Hunting Tactics of Peregrines and Other Falcons
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DICK DEKKER Hunting tactics of Peregrines and other falcons Promotoren Prof. Dr. R. C. Ydenberg Hoogleraar Faunabeheer, Wageningen Universiteit Prof. Dr. H.H.T. Prins Hoogleraar Resource Ecology, Wageningen Universiteit Promotiecommissie Dr. W. Cresswell University of St. Andrews, Scotland Prof. Dr. R.W. Goldbach Wageningen Universiteit Dr. H. Hakkarainen University of Turku, Finland Dr. B. Nolet Nederlands Instituut voor Ecology (KNAW), Maarssen Dit onderzoek is uitgevoerd binnen de onderzoekschool C.T. de Wit Graduate School for Production Ecology and Resource Conservation Hunting tactics of Peregrines and other falcons Proefschrift Ter verkrijging van de graad van doctor op gezag van de Rector Magnificus van Wageningen Universiteit Prof. Dr. M.J. Kropff in het openbaar te verdedigen op woensdag 18 februari 2009 des namiddags te vier uur in de Aula. Dekker, D. Hunting tactics of Peregrines and other falcons. Ph.D. thesis, Wageningen University, Wageningen, The Netherlands with a summary in Dutch. ISBN: 978-90-8585-328-2 Hunting tactics of Peregrines 5 TABLE OF CONTENTS Abstract ............................................................................................................... 6 Voorwoord........................................................................................................... 9 Samenvatting ..................................................................................................... 13 Foreword............................................................................................................ 16 Chapter 1. Introduction, study areas, and methods........................................... 19 Chapter 2. Peregrine migrations in Alberta ...................................................... 30 Chapter 3. Hunting strategies of migrating Peregrines..................................... 41 Chapter 4. Duck hunting by migrating and wintering Peregrines..................... 60 Chapter 5. Foraging habits of wintering Peregrines ......................................... 66 Chapter 6. Marine Peregrines hunting seabirds ................................................ 72 Chapter 7. Over-ocean flocking by wintering Dunlins..................................... 77 Chapter 8. Peregrine predation on Dunlins in relation to ocean tides............... 85 Chapter 9. Peregrine prey selection and eagle interference .............................. 92 Chapter 10. Cooperative hunting by parent Peregrines and their fledglings.. 101 Chapter 11. A comparison of Peregrines and Merlins hunting small shorebirds and passerines............................................................ 115 Chapter 12. Nest site competition between Peregrines and Prairie Falcons ... 123 Chapter 13. Gyrfalcons and Prairie Falcons hunting city pigeons.................. 135 Chapter 14. Gyrfalcons and Bald Eagles hunting wintering Mallards............ 144 Chapter 15. Herfsttrek en jachtgewoontes van de Slechtvalk in Friesland...... 150 Chapter 16. Discussion and synthesis............................................................. 163 Acknowledgements ......................................................................................... 177 Literature Cited................................................................................................ 179 Curriculum Vitae…………………………………………………….………..192 6 Hunting tactics of Peregrines ABSTRACT This dissertation describes the foraging habits and capture rates of four species of bird-hunting falcons; Peregrine (Falco peregrinus), Merlin (Falco colum- barius), Gyrfalcon (Falco rusticolus), and Prairie Falcon (Falco mexicanus). Eight of the nine study areas were situated in western Canada in widely different habitats, and the observation periods intermittently included all seasons over 44 years, 1965–2008. The various chapters report and compare the location-specific hunting methods and choice of prey of these falcons in the following scenarios: (1) Migrating Peregrines hunting waterbirds at Beaverhills Lake, a large wetland in central Alberta; (2) Migrating Peregrines and Merlins capturing small shore- birds and passerines at Beaverhills Lake; (3) Breeding Peregrines that launched their hunts from the high chimneys of an industrial powerplant by a large Alberta lake and selectively took gulls; (4) Marine Peregrines nesting on Pacific island cliffs and preying on seabirds; (5) Peregrines and other raptors hunting wintering Dunlins (Calidris alpina) and ducks at Boundary Bay on the Pacific coast of British Columbia; (6) Peregrines specializing on teal and American Wigeon (Anas Americana) wintering on coastal farmlands; (7) Territorial pairs of Peregrines and Prairie Falcons competing for prey and nest sites on a sympatric breeding range along an Alberta river; (8) Prairie Falcons and Gyr- falcons wintering in the city of Edmonton and capturing Rock Doves (Columba livia); (9) Gyrfalcons hunting Mallards (Anas platyrhynchos) wintering on Alberta farmlands; (10) Klepto-parasitic interference from eagles and buteo hawks with hunting Peregrines and Gyrfalcons, and intra- and interspecific prey theft between falcons. The largest set of data pertains to the Peregrine, which was studied in all sea- sons and habitats except during winter in central Alberta. At Beaverhills Lake, Peregrines on spring migration attacked avian prey on 674 occasions and cap- tured 52, a success rate of 7.7%. Success rates for adult falcons and spring im- matures were respectively 9.8% and 7.1%, not significantly different. Fall im- matures had a success rate of 2.4%, but the sample was small. Waterfowl and shorebirds made up 94% of prey taken. Stealth approaches and long-range sur- prise attacks were the major strategies in 70% of hunts. Hunting Peregrines often soared at great altitudes and stooped low to attack flying prey or to flush sitting prey. The effectiveness of surprise strategies at the lake was facilitated by shoreline vegetation. In addition, Peregrines used a number of other, more un- usual, hunting methods. A few prey were taken on the ground or in shallow water, all others were seized in the air and borne down. The majority of birds Hunting tactics of Peregrines 7 caught failed to use escape tactics routinely employed by their kind. Lone prey individuals were more often killed compared to individuals in flocks. Peregrines wintering on the Pacific coast attacked Dunlins 652 times with 94 captures, representing an overall success rate of 14.4%. Adult falcons achieved 26.8% success in 164 hunts, significantly higher than the 9.0% in 399 hunts by first-year Peregrines. The hunting strategies of these wintering falcons differed from the tactics deployed at Beaverhills Lake. The open coastal mudflats did not facilitate a stealth approach. Only 35% of Peregrine hunts at Boundary Bay were surprise attacks and 62% were open attacks on flocks. Hunts over the mudflats or ocean had a success rate of 11%, as compared to 44% over the shore zone. Some very long and persistent pursuits of sandpipers were made by immature Peregrines and Merlins. During high winter tides that inundated all mudflat habitats, the Dunlins de- parted and stayed well offshore for periods of 1.5 to 6.5 hours. Termed over- ocean flocking, this flight behaviour is believed to be an anti-predator strategy. After the tide began to recede the Dunlins landed in the shore zone where they were at their most vulnerable to surprise attacks by Peregrines. Captures recor- ded per hour of observation peaked at 0.25/hour in the two hours following the crest of the high tide, four times higher than two hours prior to high tide. The hypothesis is advanced that nearness to vegetation increases the Dunlin’s pre- dation risk, which was strongly supported by the data. The hypothesis that the hunting success rate of breeding Peregrines is greater than that of migrating or wintering falcons was supported by data collected at an Alberta nest site. The adult pair had an overall rate of 30.3% in 386 hunts. Over the course of the 7-year study the rate increased from 21.9% in the first year to 39.1% in the seventh year. The majority (77%) of attacks were initiated from high soaring flight, and the falcons used the exhaust of the plant’s smokestack to gain height for soaring. Of 117 prey captured 62 (53%) were Franklin’s Gulls (Larus pipixcan), and 85% of a total of 81 gulls known to have been killed by these falcons were juveniles. Peregrines breeding on an island off the northwest Pacific coast hunted sea- birds such as murrelets by direct aerial pursuit or by striking them on the water. If the swimming bird was hit and crippled by the attack, the falcon turned back to pick it up from the surface of the water. The success rate of these falcons was 22% in 73 observed hunts. In hunting small passerines and small shorebirds, the Peregrine’s success rate was 8.2% in 647 hunts as compared to 12.4% in 354 hunts by Merlins. The Merlin was significantly more successful than the Peregrine in capturing small passerines (12.2% vs. 3.8%) but not in hunting small shorebirds (12.6% vs. 8.8%). The majority of ducks (82%) taken by Peregrines were, in order of frequency, American Wigeon, Northern Pintail (Anas strepera), Northern Shoveler (Anas clypeata), and teal (spp.). Mallard hens were rarely captured but no drakes were seen killed. Mallards of both sexes were preyed upon by Gyrfalcons wintering in Alberta. Their success rate in 70 hunts was 22.8%. Gyrfalcons wintering in Ed- 8 Hunting tactics of Peregrines monton made 141 attacks on Rock Doves with a success rate of 10.6%, signifi-