| Summary: | In ‘over-ocean flocking’ (OOF) shorebirds fly at high tide in place of traditional ground-based roosting. I used radar, theodolite, and video to study OOF by Pacific dunlins (Calidris alpina pacifica) wintering at Boundary Bay, British Columbia. OOF occurred only during daytime high-tides, and did not occur in darkness. Its mean duration was 187 ± 80 (SD) min, shortened in relation to daily temperature, and did not take place when ice in the bay facilitated roosting far from shore. Flocks flew on average 666 m from the shoreline. OOF flight had much gliding (14%) and was close to the minimum power speed, while transit flights between foraging sites had little gliding (1%) and was much faster than the maximum-range speed. These results support the hypothesis that over-ocean flocking is an anti-predator behaviour involving trade-offs between minimizing energy expenditure and maximizing safety from diurnal predators.
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