Do staging semipalmated sandpipers spend the high-tide period in flight over the Ocean to Avoid Falcon Attacks along Shore?

The interaction of aerial predators and migrant Semipalmated Sandpipers (Calidris pusilla) was studied at Mary's Point in the upper Bay of Fundy, New Brunswick, Canada, during August of 2009 and 2010. Peregrine Falcons (Falco peregrinus) were locally reintroduced and increased from one active nest site in 1989 to 27 in 2010, which coincided with a decline of sandpipers roosting at Mary's Point from an annual mean of 161,000 in 1976â€"1982 to 15,000 or less during this study. Mean roosting time of flocks was 33 min (range = 10 to 40 min; N = 6). Sandpipers returned to the beach 1 h:36 min after high-tide (range = 1 h:10 to 2 h:13 min; N = 5), but were soon flushed again by falcons. On ten of 19 days, during part of the high-tide period, flocks of sandpipers remained in flight over the ocean. Termed Over-Ocean Flocking (OOF), this behavior was seen on days when spring tides inundated all beach habitat, and also at lower tides, which supports the hypothesis that OOF is an antipredator strategy intended to avoid surprise attacks by falcons near the shore. Raptors sighted during 128 hours afield included 226 Peregrines, 20 Merlins (Falco columbarius) and two Sharp-shinned Hawks (Accipiter striatus). At 1.0â€"3.2 Peregrine sightings/h-1 (mean 1.8) the level of disturbance is high and supports the hypothesis that the decline in roosting sandpipers at Mary's Point is linked to predation danger