Social foraging in cliff swallows: local enhancement, risk sensitivity, competition and the avoidance of predators

Abstract

Colonically nesting cliff swallows, Hirundo pyrrhonota, in southwestern Nebraska, U.S.A., often forage socially, in groups of two to more than 1000 individuals, on ephemeral concentrations of insects. Social foraging was promoted through information transfer among individuals at fixed colony sites, and through local enhancement in which foraging activities of some individuals attracted others to a food source. Cliff swallows feeding in groups had greater average foraging success (prey capture attempts per min) than birds feeding solitarily. Variance in prey capture success, and thus foraging risk, declined with group size and was greatest for solitary foragers. Solitary cliff swallows whose foraging success equalled or exceeded that expected from social foraging remained solitary during a given foraging bout. Solitary foragers whose success fell below that expected from social foraging joined foraging groups. For individuals foraging socially there was no consistent relationship between foraging success and group size, indicating little competition among group members for food and little resource depression. Social foraging in cliff swallows is probably not an adaptation mainly to decrease individuals' risk of predation because foraging success did not differ for birds foraging on the edges versus in the centres of groups, and individuals foraging on the edges did not try to move toward the centre. These data are consistent with risk-sensitive foraging models and provide the first empirical demonstration with free-living animals that social foraging is associated with decreased individual variance in foraging success.