We studied the diving and surfacing synchrony of foraging flocks of wintering surf scoters (Melanitta perspicillata). Our data support the hypothesis that synchronous diving is an adaptation that reduces kleptoparasitism by glaucous-winged gulls (Larus glaucescens), which frequently attend foraging flocks. We developed a statistical method for measuring and comparing synchrony between flocks, and applied it to videotape records of 30 flocks. The results show that diving and surfacing are highly synchronous, and that there is a large variation between flocks in the degree of synchrony exhibited. The most pronounced effect is for surfacing synchrony to be higher in the presence of gulls. This seems to arise because individual birds curtail their dives so that less synchrony is lost between diving and surfacing during a group dive. This curtailment of dive length may lead to a reduction in the average size of prey captured.
Observations of Above-Surface Littoral Foraging in Two Sea Ducks, Barrow's Goldeneye, Bucephala islandica, and Surf Scoter, Melanitta perspicillata, in Coastal Southwestern British Columbia
