Over-ocean flocking by Dunlins, Calidris alpina, and the effect of raptor predation at Boundary Bay, British Columbia

Abstract

Flocks of Dunlins (Calidris alpina) often exceeding 10 000 birds, wintering at Boundary Bay, British Columbia, remained airborne over the ocean for 2-4 hours when their intertidal feeding grounds were inundated. 1 postulate that this behaviour is an adaptation to avoid surprise attacks by raptors over the saltmarsh where open roosting habitat is absent. Peregrine Falcons (Falco peregrinus) attacked Dunlins 302 times with 28 captures, a success rate of 9.3%. Surprise hunts over the saltmarsh had a success rate of 33.0% in 15 hunts, significantly higher (P<0.05) than the 8.0% in 287 hunts over the open mudflats. All 29 hunts by Merlins (Falco columbarius) began as surprise attacks although all five kills were made after long pursuits.