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Hunting tactics of Peregrines and other falcons - Section: 13 Page: 98 - Peregrine prey selection and eagle interference

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Huntin tactics of Pere ines

Based on the examination of prey remains, a high proportion of shorebirds killed by raptors were reported to be juveniles (Whitfield 1985; Warnock 1994). Kus et al. (1984) suggested that this might be due to age-related differences in Dunlin flocking behaviour. I postulate that the mechanics of prey selection appear to be very simple. The majority (69'/o) of Dunlins captured during this study were taken from the outside or the tail end of flocks (Figure 9.1). Flocking is a well-known predator avoidance behaviour of open-country birds. Their drawing together into dense aerial formations is the result of each bird's instinctive desire to find safety in the centre of the flock (Tinbergen 1951). I speculate that adults are more proficient at this than juveniles, which are left on the outside of the flock. Several studies have reported age-related segregation of birds in roosting and feeding flocks (Newton 1998). If Peregrines selectively remove juvenile Dunlins from a wintering population because they are easier to capture than adults, then the proportion of vulnerable juveniles in that population declines over the course of the winter. Furthermore, juveniles that are attacked on a daily basis and manage to survive for several months might learn to become more vigilant and avoid capture. These hypotheses seem to be supported by the data. The respective hunting success rates of Peregrines for November and January declined from 18.6'/o (n = 134) to 12.4'/o (n = 404), although the difference is not significant (G = 2.30, P >0.05).

The klepto-parasitic habits of Bald Eagles are well-known, particularly at the expense of Peregrines that hunt ducks or seabirds (Anderson and DeBruyn 1979; Dekker 1995; Dekker and Bogaert 1997). Bald Eagles also klepto-parasitize Merlins. During this study, Merlins were seen to capture 14 Dunlins, two of which were pirated by pursuing eagles. Buchanan (1988) reported similar instances and he postulated that the threat of losing prey to larger raptors resulted in Merlins engaging in hunting flights of shorter duration if potential kleptoparasites were present. Gyrfalcons (Falco rusticolus) wintering in Alberta were also often robbed of ducks by eagles and avoided hunting at localities where Bald Eagles sat on prominent perches (Dekker and Court 2003).

In conclusion, I suggest that the ubiquitous presence of eagles at Boundary Bay discourages wintering Peregrines from hunting ducks on the tide flats. The possibility exists that Peregrines may hunt ducks more often at night or at inland localities where eagle presence is lower than at the coast. Adult female Peregrines, which had perched on the coast for much of the day, flew inland at dusk when wigeon leave the coast to feed on inland meadows (Dekker 1999). During November, when eagles are far less numerous at Boundary Bay than in January, several immature Peregrines persistently hunted ducks in the study area (Dekker 1998; and unpublished data). As the winter progresses however, I surmise that these juveniles eventually stop hunting ducks on the tide flats and instead concentrate on prey such as the Dunlin that can easily be transported over long distances, out of reach of klepto-parasitic eagles.

Tag: anderson ducks eagle eagles flock flocking flocks gyrfalcons peregrines seabirds shorebirds wigeon



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