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Hunting tactics of Peregrines and other falcons - Section: 13 Page: 93 - Peregrine prey selection and eagle interference

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Huntin tactics of Pere rines

Dekker and Bogaert 1997). But in marshes and agricultural regions, female Peregrines tend to kill larger prey than do males. For example, in central Alberta and coastal regions of British Columbia and Washington, migrant or wintering female Peregrines predominantly killed ducks, while the males took sandpipers and small passerines (Anderson and DeBruyn 1979; Dekker 1980, 1995).

In the estuaries of western North America, a common prey of wintering Peregrines is the Dunlin (Calidris alpi na). Based on the remains of Dunlins killed by falcons, researchers in Washington and California reported that the majority are juveniles, which might be related to age-related flocking behaviour (Kus et al.

1984; Warnock 1994). This paper details the hunting methods of both sexes of Peregrines attacking flocks of Dunlins. It also reports on the duck hunting habits of these Peregrines with particular reference to klepto-parasitic interference from Bald Eagles (Haliaeetus leucocephalus).


The study area is at Boundary Bay, which is part of the Fraser River delta (49'05'N, 123'00'W) in southwestern British Columbia. The bay is 16 km across and the intertidal zone is roughly 4 km wide at the lowest ebb. The tidal mudflats are bordered by a narrow strip of saltmarsh and a dike that protects low-lying agricultural fields inland. Boundary Bay is a major stopover for migratory waterbirds and a wintering refuge for circa 20,000 ducks and 40,000 Dunlins. The only other shorebird to winter in the bay in some numbers (circa 1,000) is the Black-bellied Plover (Pluvialis squatarola). Bald Eagles are common year-round and increase locally to 50 150 during January-February (Dekker 1999). For a more detailed description of the Fraser delta and its avifauna, see Butler and Campbell (1987); Butler (1994); Butler and Kaiser (1995).

Between early November and mid February 1994-2003, I spent part or all of 151 days in the study area for a total of 940 hours. I walked the dike or, during rain and strong winds, sat in a parked vehicle at a vantage point from where the tide flats were visible. To study the interaction of Peregrines and their prey species, I used three principal methods: (I) flocks of ducks and shorebirds were monitored for alarm behaviour such as sudden flushing; (2) the area was frequently scanned through binoculars to spot flying Peregrines; and (3) perched Peregrines were observed for varying lengths of time in the hope of seeing them start a foraging flight.

Peregrines were classified as either adult or immature, based on dorsal colour and ventral markings. Males and females could only be separated with certainty, respectively at the lower or higher end of their size range, because recorded weights of the heaviest male Peregrines overlap with those of the lightest females, particularly between the subspecies occurring in western North America (Brown and Amadon 1968; White et al. 2002). Some falcons appeared typical of F. p. pealei (Beebe 1960), others of F. p. anatum (Palmer 1988). There is no evidence that Peregrines of arctic origin (F. p. tundrius) winter in the bay.

Tag: anderson beebe duck ducks eagles estuaries falcons flocking flocks migratory peregrines shorebird shorebirds



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