Interactions of Peregrine Falcons (Falco peregrinus) and Dunlin (Calidris alpina) Wintering in British Columbia, 1994–2015

Abstract

Contemporary declines of shorebirds have been linked to environmental causes, but the role of predation in shorebird declines is not clear. From 1994 to 2015, we observed the interactions of Peregrine Falcons (Falco peregrinus) and Dunlin (Calidris alpina) wintering on the Pacific coast of British Columbia. Peregrine sightings per hour of observation declined on a seasonal basis from October to February, but increased chronologically from 0.36/hr in 1994 to 1.24/hr in 2015. Peregrines captured 205 Dunlin in 1369 attacks. Hunting success rates increased from 10.8% in October to 15.0% in the winter months. During high tide, when the intertidal zone was flooded, some Dunlin flocks spent up to 315 min in sustained flight over the ocean, apparently as an antipredator strategy, but other flocks roosted on the foreshore, or sat out the high-tide interval on agricultural fields inland. This study suggests that these antipredator behaviors allow the Dunlin population to persist at Boundary Bay as predator numbers increase.

Semi‐intensive shrimp farms as experimental arenas for the study of predation risk from falcons to shorebirds

“…Attack rate then steeply dropped during the neap tide periods, underscoring a non-linear relationship with tidal conditions. In addition, mean attack rate (0.1 attacks per hour) was within the range observed by peregrine falcons on shorebirds in other coastal areas (0.04-0.75 attacks per hour; Cresswell, 1994Cresswell, , 1996Cresswell & Whitfield, 2008;Dekker & Drever, 2016) and was highly correlated with the overall abundance of western sandpipers, its main prey, in the wetland.…”

“…Attack rate then steeply dropped during the neap tide periods, underscoring a non‐linear relationship with tidal conditions. In addition, mean attack rate (0.1 attacks per hour) was within the range observed by peregrine falcons on shorebirds in other coastal areas (0.04–0.75 attacks per hour; Cresswell, 1994 , 1996 ; Cresswell & Whitfield, 2008 ; Dekker & Drever, 2016 ) and was highly correlated with the overall abundance of western sandpipers, its main prey, in the wetland. Similarly, seven out of 10 successful attacks on shorebirds involved western sandpipers, and the observed 14% hunt success was similar to previous studies with peregrine falcons and other raptors on shorebirds, which typically varies from 7% to 30% (van den Hout et al., 2008 ).…”

“…Migratory shorebirds (mainly Families Scolopacidae and Charadriidae) face different levels of danger along the annual cycle (Ydenberg et al., 2007 ), with raptor predation being an important cause of mortality both in staging/stopover and nonbreeding areas (Dekker & Drever, 2016 ; Whitfield, 2003 ). Shorebirds respond to high predation risk by raptors in a variety of ways, including body mass reduction (Piersma et al., 2003 ) and modifying timing of primary molt and wing length (Lank et al., 2017 ; Ydenberg et al., 2007 ).…”

Gaming the system: how predators induce prey to make themselves more vulnerable

“…OOF is easily distinguished from the other types of flight used by dunlins (foraging flights, transit flights, escape flights; see Reurink et al, 2016). It involves all or most of the dunlins present, and the large flock is usually easily visible as it circles aloft over the bay (Dekker, 1998). The flight mode is distinct, with much gliding, and low speed (Hentze, 2012).…”

“…Dunlins and other shorebirds gather in large, dense groups to sleep and preen while the tide is high ('roosting'). At Boundary Bay Pacific dunlins largely abandoned this traditional form of roosting during the 1990s, and instead took up 'over ocean flocking' ('OOF' or 'high tide flight'; Dekker, 1998;Ydenberg et al, 2010). Diurnal high tide periods are now often spent circling in a loose flock high over the bay several hundred metres from shore.…”