Raptor Predation on Wintering Dunlins in Relation to the Tidal Cycle

At Boundary Bay, British Columbia, Canada, Peregrine Falcons (Falco peregrinus) captured 94 Dunlins (Calidris alpina) in 652 hunts. The two main hunting methods were open attacks on flying Dunlins (62%) and stealth attacks on roosting or foraging Dunlins (35%). Peregrines hunted throughout the day, yet the kill rate per observation hour dropped 1-2 hr before high tide and peaked 1-2 hr after high tide. The drop in kill rate coincided with the departure of the mass of Dunlins for over-ocean flights lasting 2-4 hr. The peak in kill rate occurred just after the tide began to ebb and the Dunlins returned to forage in the shore zone. The hypothesis that closeness to shoreline vegetation is dangerous for Dunlins is supported by three converging lines of evidence: (1) the high success rate (44%) of peregrine hunts over the shore zone compared to the rate (11%) over tide flats and ocean; (2) the high kill rate per observation hour at high tide; and (3) the positive correlation of kill rate with the height of the tides. Seven of 13 Dunlins killed by Merlins (Falco columbarius) and all five Dunlins killed by Northern Harriers (Circus cyaneus) were also captured in the shore zone.