Wintering Raptors and Their Avian Prey: a Study of the Behavioural and Ecological Effects of Predator-Prey Interactions
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WINTERING RAPTORS AND THEIR AVIAN PREY: A STUDY OF THE BEHAVIOURAL AND ECOLOGICAL EFFECTS OF PREDATOR-PREY INTERACTIONS. Will Cresswell This thesis has been composed by myself and represents my own work except where acknowledged. Thesis submitted for the degree of Doctor of Philosophy University of Edinburgh 1993 - 1 - / ABSTRACT Sparrowhawks, peregrines, and merlins were observed hunting known prey populations (mainly redshank, dunlin and skylarks) at the Tyninghame estuary (East Lothian, Scotland). Raptor predation was shown to be the most significant cause of mortality in some wader species. Kieptoparasitism of raptors carrying prey, by carrion crows, significantly increased the over winter mortality of some waders. Redshank populations were most affected by raptor predation. No selection for body size in redshank was found, but juveniles were more likely to be killed by raptors. This was a consequence of adult redshank risk-averse foraging, and excluding juveniles from low- risk, and low-feeding profitability areas. Juveniles, even though feeding in a relatively profitable area compared to adults, still showed risk-prone foraging within that area. Flocking reduced an individual redshank's probability of being killed by a raptor. Larger flocks were preferentially attacked, but an attack was significantly more likely to succeed on a smaller flock. Within a larger flock a redshank was less at risk through the "dilution" effect, vigilance effects (which were shown to be a direct consequence of flock size) and probably also the "confusion" effect. Redshank did not gain any foraging benefits within larger flocks. Redshank responded differently when attacked in a similar way by the three species of raptor. The response that was most likely to lead to escape from a sparrowhawk was most likely to lead to capture on peregrine attack, showing that raptor discrimination was important. Skylarks used song as a pursuit-deterrent signal on attack by merlins. Other anti-predation responses also influenced the probability of capture, and their use probably depended on the condition of the skylark. All three raptors were generalist avian predators with overlapping diets, although merlins specialised on skylark and dunlin. Interspecific raptor size differences and intraspecific sexual size dimorphism were reflected in differences in the size of prey chosen. The three raptor species differed in their attack characteristics, but prey capture success rates were similar and generally low. Attack success rate was determined mainly by prey behaviour, condition of the prey, and time since the previous hunt. Attack rates depended on availability of prey, temperature and time of day. Prey was chosen according to size, numerical availability and vulnerability of a species (which was shown to consist of attack rate, success rate, conspicuousness of the prey, and association with vulnerable species). The three raptor species were probably energy-intake minimisers during the winter: most of the daily time budget was spent inactive, and much time was available for the usually very brief hunts. Most mobbing of raptors was probably kieptoparasitism. Kleptoparasitism risk between raptors and from crows was high: much of sparrowhawk feeding behaviour reflected this risk. Interactions between raptors appeared to be based on the relative risk each species represented as a kleptoparasite. - 2 - ABSTRACT Raptor predation was studied by direct observation of sparrowhawks, peregrines, and merlins, hunting a known prey population, and subsequent recovery of kills. Raptor predation was shown to be the most significant cause of mortality in most wader species. KleptoparasitiSm of raptors carrying prey, by carrion crows, significantly increased the over winter mortality of some waders. Redshank populations were most affected by raptor predation; over 50% of the total population and over 90% of the juvenile population were taken in two winters. No selection for body size in redshank was found, but juveniles were more likely to be killed by raptors. This was a consequence of adult redshank risk-averse foraging, and excluding juveniles from low-risk,and low-feeding profitability areas. Juveniles, even though feeding in a relatively profitable area compared to adults, still showed risk-prone foraging within that area. Flocking reduced an individual redshank's probability of being killed by a raptr. Larger flocks were preferentially attacked, but an attack was significantly more likely to succeed on a smaller flock. Within a larger flock a redshank was less at risk through the "dilution" effect, vigilance effects (which were shown to be a direct consequence of flock size) and probably also the "confusion" effect. Redshank did not gain any foraging benefits within larger flocks. Reduced individual risk of predation appeared to be the main reason for flocking. Redshank responded differently when attacked in a similar way by the three species of raptor. During an attack the probability of capture depended on the escape response. The response that was most likely to lead to escape from a sparrowhawk was most likely to lead to capture on peregrine attack, showing that raptor discrimination was an important determinant of over-winter survival. Alarm calls were relatively unimportant as an anti-predation behaviour in redshank. Skylark, in contrast, used song as a pursuit-deterrent signal on attack by merlins. A merlin was more likely to give up sooner and to be unsuccessful if the skylark being chased had sung at the beginning of the attack. Skylarks used other anti-predation responses on merlin attack, which also influenced the probability of capture. All three raptors were generalist avian predators with overlapping diets, although merlins specialised on skylark and dunlin. Interspecific raptor size differences and intraspecific sexual size dimorphism were reflected in differences in the size of prey chosen. Sparrowhawks mainly made single attacks per hunt, at prey on the ground; peregrines made several attacks per hunt, at prey in flight; merlins were intermediate. Chase lengths were mostly very short except for some merlin attacks on skylark. Prey capture success rates were low and depended mainly on prey behaviour, condition of the prey, and time since the previous hunt. Attack rates depended on availability of prey, temperature and time of day. Prey was chosen according to size, numerical availability and vulnerability of a species (which was shown to consist of attack rate, success rate, conspicuousness of the prey, and association with vulnerable species). Most mobbing of raptors could be interpreted as kleptoparasitism. Kleptoparasitism risk between raptors and from crows was high: much of sparrowhawk feeding behaviour reflected this risk. Raptor-raptor interactions appeared to be based on the relative risk each species represented as a kieptoparasite. The three raptor species were probably energy-intake minimisers during the winter: most of the daily time budget was spent inactive, and much time was available for the usually very brief hunts. - 2 - TABLE OF CONTENTS Acknowledgements 8 Chapter 1: General Introduction 11 Introduction ...................................... 1.2 Predator Responses to Prey Population .............11 1.3 Optimal Foraging..................................1 2 1.4 Density Dependent Effects of Predator Numbers on Prey Numbers ...........................14 1.5 Behavioural. Responses by Prey to the Risk of Predation .........................................16 1.6 Behavioural Responses of Predators to Anti-predation Behaviour ..........................21 1.7 Evolutionary Consequences of Predator-Prey Interactions ......................................24 1.8 The Aims of the Study.............................26 Chapter 2: Methods 2.1 Study Site ........................................27 2.2 Recording Raptors .................................30 2.3 Time Budget Data ..................................32 2.4 Attack Data .......................................34 2.5 Prey of Raptors 2.5.1 Census Methods ....................................39 2.5.2 Kills ............................................. 4 0 2.6 Redshank Data 2.6.1 Biometrics ........................................46 2.6.2 Number of Redshank Present ........................49 2.6.3 Ageing of Redshank ................................50 2.6.4 Territorial Interactions ..........................51 2.6.5 Flock Size Availability........................... 51 2.6.6 Spacing and Distance to Cover.....................52 2.6.7 vigilance and Feeding Data: Winters 1 and 2 .......52 2.6.8 Feeding Data: Winter 3 ............................55 2.6.9 False Alarm Flight and Alarm Call Data ............59 2.6.10 Approach Distances ................................61 2.7 Additional Data ...................................62 2.8 Statistical Analysis ..............................62 - 3 - Chapter 3: Raptor Predation on Wintering Wader Populations at the Tyninghame Estuary, East Lothian 3.1 Introduction ...................................... 64 3.2 Results 3.2.1 Predators ......................................... 67 3.2.2 Waders as prey .................................... 71 3.2.3 Estimates of Mortality ............................ 76 3.2.4 Redshank Population ............................... 80 3.2.5 Selection of Redshank by Raptors .................. 85 3.2.6 Dunlin Population ................................. 8 7 3.3 Discussion 3.3.1 Is Tyninghame Unusual in Terms of Raptor Predation on Waders' .............................