Flock size and density influence speed of escape waves in semipalmated sandpipers

How animals in groups coordinate their movements to produce cohesive collective patterns has been the subject of many models but remains poorly documented empirically in the field. I investigated waves of escape in flocks of a shorebird roosting on beaches to pinpoint factors that influence coordination in the initiation of group movements. In roosting flocks of semipalmated sandpipers, Calidris pusilla, the escape of a single alarmed bird triggered similar responses in nearby companions, eventually propagating through the whole flock as a wave. Among flocks, wave speed through the whole flock increased with size but decreased with density over nearly three orders of magnitude in size and density. Large flocks are attacked more often by predators, and individuals in such groups may therefore respond more quickly to the escape of companions. The negative effect of density suggests that birds escaped more slowly in denser flocks to avoid collision. This interpretation is bolstered by the fact that wave speed increased with density at the edges of the flock, where the risk of collision is reduced. Waves propagated faster at the edges of the flock than through the whole flock, suggesting a selfish herd mechanism whereby edge individuals try to avoid being left behind or overtaken by others. Findings suggest that birds can integrate a host of factors to adjust escapes in a coherent fashion very rapidly.