FALCON HUNTING BEHAVIOR DURING SPRING MIGRATION AT GRAYS HARBOR AND WILLAPA BAY, WASHINGTON, USA

The hunting behavior of falcons, particularly when hunting shorebird prey species, has been investigated in numerous studies, most of which were conducted during nonbreeding periods. Relatively few studies of falcon hunting behavior, however, have been conducted during spring migration. Between 2006 and 2025, I observed hunting activity by Peregrine Falcons (Falco peregrinus) and Merlins (F. columbarius) during a long-term study of shorebird migration at Grays Harbor and Willapa Bay, Washington. On 80 of 267 field dates, I observed 146 instances of hunting activity, which I defined as hunting effort with or without a known ending, and including observations of falcons with prey even if the hunting flight was not observed. Hunting activity was observed by Merlins (13 instances) between 21 April and 10 May, and Peregrine Falcons (133 instances) between 7 April and 22 May, during the observation period. The median date of the last hunting activity by Peregrine Falcons across all years was 15 May. Prey were captured from single- or mixed-species flocks and included Red Knot (Calidris canutus), Sanderling (C. alba), Western Sandpiper (C. mauri), Dunlin (C. alpina), and Short-billed Dowitcher (Limnodromus griseus). Some shorebirds dived to or were knocked to water or mudflats by falcons; 8 of these shorebirds survived, whereas 6 were captured. I observed more hunting activity than expected at Willapa Bay and less than expected at Grays Harbor, perhaps because a Peregrine Falcon breeding territory at Grays Harbor was occupied in most years and the resident falcons may have excluded others from the area. The hunting success of Peregrine Falcons was 47% (39 successes in 83 hunting flights with known endings), and 43% (3 successes in 7 hunting flights) for Merlins. The success of Peregrine Falcons was comparable to that of other studies in Washington but higher than in most other studies globally; differences are largely due to different definitions of hunting behavior. Peregrine Falcons were challenged for possession of prey by other Peregrine Falcons 2 times and Bald Eagles 3 times. These data represent a baseline of predator-prey interactions between falcons and shorebirds in coastal Washington during spring migration. The magnitude and variation of predation risk along the migratory pathway is poorly understood and should be investigated more comprehensively.