High-tide flight by wintering Dunlins (Calidris alpina): a weather-dependent trade-off between energy loss and predation risk

Migratory shorebirds wintering or staging on ocean coasts collect at high tide on roosting sites that remain above the flood line. However, some species of Calidris sandpipers spend the high-tide interval in flight over the ocean. In the winters of 2006–2012, the characteristics of high-tide flight by Dunlins (Calidris alpina (L., 1758)) were studied at Boundary Bay, British Columbia, Canada. At wind speeds of 1–6 m/s, flocks of Dunlins remained airborne over the ocean for up to 4 h at altitudes of >30 m. If winds were >10 m/s, the Dunlins coursed low over the waves. Ambient temperature was a significant determinant in the occurrence and duration of high-tide flight. In October andNovember, the Dunlins spent just asmuch time in flight before as after high tide, but in January, flight duration was 43% shorter after high tide than before high tide. The mean January temperatures were significantly lower than in October and November. The Dunlins were hunted by Peregrine Falcons (Falco peregrinus Tunstall, 1771), which captured 81 prey in 494 attacks. The maximum kill rate of 0.28 captures per hour of observation was recorded in the second hour after high tide, which suggests that predation risk is greatest for Dunlins that return early from high-tide flight.