting period, nestling body condition, or probability of recruitment of young birds (Fargallo et al. 2014). Indeed, protoporphyrin should provide protection to the shell where eggs are laid in particular humid substrates which make them more susceptible to increased trans-shell bacterial infection (Cassey et al. 2012; Fargallo et al. 2014). Horváth (1955) observed a breeding red-footed falcon population in Hungary where less pigmented eggs are produced by younger parents. In birds, increased protoporphyrin pigmentation in eggs is reflective of higher age/quality of females (Martínez-Padilla et  al. 2010). Accordingly, highly pigmented eggs have higher hatching success and egg mass, but they are costly to produce (Martínez-Padilla et al. 2010). However, the level of pigmentation has been shown to change because of environmental influences, including food availability (Hodges et  al. 2020). Indeed, female falcons that laid highly pigmented eggs are those mated with males in better condition (Martínez-Padilla et al. 2010). Eggshell thickness decreases during incubation because of the important effect of calcium uptake by the embryo during development, and also eggs laid later in the sequence had significantly thinner eggshells (Castilla et al. 2010a). Thicker membranes are associated with larger, heavier, and relatively wider eggs but egg-laying sequence, female age, and the study zone did not explain the observed variation of membrane thickness in falcons (Castilla et al. 2010b).

2.6.4  Re-laying Falcons are single-brood species, but, when forced, most pairs re-nested on their original territory (Bowman and Bird 1985). In peregrine falcons, the frequency of re-laying was highest if eggs were experimentally removed after 7–10 days of incubation (Fyfe 1976). A captive American kestrel laid five clutches within 61-day period, but it showed a significant decrease in clutch size between laying (Porter and Wiemayer 1972). Multiple replacements are rare such as the laying of four clutches within a 2-month period by an urban peregrine falcon pair (Herbert and Herbert 1965). However, female parents invested less in second broods because they are more susceptible to costs of reproduction than males (Dawson and Bortolotti 2008).

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Re-nesting intervals were shorter in older and experienced individuals, but no differences existed between first and replacement clutches for fertility, egg dimensions, hatchability, and fledging success (Bird and Laguë 1982a, b; Bowman and Bird 1985). As well, female common kestrels that bred twice in the same area had highly repeatable egg size, and the main part of the observed variation was likely to be due to among female differences (Valkama et al. 2002). In extreme environment such as Arctic and subarctic zones, re-nesting is not likely to be successful (Cade1960). Second-clutch American kestrel males were smaller at hatching than males from first clutches, but this is not the case for the females (Bowman and Bird 1985). Interestingly, first-clutch birds fledged significantly younger than second-clutch birds (Bowman and Bird 1985). In addition, males fledged earlier than females in first clutches, but the sexes fledged simultaneously in second clutches (Bowman and Bird 1985).

2.7  Clutch Size Depending on the species, clutch size in falcons is usually no more than five eggs (till eleven eggs such as in a nest box of common kestrels; Bárta 1990). Clutch fixation occurred sooner after the first egg, around day 2 of laying (second egg) and day 6 (fourth egg), respectively (Meijer 1988; Beukeboom et al. 1988). In fact, laying common kestrels did not respond to eggs added to the nest after the second egg was laid (Meijer 1988). Also, egg removals during laying resulted in the production of extra eggs only in early breeders early in the laying period (Meijer 1988). Thus, the proper timing of reproduction in seasonal breeders is a critical component of reproductive success and fitness (Aparicio 1994a, b; Davies and Deviches 2014). Overall in kestrels, the mean clutch sizes decrease as the laying period progresses (from 6.5 to 3.2 eggs; Cavé 1968; Beukeboom et al. 1988; Daan et al. 1989,1990; Sockman and Schwabl 2001a). Generally, this phenomenon is attributed to differences in age and/or parental quality and food availability (Bond 1946; Cade 1960; Cavé 1968; Aparicio 1994a, 1994b, 1998). Thus, the clutch size is not dependent on environmental conditions directly but only through the time of laying of the clutch (Cavé 1968). Ultimately, Cavé (1968) found that the mean time of laying and the number of breeding pairs are correlated which means that the mean yearly clutch size and the number of clutches are correlated. In short, females that lay earlier in the year produce (1) larger clutches and (2) fledglings with higher survival rates, and (3) they recruit more offspring to the breeding population (Klomp 1970; but see contra Costantini et al. 2009). This indicates that timing of laying is, in part, a property of individual females, independent of environmental factors (Sockman and Schwabl 2001a). In fact in common kestrels, reserves are used to initiate egg-laying and are depleted such that egg size declines with laying sequence (Aparicio 1999).

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Several constraints such as scarce food availability and bad weather conditions (rainfalls and low temperature) forced common kestrels to start laying late but also produce smaller clutches (Cade 1960; Cavé 1968). Under these conditions also the number of breeding pairs is low (Cavé 1968). Ultimately, food availability restricts both clutch size and laying date in the kestrel in both natural and experimental conditions (Dijkstra et al. 1982). In Eleonora’s falcons, the clutch size is correlated with number and total weight of prey present in a nest during the time of egg-laying and incubation (Wink et al. 1980b, 1985). Thus independently of laying date, falcons will maximize their fitness laying a clutch since as large as the number of young that can be adequately fed (Wink et al. 1980b; Aparicio 1994a). The seasonal decline in clutch size may partially offset a predictable seasonal decline in the reproductive value of offspring (Sockman et  al. 2006). Thus, the developmental sibling hierarchy among offspring may hedge against unpredictable changes in resource availability and minimize energy expenditure in raising a brood (Sockman et al. 2006). Indeed in common kestrels, the largest broods, both early and late, had a lower survival than the more common smaller broods (Cavé 1968). Food supply to the female is dependent of the hunting success of the male falcon, and the regulation of clutch size by the hunting success of the male seems to be of a high adaptive significance (see Sect. 2.4; Wink et al. 1980b). In fact, supplemented females laid larger clutches with shorter intervals between eggs (Aparicio 1994b).

2.7.1  Extra Eggs The production of extra eggs ceased at 4 days before the normal final egg (Beukeboom et al. 1988). Clutches, such as in lesser kestrels, appear excessive as only 3% of them yield as many young as eggs laid (Aparicio 1997). The insurance egg hypothesis proposes that extra eggs are an insurance against the failure of any egg, but parents may suffer reproductive costs when all eggs hatch (Aparicio 1997).

2.8  Incubation Incubation by females begins shortly before the clutch is completed and rose from 0 to 98% of time in the course of the laying phase (Beebe 1960; Cavé 1968; Beukeboom et al. 1988; Ivanovski 2003). For instance, incubation of clutches of five eggs usually began with the laying of the fourth egg (Porter and Wiemayer 1972). This rise started sooner and was more rapid in late nests and double-brooding nests due to rapid follicle reabsorption and hence in clutch size reduction (Beukeboom et al. 1988). Small and large falcons show a fairly constant incubation length (28–31 days; Olendorff 1971). Females alone seemed to incubate during the night (Carlier 1993).

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In Eleonora’s falcon at ambient temperatures of 45 °C, unattended eggs reach lethal values of more than 43 °C (Wink et al. 1980a). Brood patches, areas of skin which become devoid of feathers, are highly vascularized for heat transfer from the parent bird to the eggs (Olendorff 1971). A female needs 50–60 min of incubation to raise the temperature of cooled eggs by 7.5 °C (Wink et al. 1980a). Equally males have brood patches, but their contribution is variable but always smallest compared to females (Horváth 1955; Cade 1960). In fact, the female does 90% of the incubation; hence she had much control over future hatching patterns (Wink et al. 1980a; Wiebe et al. 1998a). The proportion of daily incubation increased monotonically with the laying sequence (Wiebe et al. 1998a). Female kestrel shifts averaged 38 minutes period to increase to 200 minutes prior to hatching (Wassink 2008). Incubation by the female (excluding the night) increased of 40% from early phases to prior to hatching, whereas daily effort by the male declined of 90% (Wassink 2008). Unusual patterns of incubation (less than 35%) were most common in females with poor body condition indicating that females are reluctant to jeopardize their own condition (Wiebe et al. 1998a). As consequence in lesser kestrels, mean daily maximum temperature during incubation affected hatching success negatively but only for females in poor condition (Serrano et al. 2005).

2.8.1  Desertion Desertion of clutches occurs fairly frequently, and it is the most important mortality factor for the eggs (see also Sect. 2.14; Cavé 1968; Gombobaatar et al. 2004). The higher proportion of the clutches of kestrels is deserted in seasons with a scarcity of prey probably due to the failure of the male to bring the female enough food (Cavé 1968). Nest desertion in the early stages of reproduction should depend on females in poor body conditions. Although extra food did not significantly affect adult body condition or oxidative status of females, it reduced their chances of nest desertion by decreasing laying costs of heavier eggs for females in poor body condition (Podofillini et al. 2019).

2.9  Hatching Hatching is a crucial event, and the female parent sits very close on pipped eggs (Cade 1960). The time required for each clutch to hatch varied from 2 to 5 days (Horváth 1955; Olendorff 1971). Red-footed and gyrfalcons carry their eggshells from the nest as well as excess food (Horváth 1955: Wayre and Jolly 1958). Overall in common kestrels, eggs did not hatch in the order they were laid and that, although laying order was not sex-biased, females hatched earlier than did males (Blanco et al. 2003a).

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2.9.1  Hatching Failures A variable proportion of the eggs in the fully incubated clutches do not hatch, and most of these are considered to be non-fertile (Hickey 1942; Bond 1946; Vaughan 1961; Cavé 1968). Decreased hatchability represents a cost for the parents in terms of immediate fitness, since it reduced both their number of young fledged and recruits in the breeding population, even when controlling for clutch size (Serrano et al. 2005; Ortego et al. 2010). Overall, the intrinsic properties of individuals or genetic similarity between the parents seem to have no effect on hatching success (Serrano et al. 2005; Ortego et al. 2010). As well, the parent’s choice of cleaned nests (i.e. nest boxes) was irrelevant (Fehérvári et al. 2015). Indeed, hatching failure is simultaneously influenced by several factors acting in a complex way.

2.9.2  Hatching Asynchrony Eggs generally hatched in the order they were laid and both total hatching span of the clutch and the pattern of eggs hatching on certain days corresponded well with incubation behaviour (Wiebe et al. 1998a; Sockman et al. 2000). The rising of the prolactin (a hormone that controls the number of eggs per clutch and the intensity of brood care) enhances the expression of incubation behaviour in a falcon that shows hatching asynchrony (Sockman et al. 2000). It seems that yearling or small females in poor physically conditions were more likely to have such irregular hatching patterns (Bortolotti and Wiebe 1993; Wiebe et al. 1998b). Order of birth has profound consequences on offspring during development and can have effects on individuals later in life (Bortolotti 1986; Martínez-Padilla et al. 2017). For example, the level of lipids in the last-hatched nestling can be affected by the food restriction imposed by hatching order (Massemin et al. 2002). Thus, early hatching may be advantageous to gain a high rank in the size hierarchy within the brood independently of the effect of sex on fledgling mass (Blanco et al. 2003b). Accordingly when broods were food-supplemented, disadvantages of last-hatched nestling disappeared due to reduced sibling competition (Massemin et al. 2003). Overall, first-hatched chicks showed higher survival probability and lifetime reproductive success than their siblings where mainly the smallest nestling did not survive to fledging (Burnham and Mattox 1984; Anderson et  al. 1993b; Martínez-­ Padilla et al. 2017). In an Arctic peregrine population, about half of the last-hatched chicks died within their first 5 days of life by starvation (Court 1986). Nevertheless, surviving last-hatched chicks of asynchronous broods show that standard growth rates and mortality did not appear related to the food available to the parents (Court 1986). This could be explained by the higher body condition attained by first-­hatched chicks at the end of the nesting period, perhaps due to an enhanced competitive advantage for food over their siblings at the time of hatching (Court 1986; Anderson et al. 1993b; Martínez-Padilla et al. 2017). In addition, differential survival of last-hatched chicks

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in asynchronous broods was linked to differences in the attentiveness of adult females (feeding rate and time spent brooding), prey size, and, possibly, nest ledge size (Court 1986). On the other hand, parents, in order to maximize the survival of the entire brood, could allocate the larger sex to late-laid eggs and egg-related resources to the smaller sex (Bortolotti 1986). Kestrels have small synchronous clutches in stable (even if low) abundance years of voles and large asynchronous clutches under food stress (Wiebe and Bortolotti 1994a; Wiebe et al. 1998b; Wiehn et al. 2000). According to Lack (1954), the unpredictable vole numbers may make hatching asynchrony an important reproductive strategy for kestrels. In fact, parents at synchronous nests fed their broods at a higher rate and success than at asynchronous broods (especially in the final part of nestling period) both when food was limiting and when food conditions were substantially improved by food supplements (Wiebe and Bortolotti 1994b; Wiehn et al. 2000). Accordingly, synchronous broods require more energy to rear than asynchronous broods with the same number of young (Wiebe and Bortolotti 1994b).

2.10  Nestlings The female brooded their chicks non-stop until the smallest young had reached its seventh or eighth day of life (Bijlsma 1993). In fact, nestlings obtain the capability of thermoregulation 6 days after hatching (Wink et al. 1980a). However, brooding of the female important for a longer period, however, to prevent chilling of the young at low temperature during the night or in period of rainfall (Wink et al. 1980a; Bijlsma 1993). Parents offered food to newly hatched chicks that, in turn, beg parents for them (see Chap. 6). Chicks begin to imprint on the sounds made by the parents and, ultimately, on the parents themselves as their visual acuity improves (Jones 2001). Anything that is not considered parent or siblings is something to be feared (Jones 2001). The visual recognition of the area and its surroundings was formed, irreversibly, from the age of 15–20 days (Brosset 1973). Overall, the mortality in the first week after hatching was higher than that during the remaining time spent in the nest (Cavé 1968). Accordingly, survival indices for offspring, both in the nest and after fledging, generally decline with the progress of season (Daan et al. 1989). Large broods caused (1) increased daily hunting activity of the parents, (2) reduced growth rate of the nestlings, (3) increased nestling mortality, and (4) enhanced weight loss in the female parent (Dijkstra et  al. 1990b). Inversely, small broods caused an increased food intake by the nestlings (Dijkstra et al. 1990b). In fact, kestrels raised in reduced broods were in better condition as indicated by large tail subterminal bands, a melanin-based trait useful for this purpose (Piault et al. 2012). In addition, nestling kestrels with a larger black tail band were more aggressive and more reactive to potential predators (van den Brink et al. 2012).

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Success in sibling competition is one of the main factors determining individual body condition in the early stages of life and consequently offspring survival and fitness (Vergara and Fargallo 2008b). Nestlings’ body conditions should be evaluated through several biochemical parameters. For instance, increased levels of testosterone tended to negatively affect body condition, reduced cell-mediated immune responses in male and female nestlings, and also diminished the expression of grey rump coloration in male nestlings (Fargallo et  al. 2007). As well, high levels of oxidative damage and stress-induced increases in corticosterone were negatively correlated with body condition of rearing nestlings (Sockman and Schwabl 2001b; Casagrande et al. 2011). Probably, intra-brood sibling competition could play a role in determining oxidative stress especially on males (Costantini et al. 2006).

2.10.1  Growth of Nestlings Nestling growth rates and size at fledging greatly influence future survival and reproductive success (Braziotis et al. 2017). In common kestrel, the growth of body weight, tarsus, and culmen was retarded when the feathers emerged coinciding with an intensification of the metabolic activity (Veiga 1985). The length of the 4th primary seems to be the most useful character for age identification (Voříšek and Lacina 1998). In peregrines after the 21 days, females only continued to grow, and by the end of the nestling period, their total weight and food intake exceeded those of males by 45% and 25%, respectively (Boulet et al. 2001). Body size differences typically influence sibling competition among nestling birds (Fig. 2.6). Overall in falcons, females were significantly heavier and had longer bills and total length than males, whereas males were similar to females in tarsus growth (Braziotis et al. 2017). As consequence, female nestlings require a greater amount of food than do males (Wiebe and Bortolotti 1992; Anderson et al. 1993a; Boulet et al. 2001). Thus, parents must supply more food to satisfy daughters’ needs than to satisfy sons (Anderson et al. 1993a). Nevertheless as for peregrines, they need less food than expected on the basis of body mass due to a greater growth efficiency (Boulet et al. 2001). At 26 days old, 89% of fledglings express their sex through the body size and plumage coloration (Picozzi 1983; Bird and Laguë 1982c; López-Rull et al. 2016). Thus, any differences may not be apparent until the latter part of the nestling period (Boulet et al. 2001). Growth curves revealed differences in parental quality between nests and also the distribution of parental care among siblings (Braziotis et al. 2017). During low prey years, kestrel chicks of both sexes can show a poor health state, but males seem to be more affected by food scarcity than females (Cavé 1968; Laaksonen et  al. 2004b). Although the heavier female nestlings are clearly more competitive, this advantage becomes negligible when parents delivered large prey (Cavé 1968; Anderson et al. 1993b; Boulet et al. 2001). In fact, females showed a clear superiority over male sibling only when food delivered by parents is prey small enough to be seized and swallowed immediately (Anderson et al. 1993b). Thus, variable avail-

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Fig. 2.6  Early hatching may be advantageous to gain a high rank in the size hierarchy within the brood independently of the effect of sex on nestling mass. (credit: Ugo Mellone)

ability of the main large prey from one season to another suggests that similarly variable growth and survival rates of nestlings from one season to the other can be expected (Cavé 1968; Anderson et  al. 1993b). Ultimately, nestlings that received less food stay longer in the nest or leave it at lower weights (Cavé 1968).

2.10.2  R  elationships Between Survival and Environmental Conditions 2.10.2.1  Nestlings Mortality Most nestling mortality occurred in the first week after hatching until the chicks were 2 weeks old (Cavé 1968; Wiehn et al. 2000). Large broods show lower nestling survival than small ones, and individuals of experimentally enlarged group were in poorer body condition than those of reduced groups (Cavé 1968; Fargallo et  al. 2002). The major risk depends upon the shortage of food supply for the nestlings (Cavé 1968). In fact, lower cell-mediated immunity exhibited by chicks (especially males) indicates chick starvation due to food restrictions (Fargallo et al. 2002). Other reported casualties include acute dehydration (in artificial nests), casual accidents, and diseases (Horváth 1955; Zajíc 2003; Catry et  al. 2011). Schilling et  al. (1981) reported a very high nestling mortality (>70%) in nests infested by ticks vectors of diseases. Inversely, dipterans that feed on blood seem to have no impact on nestlings and the breeding success of common kestrels (Kaľavský and Pospíšilová 2010).

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2.10.3  Sex Ratio In many species of birds, the sex ratio is about equal at hatching (Lack 1954). Several factors may modify the primary sex ratio, including differential offspring costs by sexes, mortality of offspring, or parent mortality. Presumably, high dimorphism of falcons promotes differential costs in rearing male and female offspring and unequal competitive ability among siblings (Fargallo et al. 2003). Thus, parents can allocate food resource to their offspring in relation to the intersexual size dimorphism (Fargallo et al. 2003). However, in peregrines and common kestrels, no differences arose in prey delivery rate, biomass of prey brought to the nest, body mass, or heterophile/lymphocyte ratio of the parents (Boulet et  al. 2001; Laaksonen et al. 2004b). In long-term studies on merlins and peregrines, sex ratios did not significantly differ from 1:1 but occasionally on an annual basis (Rebecca et al. 1992; Restani and Mattox 2000; Burnham et al. 2003; Gahbauer et al. 2015). Overall, the number of males in broods of common kestrels declined with progressive date of birth (Dijkstra et  al. 1990a). In fact, falcon breeding populations can produce a large number of females where early laying females likely produce large offspring that produce daughters (Cavé 1968; Olsen and Cockburn 1991; Griggio et  al. 2002; Ristow and Wink 2004; Ristow et al. 2004). Inversely, parents in poor physical condition or during food shortage should produce more males (Wiebe and Bortolotti 1992). In fact, sons of both poor and good condition would be more valuable for parents than daughters (Aparicio and Cordero 2001). Indeed in low prey years, female chicks in all-female broods fledged in poorer condition than those in mixed-sex broods (Laaksonen et al. 2004b). Thus when food is abundant, broods biased towards the larger sex can be reared successfully, while they may suffer under food shortage situations (Laaksonen et al. 2004b). Ultimately, the timing of birth and food availability may select for biased offspring sex ratios when they differentially affect the reproductive value of male and female young (Laaksonen et al. 2004a).

2.10.4  S  iblicide, Cannibalism, Infanticide, and Conspecific Nest Predation As the falcon nestling grows, it begins to recognize its siblings as competitor for food (Jones 2001). Siblicide may occur fairly regularly in peregrine, saker, and gyrfalcon broods that are food-stressed (Cade 1960; Tordoff and Redig 1998; Ellis et al. 1999). Cannibalism has previously been documented for five species of falcons including the peregrine (Ellis et al. 1999). Its frequency can vary between years, especially in territories with fewer prey and lower prey delivery rates (Bortolotti et al. 1991). Adult falcons have been observed feeding dead young to their siblings, but carcasses were eaten in at least 20–60% (Bortolotti et al. 1991).

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Franke et al. (2013) documented an infanticide by a female peregrine falcon that, during a period of intense rainfall, killed and partially consumed its own two distressed nestlings. Bélisle et al. (2012) reported about an injured juvenile peregrine falcon that grabbed from the nest a chick from the new brood made by her parents. Eleonora’s falcons and lesser kestrels perform nonparental infanticide, followed by cannibalistic feeding of the carcass to its nestlings (Negro et  al. 1992b; Hadjikyriakou and Kirschel 2016). In these social falcons the colony size, nest density, and food availability during the breeding period are crucial factors for the occurrence of these feeding strategies (Negro et al. 1992b; Steen et al. 2016). In fact during period of low prey availability, infanticide affects significantly breeding outputs (Gangoso et  al. 2015). Interestingly in a polymorphic falcon such as the Eleonora’s falcon, it usually occurs between nearby nests and is maximized in dense areas, mostly occupied by pale falcons (Gangoso et al. 2015).

2.10.5  Adoptions of Nestlings Falcon parents can adopt alien chicks (Gallo 1974; Tella et al. 1997; de Nie 2014). Adult lesser kestrels seemed unable to finely discriminate between their own and alien chicks (Aparicio 1997; Tella et  al. 1997; Bonal and Aparicio 2009). Alien nestlings are accepted, but they did not improve their body condition and survival (Tella et al. 1997; de Nie 2014). They returned to the nest shared with natural sibling where they received care from their natural parents (Anctil and Franke 2013). Highest percentages of adoptions in colonies of lesser kestrels should be due to recent occupation of a new breeding habitat (Tella et al. 1997).

2.10.6  Parasites and Infectious Diseases Nests used in consecutive years may become heavily infested by ectoparasites that potentially reduce survival of nestlings and adults (McFadzen and Marzluff 1996). In addition, Piross et  al. (2015) observed in a breeding population of red-footed falcons interannual fluctuations of ectoparasites (cf. louses) that presume a host– parasite interaction. Nevertheless, effects on breeding performances should be negligible (Zuberogoitia et al. 2015). Common and lesser kestrels, peregrine falcons, hobbies, and merlins yielded bacteria Escherichia coli but also Staphylococcus spp., Mycobacterium spp., and Pseudomonas spp. as well as viruses (Vidal et al. 2017). Effects of virus infections in nestlings can be mitigated by maternal antibodies (Soltész et  al. 2017). Nevertheless, hatching order negatively influences antibody levels in broods with seropositive nestlings (Soltész et al. 2017). Although malaria infections in male and female parents had no effect on clutch size, hatching success, or nesting success, parental effort by males may be limited by this parasite due to their higher investment during the chick rearing period (Ortego et al. 2008a).

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2.10.6.1  MHC The diversity of pathogens to which a population is exposed as well the rate of exposure promotes the presence of appropriate countermeasures (Gangoso et  al. 2012; Minias et al. 2017). Genes of the major histocompatibility complex (MHC) play an essential role in the adaptive immune response by coding for molecules that recognize and present antigenic peptides to T lymphocytes (Gangoso et al. 2012). Thus, the great diversity of MHC genes should recognize and fight a broader range of pathogens and parasites (Minias et al. 2017). We expected that birds with social behaviour have elevated transmission rates of pathogens due to breeding in dense aggregations. Likewise, migratory birds are exposed to a more diverse fauna of pathogens and parasites. In fact, solitary and resident falcons such as the peregrine and common kestrel show a low MHC diversity than the facultatively colonial lesser kestrel (Alcaide et al. 2007, 2008; Gangoso et al. 2012). Interestingly, the Eleonora’s falcon, a migratory and colonial raptor with a very specialized MHC, shows low parasite infections on adults and none on nestlings (Gutiérrez-López et al. 2015).

2.11  Parental Care and Investment Overall productivity in breeding population depended mainly on parental care rather than nest-site quality per se (Xirouchakis et al. 2012). Accordingly, environmental factors such as wind speed, rainfall, and ambient temperature do not influence directly parental daily energy expenditure but indirectly upon hunting efforts and prey availability (Deerenberg et al. 1995). Indeed, the most critical period is the second nestling week, when both female and nestling fat reserves will decrease to low levels (Brodin et al. 2003). Food availability during the developmental phase allowed fat reserves to females providing a solution to this conflict and is essential for successful breeding (see Sect. 2.2.3; Brodin et al. 2003). However, opportunities for obtaining body reserves may be limited in falcon population that exhibits a migratory habit (Jönsson et al. 1999). In addition, increased effort may make parents susceptible to infection, especially when food was less available (Wiehn and Korpimäki 1998; Wiehn et al. 1999).

2.11.1  Role Specialization Parental efforts made by male and female falcons are shaped by morphology (i.e. reversed size dimorphism) that, in turn, regulates the behavioural relationships among partners. For example, male peregrines spent 12–33% of the daytime in incubation, but after hatching dominant females do not permit it even when they leave the nest (Cade 1960; Carlier and Gallo 1989; Hubert and Carlier 1992). As in the courtship rituals, contacts between male and female become easier through the

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medium of food, and the male learnt to perform prey transfers in order to incubate (Carlier and Gallo 1994). Although male peregrines have less efficient heat transmission capabilities, due to undeveloped brood patches, the investment during incubation may be related to the reinforcement of mate bonding and hence avoidance of divorce (Zuberogoitia et al. 2018a). Immediately after egg pipping, the brooding status changes from the point of view of the parents (Carlier and Gallo 1989). Then, the male shows a submission display with the head down, and, as expression of dominance, female forced her mate to begin hunting (Cavé 1968; Hubert and Carlier 1992; Feldsine and Oliphant 1985). In this period, the female is the only one who feeds the chicks, and it is only at this moment that food contribution from the male occurs (Carlier and Gallo 1989). Accordingly, the female can show aggression towards the male when it ­delivered the prey directly at the nest than when it delivered the prey outside the nest (Cavé 1968; Carlier 1993; Nodeland 2013). Under normal circumstances, the female broods during the first 2 weeks and leaves the young only if hunting is absolutely necessary (Brodin et al. 2003). Ultimately, the dominance of the females over their mates partly conceals the actual parental motivation of the males avoiding conflicts due to mate’s aggressive competition for duties (Carlier and Gallo 1994). Overall, the male hunts and delivers food for the family, and the female lays eggs, incubates, feeds, and protects the young during their first 2 weeks of life. From the third week onwards both male and female cooperate in supplying food to the offspring (Cavé 1968; Masman et al. 1989, Sodhi 1993; Tolonen and Korpimäki 1994). For example, female gyrfalcons conduct incubation (94%), brooding (99%), nestling feedings (89%), food caching (93%), and retrieval of cached prey (96%; Bente 1981). Males provided more food items to the young than females during the day (Donázar et al. 1992). Accordingly, female falcons have compact home ranges, whereas males have very elongated home ranges and mean distance from nest almost three times as elevated as females’ one (Gustin et al. 2014). Alternatively, males perform a large number of short foraging trips, while females made a few long ones (Hernández-Pliego et al. 2017). Prey biomass delivered by males did not increase significantly with distance from the nests, however (Sodhi 1992). Indeed, different breeding stages and prey availability influence length and duration of foraging trips (Sodhi 1993). In fact during the nestling period, males flew more in longer foraging trips, and those with larger broods spent more time flying than males with smaller broods (Sodhi 1993). The energy needs of the young peak early in the nestling period, but they still cannot thermoregulate and therefore need brooding from the female (Brodin et al. 2003). In fact, the only significant factor consistently affecting offspring condition and survival was weather (Dawson and Bortolotti 2002; Zuberogoitia et al. 2013). Thus the female faces a conflict between the need to brood the young and the need to hunt to provision them with food (Brodin et al. 2003). In addition, female participation in the foraging effort was greatest during food shortage and rainy days when males bring significantly less prey to the nest (Jenkins 2000; Chavko and Krištín 2017). Nevertheless, this allocation of the female effort can compromise the security of the nest (Jenkins 2000). In fact, the nest defence intensity by females increase

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with offspring number but not for males (Tolonen and Korpimäki 1995). However, the main female role is related to prey size and prey type (Sonerud et al. 2014). In fact, nestlings are unable to ingest larger prey unassisted, and the female partitioned prey and fed offspring for a longer portion of the rearing period as prey size increased (Sonerud et al. 2014). Ultimately, this role supports the female-biased size dimorphism among falcons, which becomes larger as diet changes from small prey (i.e. insects) via medium prey (reptiles and mammals) to birds and as relative prey size increases (Sonerud et al. 2014).

2.11.2  Prey Provisioning Parents bring more food when the preferred prey are abundant; otherwise they can compensate for this scarcity exploiting alternative food sources, increasing their hunting efforts or the efficiency of their hunting behaviour (Cavé 1968). Experimentally, common kestrels in the boreal forest would be unable to raise an average brood solely on insects or lizards, unlikely to do so solely on shrews, but able to do so solely on voles in a vole peak year (Steen et al. 2011). On the other hand, in warm Mediterranean areas, kestrels successfully feed their young mainly on insects, without reducing fledgling production in relation to populations where voles form the bulk of the nestling diet (Gil-Delgado et al. 1995). The number of prey delivered to the nest increased twofold in the first week after hatching and then slightly increased in the second week and in the third week; from then on, prey delivery remained more or less stable (Wassink 2008). Number of foraging trips made by males per hour increased from the incubation to nestling period and then declined during the fledging period (Sodhi 1993). Parents responded to supplemental food by decreasing the number of visits to the nest (Wiebe and Bortolotti 1994b). In particular, female parents responded to food supplements by decreasing their hunting effort and prey delivery rate, whereas the hunting effort and/or prey delivery rate of males did not change (Wiehn and Korpimäki 1997; Dawson and Bortolotti 2002). In addition, supplemented females only had higher return rates than males, suggesting significant effects of food on female survival (Dawson and Bortolotti 2002). When food was abundant, food caching was a regular phenomenon. In sooty falcons nesting in arid environments, surplus prey items are cached near the nest and used when food is scarce during the hottest hours of the day (Frumkin 1993). In merlin, food caching is common throughout the breeding cycle and is performed by both sexes (Oliphant and Thompson 1976). The diet of chicks is less diverse compared to adults, but different resource allocation between chicks and adults might also result from different energy requirements (see also Sect. 2.11.3; Catry et al. 2016). For example in small falcons, preys

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consumed by nestlings are larger in size than consumed by adults (Sarasola et al. 2003). In fact after the half of the nestling period, females avoid splitting prey among their nestlings but leave them intact inside the nest. Interestingly, experimentally supplemented offspring therefore did not fledge in better condition or have higher survival rates than non-supplemented nestlings (Dawson and Bortolotti 2002). In peregrine falcons, brood size is not related to prey size, although this may play an important role (Zuberogoitia et al. 2013). Efforts made by parents for provisioning prey to nestlings varied yearly in relation to several constraints including weather conditions (Nodeland 2013; Chavko and Krištín 2017). Ultimately, low success rates were recorded in specific breeding territories where persisted adverse weather conditions (Xirouchakis et al. 2012). In fact, the probability that a falcon can return with items of the same prey type repeatedly differed between prey types but also depended on weather conditions (Nodeland 2013). In common kestrels, variations in air temperatures influence activities of terrestrial prey (i.e. lizards and shrews) and, consequently, capture rates (Nodeland 2013). Rainy days affected the daily chick feeding frequency and especially the male provisioning rates (Cavé 1968; Chavko and Krištín 2017). Nevertheless, unfavourable weather conditions constrained the win–stay strategy adopted by common kestrels against terrestrial prey, but for birds no weather variable had an effect (Nodeland 2013). The breeding season of Eleonora’s and sooty falcons is scheduled to coincide with the peak of autumn migration, thereby providing their broods with an ample supply of food (Rosén et al. 1999; Schjølberg 2006; Gschweng et al. 2008; Gangoso et al. 2013). The highest rate of prey arrival observed just after dawn is only partly derived from a major hunting effort by Eleonora’s falcons (Spina et al. 1988). It is remarkable that sooty falcons occupy, earlier in the season, unfavourable desert areas with relatively insufficient prey species (Booth 1961). Prey type and size varied in relation to nestlings’ age and energy demand, especially when their growth rate is higher (Steen et al. 2012). For example, peregrine falcons apparently maintained constant provisioning rates per nestling as brood size increased by increasing prey mass (Palmer et al. 2004). In red-footed falcons, ca. 40% of small prey (i.e. insects) are consumed immediately, but larger prey are almost always carried directly to the nest site (Palatitz et al. 2015). However, a parent can decide to prepare or not a prey (i.e. consuming removed prey parts) in order to increase the overall provisioning rate (Sodhi 1992). Parents usually provide smaller food items to younger than to older nestlings (Slagsvold and Sonerud 2007). In addition, they often remove the head of the prey prior to providing the remaining body to the nestlings (Slagsvold and Sonerud 2007). However, only larger prey (i.e. birds, voles) than small prey (i.e. lizards, shrews) are decapitated (Steen et al. 2010). Thus, it is presumed that kestrel nestlings are unable to swallow, digest, or egest skulls from larger prey (Steen et al. 2010). Ultimately, parents dismember large prey for small nestlings and provide smaller prey to older nestlings when they start to feed unassisted (Steen et al. 2012; Sonerud et al. 2013).

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2.11.3  Parental Investment Successful individuals vary greatly in productivity, which is correlated with life span (Tordoff and Redig 1997). In fact, peregrine falcons that start breeding as yearlings are likely to have greater lifetime reproductive success than birds entering the breeding pool as adults (Zabala and Zuberogoitia 2015). Nevertheless, survival of the parents is negatively correlated with increasing clutch size (Masman et al. 1989; Dijkstra et al. 1990b). Indeed, the investment strategies of males and females are determined by different proximate mechanisms (Dawson and Bortolotti 2008). Experimentally, parental daily energy expenditure is positively associated with the increasing number of young in the brood (Deerenberg et al. 1995). This is apparent in males, whereas females are influenced by nutritional condition of individual offspring (Dawson and Bortolotti 2003, 2008). Male parents responded to brood size variation and adjusted their provisioning behaviour accordingly (Dawson and Bortolotti 2003). In fact, they show a high metabolic rate (>20% than females) which ultimately limited their expected longevity (Masman et  al. 1988). Males’ peak of energy expenditure corresponds to mate and offspring food provisioning, whereas females peak when they actively participate in feeding (Masman et  al. 1988; Sodhi 1993; Tolonen and Korpimäki 1994). Ultimately, variation in breeding success is explained by individual identity, particularly male identity (Zabala and Zuberogoitia 2014). Conversely, female parents did not adjust provisioning in response to brood size (Tolonen and Korpimäki 1994; Dawson and Bortolotti 2003). They show a conservative strategy aimed at maintaining and rationalizing fat reserves collected during the pre-reproductive phase (Wiebe et al. 2000; Xirouchakis et al. 2012). In fact, females stopped hunting during courtship and were fed by the males until the young were 2 weeks old (Meijer et  al. 1989). The deposition of fewer body reserves by late females may let them gain time to advance laying date, maintaining more reserves may buffer early females against adverse weather conditions (Dijkstra et al. 1988). Thus, the advance of laying date is related to individual nutritional conditions (Dijkstra et al. 1988). Masman et al. (1989) demonstrated that male common kestrels strongly respond to food shortage in the nest increased their daily rate of food delivery to the nest and their flight activity. Nevertheless, the improvement in hunting efficiency by late breeding males is slower than early males (Meijer et al. 1989). Likewise, late breeding females are more prone against episodes of food shortage due lower food intake and lower body mass than early breeders (Meijer et al. 1989). After laying, female lost parts of its body mass but maintained the same level until the young hatched, whereas males showed a gradual decrease in body mass during the whole reproductive phase (Village 1983; Dijkstra et  al. 1988). Nevertheless, from hatching onwards, female weights dropped sharply to a minimum level especially during moult (Village 1983; Dijkstra et al. 1988). In fact, both sexes had higher body mass before breeding than after breeding (Dijkstra et al. 1988).

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2.11.4  Parental Favouritism Differential deposition of resources invested in initial eggs of different sex can promote favouritism towards early hatched chicks or avoid any favouritism by producing siblings of each sex with similar mass (Blanco et al. 2003a). In fact, daughters from initial eggs hatched earlier than sons from initial eggs, which may enhance survival of smaller siblings (males) hatched later (Blanco et al. 2003a). Accordingly, the duration of the embryonic period decreased with laying order, and clutch size and female embryos show the shorter embryonic period that allowed them to hatch earlier in the hatching sequence (Blanco et  al. 2003a). By the way, difference in hatching time in favour of females may be sufficient to compensate for their higher requirements that give them a head start in the intra-brood size hierarchy (Fargallo et al. 2003; Blanco et al. 2003a). Ultimately, this mechanism avoids broods with unbiased sex ratios and allow a fledging sex ratio remarkably similar to 1:1 (Blanco et al. 2003b).

2.11.5  Helpers In peregrine falcon and merlins, a certain number of pairs can include a mainly and adult helper; almost all of these are males (Monneret 1983; James and Oliphant 1986). They contribute to increase breeding success and survivorship of the recipients through the defence of the nesting area, food transfer, and prey caching (James and Oliphant 1986). Possible benefits for helper should be linked with their kinship with the brood helped (Monneret 1983; James and Oliphant 1986).

2.12  Fledglings Although newly fledged chicks attain the independence from the nest, they do not catch live prey and chase parents for food. In lesser kestrels, physiological response to stress was higher in males than in females, independently of physical condition but related to individual heterozygosity (Ortego et  al. 2009a). Overall, fledgling body mass seemed to be influenced by pre-hatching or post-hatching maternal effects and by the nest where the bird was reared (Tella et al. 2000b).

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2.12.1  Post-fledging Dependence Period The post-fledging dependence period (PFDP) is the crucial stage when juveniles are still dependent from parents and move around the nest site with the family which occurs from the fledging day to the first of leaving the natal area (Morrison and Wood 2009). Body conditions of fledglings increase with the length of the PFDP where first fledglings received more food than others, and food per fledgling decreased with brood size (Boileau and Bretagnolle 2014). Accordingly, the survival of released saker falcons is positively associated with the increasing length of time spent in the hack and feeding stations vicinity (Dixon et al. 2020). In common kestrels, male parents decreasing their delivery rate with time and females stopped feeding their young 3 days after they fledged (Boileau and Bretagnolle 2014). Sibling competition still continues during the PFDP, but male fledglings showed a higher competitive capacity than females capturing larger prey items delivered by parents (Vergara and Fargallo 2008b). In fact, male fledglings of the common kestrel showing greyer coloration in the rump (an index of competitive capacity) have longer PFDPs than browner males (Vergara et al. 2010). However, before fledglings became independent, social play in the form of chases among fledglings, manipulative play with objects, and the capture of insects by the fledglings were observed (Bustamante 1994). The duration of post-fledging dependence period should vary by species (Cavé 1968; Komen and Meyer 1989; Rebecca et al. 1992; Bustamante and Negro 1994; Bondì et al. 2018; Karyakin et al. 2018). Indeed, parent quality can modulate the PFDP duration mediated by food conditions (Vergara et al. 2010). In fact, fledglings raised by higher-quality parents showed longer PFDP (Vergara et  al. 2010). Nevertheless, increased parental expenditures on offspring during the PFDP may represent a potential cost of reproduction in breeding males (López-Idiáquez et al. 2018).

2.13  Turnover and Recruitment In agreement with the life history theory, reproductive performance of both sexes improved with age (Espie et al. 2000; López-Idiáquez et al. 2016). In females this is due to a strong selective pressure upon non-competitive breeders, whereas males improve within individuals early in their life along with hunting skill (Espie et al. 2000). Overall, longer-lived birds produced more offspring over their lifetimes and thereby had a greater probability of producing recruits (Espie et  al. 2000). Nevertheless, the probability of an individual falcon surviving to become a breeder was not related to size of the brood from which it fledged as well the breeding success of the previous year for adults (Tordoff et al. 2000; Ponnikas et al. 2017). Small falcons show a high rate of turnover related mainly to food supply and seasonal patterns of migration (Village 1985; James et al. 1989). In common kes-

2.14 Total Failures

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trels, the mortality rate of the first year accounted up to 60% with a higher rate in migratory species (Schifferli 1964; Cavé 1968; Snow 1968). In sooty falcons, only about 12 % of fledglings survive to the average age of first breeding (3.8 years) resulting in low recruitment to the breeding population (McGrady et  al. 2016). Minimum first year survival for the peregrine is < 25% (Tordoff and Redig 1997). Predation, starvation, and accidental deaths are the main causes of mortality that however decline to about >35% from the third year onwards (Snow 1968; Young et al. 2009; Nemček et al. 2014; Rahman et al. 2015). The death rate of adult falcons is in average 9–24% (Mearns and Newton 1984; Court et al. 1989; Ristow et al. 1991; Oliphant et al. 1993). Nevertheless, rates in common kestrels reach 35–45% (Schifferli 1964; Cavé 1968). Mean annual turnover in peregrine and gyrfalcons ranged from 15 to 26% of a breeding population, and more male than female young were recruited (Court et al. 1989; Booms et al. 2011). On the other hand, 20 years (Ratcliffe 1984; Watts et al. 2015). For example, the colonization of the Netherlands by the peregrine falcons started with males occupying potential breeding sites, often as juveniles (Fig. 3.1; Pandolfi et al. 2004; van Geneijgen 2014). Initial population growth was exceedingly slow, indicating a decrease in adult survival, and then it increased exponentially (Oliphant and Haug 1985; van Geneijgen 2014; Monneret et  al. 2019). In years of high population increase, the breeding season should be retarded possibly because of greater competition for territory that ultimately results in reduction in clutch size (O’Connor 1982). A substantial number of individuals emigrated annually from a breeding population affecting the population dynamics in other territories (Fay et al. 2019). This natal dispersal averaged shorter distances for males than for females (see also Sect. 5.3). Accordingly in the Netherlands, peregrine falcon males are more often of local origin than

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Fig. 3.1  Ranges (min, max, upper limit) of age of first breeding, lifetime breeding range, and territory occupancy of the breeding peregrine falcon population in the Netherlands during the first 24 years after colonization. (Data from van Geneijgen 2014)

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females originated mainly from breeding sites in Germany (van Geneijgen 2014). In fact, these floaters have a fundamental role on population growth, especially in small size populations (Village 1983; Newton 1988; Hunt 1998; Altwegg et  al. 2014). Ultimately, immigrants increase the average heterozygosity and the allelic diversity and decrease genetic similarity between potential mates (Ortego et al. 2007). Overall, population growth was most sensitive for change in adult survival, followed by juvenile survival, productivity of fledglings, proportion of adults that attempt breeding, and age at first breeding (Hiraldo et  al. 1996). Accordingly, it seems that declines in common kestrel populations are due to low survival rate of adults rather than brood size (Wallin et al. 1983). Thus, survival of adults was negatively density dependent (Lieske et al. 2000). A prudent approach in the evaluation of density estimates is desirable in order to avoid exaggerations due to methodological flaws (Village 1984; Koskimies 2011; Monneret et al. 2019). Brief study periods, small sample areas, and surveys restricted to the classical sites can bias results, especially for widespread and localized falcons (Village 1984; Koskimies 2011; Monneret et al. 2019).

3.3  Breeding Dispersal and Territoriality Territory occupation by falcons varied markedly. However, stable food sources can induce a relatively regular spacing of nesting places (Solonen 1993). Inversely, irregular patterns characterized nomadic falcon species such as small mammals specialists (Solonen 1993). For example, saker falcons exhibit partial nomadism when local prey populations fluctuate widely as well as gyrfalcons in the Arctic where large-scale prey crashes occur (Ellis et  al. 2011). Nevertheless, irregularities in spacing of nesting places were largely due to the uneven distribution of preferred habitats and prey distribution (Nilsson et al. 1982; Becker and Sieg 1987; Solonen 1993). In fact, the access to prey is a potential breeding cost due to additional travelling costs (L’Hérault et al. 2013). Although urban environments show a lower prey diversity than rural areas, the higher prey density and biomass improve greatly breeding outputs of peregrine falcons (Kettel et al. 2019). At extreme ratio, the number of breeding falcons depends on seasonal prey fluctuations or migration patterns (Gaucher et al. 1995; Nielsen 2011; Kassara et al. 2013). Falcons breeding at traditionally occupied nest sites used for nesting had significantly lower variation in productivity, and thus these sites were better quality sites (Wightman and Fuller 2006). In a heterogeneous landscape, these territories are stable in size and time and defended by pairs over the year even when replacements occurred (Fig. 3.2; Pettifor 1983). Aggressive behaviour against conspecifics is a costly behaviour that forms part of reproductive investment (Vergara et al. 2007). Tinbergen (1940) observed that the male common kestrel is more ready to defend the territory than the female, but territoriality decreases somewhat during breeding. Cavé (1968) experiments demonstrated that females share the defence of the terri-

3.3  Breeding Dispersal and Territoriality

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Fig 3.2  A ringed 12-year-old male peregrine falcon F. peregrinus brookei defending its territory from a perch site (Credit: Iñigo Zuberogoitia)

tory and their share increases with time. In particular after the eggs hatched, the intensity of female aggression in intrasexual encounters was related to brood size (Wiklund and Village 1992). Peregrines competed intensively for nesting territories early in the spring, but competition dropped off as breeding commenced (Court 1986). Thus, agonistic behaviour prior to laying and clutch size could play a role as an indicator of individual quality for both sexes (Vergara et al. 2007). Indeed, territorial behaviour is more pronounced when food is scarce, but it is absent when food is abundant (Cavé 1968). Accordingly, annual production is a function of spacing of pairs, which is set during courtship and pre-laying, when prey availability is at its yearly low and when males must forage for both members of the pair (Poole and Boag 1988). The perception of the risk of nest predation can induce a response by falcon pairs (Greenwood and Dawson 2011). In fact, nest predation is one of the primary causes of nest failure in birds and also for falcons (Greenwood and Dawson 2011). Accordingly, breeding life span and nest predation were the most important components of variance in lifetime reproductive success, especially for female falcons (Wiklund 1995). In merlins, nest predation can result in lower return rate by females, but not males (Wiklund 1996). Overall, avoidance of predation might have affected the spatial structure of avian predators assemblage (Solonen 1993). For example, peregrine falcons nest-site choice is towards large cliffs that offer greater protection from predators (Newton 1988). As well as in gregarious falcons, geomorphologic characteristics are considered to offer protection terrestrial intruders (Gaucher et al. 1995; Kassara et al. 2013). Finally, the interspecific competitive effects were less evident (Solonen 1993).

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3.3.1  The Nearest Neighbour Distance (NDD) Generally, NDD declined with increasing numbers; thus increased spacing should be a mechanism for reducing intraspecific competition (Village 1982; Roberts and Jones 2004; Wightman and Fuller 2006). Nevertheless, distances can remain relatively stable in comparison with the increasing size of the population (Brown and Stillman 1998). In fact, pairs are spaced regularly where no nest-site shortage persists and distances vary in relation to food supplies, probably by limiting the number of males that were able to catch enough food to feed a mate (Newton 1988; Village 1989; Barnes et al. 2015). In addition, neighbouring common kestrels that share the same hunting grounds show differences in diet composition reflecting individual preferences or capabilities in catching some prey type regardless of their actual availability (Costantini et al. 2005). In a peregrine falcon population, Wightman and Fuller (2005) found that NDD is significantly greater than the nearest-cliff distance. Thus, spacing among occupied cliffs was probably the most important factor limiting nesting-cliff availability and, ultimately, densities (Wightman and Fuller 2005). Thus, falcons may compete for better quality habitats and may use spacing and physical features as a resource defence strategy (Wightman and Fuller 2006). Overall, reproductive success was not affected by NDD except among gyrfalcons whose reproductive success was significantly lower when conspecifics nested within 5  km than when they nested farther away (Poole and Bromley 1988).

3.4  Nest Defence by Parents Both parents increased their defensive behaviour as the nesting period advanced and the likelihood of offspring survival increased as well (Carrillo and Aparicio 2001; Sergio and Bogliani 2001). Accordingly, first broods are defended more vigorously than replacement broods (Wiklund 1990b). Overall, parents follow a circadian rhythm and attend the nest area less during the main hunting periods in the early morning and late evening (Palmer et al. 2001). In the early breeding period, merlins basically tolerated other raptors, but once the young hatched and especially after brooding ceased, they attack them regularly (Rebecca et  al. 1992). Accordingly, adult peregrine falcons are more aggressive towards other raptors flying close to their nest when they have older nestlings (Aghababyan 2006). Nevertheless, agonistic behaviour (i.e. circling silently and feinted attacks without physical contact), alarm calling while being stationary on the breeding site, and vigorous diving and calling may not discourage a larger predator (Sevink 2009). Eurasian hobbies may continue to attend the nest site for weeks also after failure (de Nie 2013). Female falcons attended and defended the nest more than males, and, as the nesting cycle progressed, female attendance decreased to levels similar to those of males (Palmer et al. 2001; Sergio and Bogliani 2001). They attacked familiar predators

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more vigorously than males as well as unfamiliar ones (Csermely et al. 2006). The intensity of nest defence by females seems to be related to the age of the young and to the brood size (Wiklund 1990a, b; Sergio and Bogliani 2001). In addition, higher attendance in the nest area by females during incubation appeared to be associated with more young fledged among successful pairs (Tolonen and Korpimäki 1995; Palmer et al. 2001). Inversely where males invested little in nest defence, nests show the highest frequency of desertions and low reproductive outputs (Wiklund 1990a, 1996). Carrillo and González-Dávila (2013), studying an island-dwelling common kestrel population, suggest that pairs with more exposed nest sites showed increased defence activities regardless of offspring value. In fact, accessible nests with high risk of predation and failure elicited a strong aggressive response (Morrison et al. 2006; Carrillo and González-Dávila 2013).

3.5  Interspecific Interactions 3.5.1  Predator–Prey Association Predator recognition by prey species is a fundamental trait that increases survival chances. For example, feeding tits evaluate the sparrowhawk as a more dangerous predator than the common kestrel (Tvardíková and Fuchs 2011). Small birds (i.e. larks) bred less often near falcon nests than farther away due to predation (Meese and Fuller 1989; Suhonen et  al. 1994; Norrdahl and Korpimäki 1998; Martínez-­ Padilla and Fargallo 2008). Nevertheless, potential large prey should be found breeding in closer proximity to falcon nest that excludes mainly other aerial predators (Wiklund 1979; Quinn et al. 2003; Hipfner et al. 2011). Likely, woodpigeons in open farmland actively preferred to nest adjacent to common kestrels and Eurasian hobbies (Bijlsma 1984; Bogliani et al. 1999; Bang et al. 2005). Indeed, the proximity of breeding common kestrels may facilitate some protection also to small species such as sparrows from other predators (i.e. corvids) which are chased away by common kestrels near the nest (Charter et  al. 2011). Thus, overall predation risks decrease near falcon nests (Norrdahl et al. 1995; Bang et al. 2005; Hipfner et al. 2011). In fact, although common kestrels fed on average ca. 6% of their chick production, curlews (Numenius arquata) nesting closer to this falcon more than expected by chance (Norrdahl et al. 1995).

3.5.2  Sympatric Falcons A certain level of competition for food and nest sites is possible in sympatric falcons. In particular, many studies focused on peregrine/hierofalcons’ (lanner, saker, gyrfalcon) coexistence (Cade 1960; Pokrovsky and Lecomte 2011; Sarà et al. 2016;

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De Rosa et al. 2019). For example, the diets of sympatric peregrine and lanner falcons overlapped by about 35% as well as a competitive interaction for breeding sites (Jenkins and Avery 1999; De Rosa et al. 2019). However, temporal and spatial segregation of habitat during reproduction might prevail over anatomical specialization for hunting and diet, allowing species coexistence (Cade 1960; Sarà et al. 2016). In addition, breeding sites with different occupancy rates by falcons showed significant differences in environmental attributes (Amato et al. 2014). In fact, the mean slope of the nest territory and the slope of the nest site are the main predictors for differentiating the cliff selection by lanner falcons and by much more competitive peregrine falcons (Amato et al. 2014). As well common kestrels and merlins that occasionally alternate nest site where both species nested very close each other (Rebecca et al. 1992). At landscape level, the range overlap between peregrine and gyrfalcons is very small (ca. 10%) in the Palearctic compared to the Nearctic, where overlap can be up to 30% (Pokrovsky and Lecomte 2011). In fact in the Palearctic ecozone, breeding gyrfalcons are mainly restricted to the forest-tundra zone and coastal rock cliffs, and they rely more often on tree nests than cliff nests, which is almost the reverse picture of the Nearctic, where gyrfalcons are able to nest further north than peregrines (Pokrovsky and Lecomte 2011). However, the recent climate change extended the annual breeding window favouring the expansion of the peregrine falcons also in northern areas occupied by gyrfalcons (Burnham et al. 2019). However at broader scales, peregrine falcon and gyrfalcon populations are generally stable (Franke et al. 2020).

3.5.3  Intra-guild Kleptoparasitism Large raptors (i.e. eagles) can steal prey caught by falcons (Dekker et al. 2012). This practice can affect hunting behaviour and prey choice of the kleptoparasitized falcon. For example, peregrine falcons increase kill rates to compensate prey losses (Dekker et al. 2012). In addition, when eagles were scarce, peregrine falcons hunt ducks and dunlins, but when eagles become numerous, they hunt dunlins only (Dekker et al. 2012). Kleptoparasitic pressure may depress also hunting success of merlins that try to minimize wasted effort avoiding lengthy and conspicuous hunting flights and increasing prey caching (Buchanan 1988).

3.5.4  Proximity with Other Competitors The spacing of breeding falcons tended to be regular, apparently as a result of intraspecific rather than interspecific factors (Poole and Bromley 1988). Nevertheless, the presence of other raptors should be a limiting factors for breeding falcons, in

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103

Fig 3.3  A female common kestrel agonistic interaction with a short-eared owl (credit: Arno van Zon)

terms of nesting space, food exploitation, and lethal interactions. As well as in winter, falcons showed extremely aggressive behaviour towards conspecifics within the boundaries of their hunting fields but also other raptors (Fig. 3.3; Cavé 1968). Interspecific relationships within the community followed a general pattern of dominance related to body mass (Martínez et al. 2008). For example, the effective breeding density of peregrine falcons and gyrfalcons is constrained by the presence of eagles (Ratcliffe 1963; Poole and Bromley 1988; Gainzarain et al. 2000; Sergio et al. 2004; Martínez et al. 2008; Johansen and Østlyngen 2011). In the same way, peregrine falcons displaced merlins from nesting areas, and the former do not nest within a 2 km radius (Rebecca et al. 1992). Eagle owl predation is also the cause of failure of peregrine falcon nesting attempts through predation of young falcons, especially where both species occurred at higher densities (Brambilla et al. 2006a, b, 2010; Krekels 2017; Lindner 2018). Indeed, the co-occurrence of peregrine falcons in high-quality nest sites affects negatively the breeding productivity of eagle owls (Brambilla et al. 2010). Nevertheless, the eagle owl always dominates at places where the two species meet (Lindner 2018). Similarly, the Bonelli’s eagle is the dominant species that exclude peregrine falcons from isolated cliffs (Gil-­ Sánchez 1999). Peregrine density is positively associated with raven density (Sergio et al. 2004). Ravens probably compete also with the gyrfalcons for nest sites, but they create nest sites suitable for later use by falcons (Bente 1981). Nevertheless, ravens can predate young peregrine falcons at their nest site when parents leave the nest without protection (Monneret et al. 2009).

104

3  Competition and Defence

3.5.5  Intra-guild Predation Mesopredator suppression made by apex predators is not an important energetic resource for falcons but rather seems mostly related to diet diversification when the main prey decreases (Lourenço et al. 2011). In peregrine falcons’ diet, other small falcons such as merlins, common kestrels, and especially Eurasian hobbies are found at 70 km) close to the equator due to rainforests interposed to the final southern African destination (see also Sect. 5.8.1; Strandberg et al. 2009a). Lesser kestrels

154

5  Dispersal Patterns

Fig. 5.6  A male red-footed falcon showing slender and pointed wings suitable for long migration patterns. (credit: Víctor Estrada-Devesa)

winter mainly in central West Africa along the Sahelian belt, but also they reach Southern African regions (Rodríguez et  al. 2009; Catry et  al. 2011; Kopij 2012; Limiñana et al. 2012a; Sarà et al. 2019b). Interestingly, there was a strong positive correlation between the longitude of lesser kestrel breeding sites from different European populations and that of the sub-Saharan non-breeding areas (Sarà et al. 2019b). Thus, the use of specific non-breeding area reduced inter-population mixing (Sarà et al. 2019b). Accordingly, private alleles support a strong connectivity between wintering in West African and Western European breeding populations than those wintering in South Africa (Rodríguez et al. 2011). In fact, these latter populations were mostly from Eastern Europe or Asian countries (Pepler et  al. 1994; Wink et  al. 2004).This spacing pattern is also adopted at individual level where lesser kestrels from the same breeding colony were widely spaced throughout the Western Sahel along the borders of Mauritania, Mali, and Senegal (Limiñana et al. 2012a). A similar scenario is also plausible for peregrines occupying the Euro-­ Asian tundra that winter in a very large front from the Mediterranean Basin to the tropical Southern Africa (Dixon 2012). The bulk of the lesser kestrels reach South Africa during October and November and depart again mainly in March (Siegfried and Skead 1971). Return movements of red-footed falcons follow a west-to-east gradient from central Europe and Ukraine to central Russia and far-eastern zones (Nightingale and Allsopp 1994). Far-eastern red-footed falcons from northern Kazakhstan have a clockwise loop migration that begins with a long westward movement around eastern Europe and a return migration farther west crossing Mediterranean bottlenecks (Katzner et  al. 2016). Migrant lesser kestrels returned to the colony from Africa gradually, adults first and then juveniles born the previous year (Negro et  al. 1991; Aparicio and Bonal 2002). Also in Eurasian hobby, adults moved before juveniles (Kjellén 1992). Merlins from Spain returned in Scandinavia following the mean passage date of

5.9  Long Movements

155

their passerine prey (Sunyer and Viñuela 1990). Eleonora’s falcons from Africa made inland movements in areas up to ca. 400 km distant from the respective breeding colonies, visiting several habitats, from forests to arable lands, probably taking advantage of high densities of insects and birds (Mellone et al. 2013a; Kassara et al. 2019). Among them, a certain number of second calendar year birds come back to their natal colonies, but others become non-breeding vagrants (Ristow et al. 1979; Ristow 2010; Ristow and Wink 1995; Corso and Gustin 2012; Mellone et al. 2013a).

5.9.1  Nocturnal Flights Overall, nocturnal flights are uncommon among raptor species and most often closely linked with barrier crossing (Meyer et al. 2003). Small falcon species such as common kestrel, Eurasian hobby, and lesser kestrel perform nocturnal flights during long sea crossing (Meyer et  al. 2003). Indeed, lesser kestrels are capable to cover long daily distances increasing the frequency of nocturnal migration rather than differences in flight speed (Limiñana et al. 2012a). In Eleonora’s falcon, nocturnal migration was observed when flying across the Sahara Desert and the Mozambique channel (López-López et  al. 2010; Hadjikyriakou et  al. 2020b). In tropical Africa, 10% of the flight segments are partly or completely made by hobbies during night hours (Strandberg et al. 2009b).

5.9.2  Feeding Habits during Migration Migrant falcons try to find a trade-off between the loss of energy, the distance from the non-breeding areas, and the predictable (barriers) and unpredictable (weather) constrains along their routes (Mellone et al. 2013b). For example, hobbies flight at different speeds especially in Europe and tropical Africa suggesting a fly-and-­forage strategy (Strandberg et al. 2009a, b). Red-footed falcons prefer lowlands covered with cropland and mosaics of cropland as stopover sites during migration (Bounas et al. 2020). In fact, they adopt a more opportunistic feeding strategies in these sites feeding mainly upon locally abundant prey (Abbingh and van Manen 1992). Also, Eleonora’s falcons show a preference to vegetation-poor areas where individuals travel at slower migration speeds (Kassara et al. 2012; Hadjikyriakou et al. 2020b). Along its 13 days/5656 km migration, an adult sooty falcons made three stopovers in East Africa characterized by moderate to sparse shrub cover associated with potential sources of water (Javed et al. 2012). Overall, intra-African migrants appear to use feeding areas for several weeks compared to Palearctic falcons that use it as nocturnal roost (Rasa 1987; Hadjikyriakou et al. 2020b). After the start of long rains in Kenya, common and lesser kestrels spend some days hunting locusts and flying termites, respectively (Rasa 1987).

156

5  Dispersal Patterns

Feeding habits during migration vary also among adult and immature falcons. Differences include daily hunting activity, aerodynamic characteristics (see Sect. 1.4), the experience in catching prey, and the development of specific search images for prey by adults (Ward and Laybourne 1985). Surprisingly, up to 50 juveniles of non-gregarious species such as the peregrine falcon can perched at very short interindividual distances (>1 m apart) when hunting (Paganini et al. 2018).

5.10  Wintering Harsh winter conditions strongly affect bird’s survival through their physiological demands but also due to limited food access and availability (Newton 1980; Canterbury 2002). Falcons exposed to these conditions show a strong interindividual and intraindividual phenotypic plasticity in basal metabolic rate (BMR) and resting metabolic rate (RMR) for both pre- and post-acclimation (Bush et al. 2008a). In wintering merlins, individuals show high metabolic levels than predicted from their mass, but they minimize foraging activity to limit energy expenditure (Warkentin and West 1990; Bush et al. 2008b). Apart from the breeding period, hobbies spent half of the year in the wintering area and the remaining half on migration (Meyburg et al. 2011). Thus, this period may influence body condition of birds and, in turn, may affect the time of laying and the size of the clutches in the subsequent breeding season (Cavé 1968; Riegert and Fuchs 2011). Breeding and wintering ranges should be completely overlapped, partially separated, or separated into two different geographical regions (up to 6000 km; Newton and Dale 1997). Falcon breeding in a single area do not necessarily follow the same migratory path and do not necessarily use the same wintering grounds (Ganusevich et  al. 2004). Indeed, individuals can respond differently to winter periods where some of them left the breeding area but others remained (see Sects. 5.2 and 5.3; Riegert and Fuchs 2011). Along 60 years of study, a small number of British peregrines had moved over 100 km (Mead 1973). Cavé (1968) reported few recoveries of common kestrels from northern countries and the maximum distance between successive captures of a bird during the same winter was 3 km. Thus, where environmental conditions are favourable, the wintering population could consist almost entirely of locally hatched birds (Warkentin et al. 1990; Adriansen et al. 1997; Śliwa et al. 2009). In southern Spain, wintering population of lesser kestrels occupy areas with milder winters than the rest of the breeding range and almost never experiences frost (Negro et al. 1991). Interestingly in Southern Bohemia, male kestrels able to feed on birds during snow cover could remain in harsh winters with an advantage during competition for breeding sites in the following spring (Riegert and Fuchs 2011). Gyrfalcons travel continually during the non-breeding period resting on icebergs and feeding on seabirds (Burnham and Newton 2011). Some adults travel long distances to the south during the fall and winter presumably due to low abundances of ptarmigan populations in northern regions (Dementiev 1960). However, the migratory status affects the onset of reproduction of falcons where resident birds

5.10 Wintering

157

have a wider choice of times to breed but migrants must adhere to very strict timing of their breeding season (see Sects. 5.1 and 5.3; Cade 1960). Falcons can maintain an individual home ranges in winter regardless of the distance travelled to reach the wintering areas (Warkentin and Oliphant 1990; Ganusevich et  al. 2004). Wintering peregrines defended a mean home range or 2000 individuals in Sicily) in August (Clark 1999; Sarà et al. 2014). Also, red-footed falcons form communal night roosts, except when breeding, and usually migrate in small to large flocks (Clark 1999). Less urbanized areas’ traditional arable land, rich of orthopterans, attracts a large number of lesser kestrels fuelling before migration (Sarà et al. 2014).

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Index

A Abundances, 18, 20, 95–98 Aerial hunting, 121, 122 Afro-Palaearctic migratory falcon, 153 Alarm calls, 129 Amino acid, 170 Anatomical and physiological adaptations, falcons digestive efficiency, 112 energetics, 115 feeding strategies, 111 gastrointestinal tract, 112 intestines, 112 larger sternum surface area, 111–112 pellets, 114, 115 prey consumption, 114 prey handling, 113 shape of beaks, 113 Androgens, 37 Anti-predator strategies, 127–129 Arctic environments, 40 Artificial colony falcons’ breeding performances, 194 lesser kestrels, 194 nest boxes, 194 nest-site provision, 195 predation rate, 194 red-footed falcon, 194, 195 Autumn migration, 142 Avian predators, 127 Avian prey, 112

B Basal metabolic rate (BMR), 156 β-hydroxybutyric acid (BUTY), 37 Bird-eater falcon, 119 Breeding coloniality, 190 Breeding dispersal, 98, 99 Breeding distribution, 96 Breeding population, 41, 42 C Carotenoid-based colouration, 177 Carotenoids, 13, 17, 18, 176 Chitin, 114 Coloniality age-dependent access, 191 benefits, 189 breeding success, 191 climate, 190 costs, 189, 190 dispersal, 191 extra-pair fertilizations, 194 growth, 191 inter-colony movements, 192 isolated colonies, immigrants, 191 land use, 190 lesser kestrel, 188 occupation, 188 paternal assurance, 194 philopatric and dispersal behaviours, 191 physiological benefits, lesser kestrels, 191 prey depletion, 192, 193

© Springer Nature Switzerland AG 2020 G. Leonardi, Behavioural Ecology of Western Palearctic Falcons, https://doi.org/10.1007/978-3-030-60541-4

201

Index

202 Coloniality (cont.) systemic oxidative stress, 191 western Palearctic falcons, 188 Colony size, 189 Communication calls, 178, 180 courtship displays, 180, 181 pair bonding, 179–181 parental–offspring communication, 177, 178 plumage colouration, 170 polymorphism, 175 sexual dichromatism, 172, 173 sexual monochromatism, 173, 174 sexual ornaments, 176 signals, 169 skin colouration, 177 Cooperative breeding, 43, 44 Cooperative hunting, 123 Copulation, 181 Corticosterone, 37 Courtship displays, 180, 181 D Dichromatic falcons, 176 Diets composition, falcons, 116–117 environmental conditions, 118 mammalian prey, 118 non-random prey selection, 116 pellets, 116 peregrine falcons, 118 prey remains, 116 prey size, 116 raptor assemblages, 116 seasonal variation, 118, 119 species level, 116 Dispersal patterns breeding seasons, 141 common kestrels, Finland, 141 fly-and-forage strategy, 143 genes, 141 individual conditions, 143 individual decisions, 145 interannual variations, 143 juvenile falcons, 145–147 long movements Afro-Palaearctic migratory falcon, 153 Eleonora’s falcons, 155 European peregrines, 153 feeding habits, migration, 155, 156 gregarious breeding species, 153 lesser kestrels, 154

male red-footed falcon, 154 migrating falcons, 153 nocturnal flights, 155 peregrine falcon breeding, 153 migration, 141, 142 natural barriers, 150, 151 philopatry, 147 pre-migration, 148–150 seasonal movements, 147, 148 species distribution, 141 spring and autumn movements, 142 weather conditions, 151, 153 wintering, 156–159 DNA-based molecular methods, 116 E Eleonora’s falcons autumn, migration routes, 152, 153 coloniality breeding colony, 191 breeding season, 189 climate, 190 costs and benefits, 189, 190 extra-pair fertilizations, 194 inter-colony movements, 192 land use, 190 occupation, 188 paternal assurance, 194 grey nest, 175 individual migration pattern, 145 migration, 143 offshore hunting, 122 philopatry, 147 Environmental oxidative stress, 170 F Falcon foraging distributions, 120 Falconidae family behavioural repertoire, 23–25 bone characteristics, 1 carotenoids, 13, 17, 18 diversification, 1–3 endemic species, 3 endemic subspecies, 10, 11 features, 1 flight performances, 20–22 fossils, 3, 4 genetic divergence, 3 genetics, 1, 4 grasslands xeric habitats, 3 habitat preferences, 2 habitat types, 1

Index haplotype diversity, 5 Hierofalco complex, 5 hybridization, 5 hybrids, 11, 12 kestrels, 3 melanin, 13, 16, 17 migration patterns, 3 monophyletic groups, 4 morphological and molecular studies, 1 morphological differences, 2 morphology, 15, 20 palearctic ecozone, 8 parrots, 1 passerine birds, 1 phenotypic traits, 15 physiological functions, 15 polymorphisms, 14, 15 RSD, 18–20 seriemas, 1 subspecies, 8, 10 systematics, 3, 12 taxonomy, 5 vision, 21–23 Western Palearctic, 6, 7 Falcons, 187 age-specific plumage morphs, 170 predators, 169 vocal communication, 180 V-type (violet) visual sensitivity, 176 Fledglings, 73, 74 Flight-hunting, 122 Follicle-stimulating hormone (FSH), 37 Food supplementation, 39 Foraging areas, 119, 120 G Glutathione (GSH), 16 Gonadal steroids, 36 Gonadotropins, 37 Gregariousness, 187 Group living artificial colony, 194, 195 benefits, 187 coloniality (see Coloniality) competition, resources, 187 costs, 187 group hunting, 187 mixed species colonies, 195 nonreproductive stage, 196 potential infection, 187 Gyrfalcons, 53, 96, 129, 156, 157, 175

203 H Heterozygosity, 47 Hunting techniques aerial attacks, 121, 122 common kestrels, 120 cooperative hunting, 123 crepuscular and nocturnal hunting activities, 122, 123 environmental factors, 120 falcons, 120 kleptoparasitism, 125 lesser kestrels, 120 perch-hunting, 121 scavenging, 125 surprise attacks, 124 Hybrids, 11, 12 I Interspecific interactions intra-guild kleptoparasitism, 102 intra-guild predation, 104 predator–prey association, 101 proximity, 102, 103 sympatric falcons, 101, 102 J Juvenile dispersal, 145–147 Juvenile peregrine falcon, 171 K Kleptoparasitism, 125 L Land use, 190 Lesser kestrels artificial colony, 194 coloniality breeding colony, 191 climate, 190 costs and benefits, 189, 190 extra-pair copulations, 194 individuals, 187 inter-colony movements, 192 land use, 190 occupation, 188 orthopters, 193 prey depletion, 192, 193 intraspecific kleptoparasitism, 125 melanin-based plumage traits, 172 mixed species assemblages, 195, 196

Index

204 nonreproductive stage, 196 philopatry rates, 147 summer concentrations, 149 weather conditions, 153 Low-cost perch-hunting, 115 Luteinizing hormone (LH), 37

thermoregulation, 63 visual recognition, 63 Nocturnal flights, 155 Nocturnal hunting, 123 O Offshore hunting, 122

M Major histocompatibility complex (MHC), 68 Mammalian-eating falcons, 111 Mammalian prey, 118, 119 Mate choice assortative mating, 47 benefits, 45 copulation and extra-pair fertilization, 46, 47 early mate replacement, 47 genetic quality, 45 gyrfalcons, 44 intersexual selection, 44 male kestrels, 45 mate-feedings, 45 territorial fidelity, 44 UV-reflecting plumage, 45 Mating calls, 180 Melanin-based colouration, 176 Melanogenesis pathway, 172 Migrant falcons, 143 Migration, 141, 142 Mixed species association, 195, 196 Moult, 38 Movement patterns, 146 N Nearest neighbour distance (NDD), 100 Nest defence, 100, 101 Nestling falcons, 178 Nestlings adoptions, 67 biochemical parameters, 64 cannibalism, 66, 67 conspecific nest predation, 66, 67 corticosterone, 64 factors, 64 growth, 64, 65 infanticide, 66, 67 mortality, 63, 65 parasites and infectious diseases, 67, 68 sex ratio, 66 siblicide, 66, 67

P Palearctic migrants, 148 Parental investment, 45, 72 Parental–offspring communication, 177, 178 Pellet composition, 115 Pellets, 114, 115 Perched red-footed falcons, 150 Perch-hunting, 121 Peregrine density, 103 Pheomelanin synthesis, 176 Philopatry, 147 Photoperiod, 36, 37 Plumage colouration juvenile plumage, 170, 171 plumage maturation, 170 polymorphism, 175 sexual dichromatism, 172, 173 sexual monochromatism, 173, 174 sexual ornaments, 176, 177 Plumage maturation, 170 Polyandry, 43, 44 Polygyny, 43, 44 Polymorphism, 14, 175 Population dynamics, 96–98 Post-fledging dependence period (PFDP), 74 Predator-prey relationship anti-predator strategies, 127–129 synchronization, 127 Pre-migration, 148–150 Prey anatomical and physiological adaptations (see Anatomical and physiological adaptations, falcons) avian, 112 diets (see Diets) foraging area, 119, 120 hunting techniques (see Hunting techniques) insects, 111 mammalian-eating falcons, 111 predator/prey relationship, 127, 129 prey choice, 125, 126 profitability, 111

Index size, and energy contents, 111 vertebrate taxa, 111 Prey choice, 125, 126 Prey depletion, 192 R Red-footed falcons artificial colony, 194, 195 behavioural plasticity, 145 breed solitarily, 188 coloniality climate, 190 extra-pair copulations, 194 land use, 190 migration routes, 148 nests, 188 spring irruptions, 149–150 weather conditions, 151 Reproductive strategies breeding population, 41, 42, 68 clutch size, 59, 60 developmental phase, 39 egg-laying characteristics, 56 early and late breeding, 54, 55 eggshells, 58 female embryos, 56 food availability, 56 formation, 56 kestrels, 57 maternal resources, 55, 56 peregrines, 57 physiological characteristics, 57 re-laying, 58, 59 environmental factors, 68 fledglings, 73, 74 food availability, 68 food supply, 35 hatching, 61–63 helpers, 73 inbreeding cooperative breeding, 43, 44 genetic diversity, 43 natal dispersal, 43 non-breeders, 44 polyandry, 43, 44 polygyny, 43, 44 siblings, 43 incubation, 60, 61 moult, 38 nest-site choice area surrounding, 52 aspect, 51

205 breeding density, 48 features, 49 height, 49, 50 occupancy and alternative, 53, 54 profitability, 49 public information, 51 slope, 49, 50 structure, 51 parental favouritism, 73 parental investment, 72 photoperiod, 36, 37 prey provisioning, 70, 71 recruitment, 74, 75 role specialization, 68–70 total failures, 75, 76 turnover, 74, 75 vertebrates, 35 weather effects, 39–41 Resting metabolic rate (RMR), 156 Reversed size dimorphism (RSD), 18–20, 116 Rock ptarmigan, 129 S Saker falcons, 126 Scavenging, 125 Seasonal movements, 147, 148 Sexual dichromatism, 172, 173 Sexual monochromatism, 173, 174 Sexual ornaments, 176, 177 Skin colouration, 177 Social signals, 177 Solitary breeder, 188 Sooty falcons, 122, 157 Spatial synchrony, 127 T Territoriality, 20, 98, 99 Trans-Saharan falcon, 190 Triglyceride (TRIG), 37 U Urban environment, 120 V Vocal communication, 180 Vocalizations, 180, 181

Index

206 W Weather conditions, 119, 151, 153 Western Palearctic falcons, 172–174 Winter diet, 115 Wintering, 20 annual rainfall patterns, 158 bird’s survival, 156 BMR, 156 breeding, 156 diets, 158

environmental conditions, 156 falcons, individual home ranges, 157 gyrfalcons, 156, 157 habitat preferences, 157 lesser kestrels, 156, 157 migrating lesser kestrels, 157 non-migratory kestrels, 158 RMR, 156 sooty falcons, 157