Foraging Strategies of American White Pelicans

Abstract

AbstractForaging strategies employed by American white pelicans were studied at a riverine site in Manitoba, Canada, during the breeding season in 1985 and 1986. Six strategies were identified during both diurnal and nocturnal foraging periods. Sit-and-wait was the least common strategy (four instances). Mobile individuals were common but had low rates of bill dipping and prey capture, as did relatively rare and uncoordinated aggregations. A degree of flock coordination occurred in following flocks, characterized by foragers following one after the other, with occasional synchronization of bill dipping among flock members. The largest number of pelicans foraged within more or less circular groups called nuclei. Synchronous bill dipping and apparent herding of prey towards shore were common within nuclei. The most highly coordinated strategy, semicircles, involved small numbers of foragers (2 to 30 birds) that maintained their positions relative to one another, usually in a semicircle but sometimes moving to a closed circular pattern. The greatest degree of synchronized bill dipping occurred in semicircles. Small inter-bird distances and synchronized bill dipping in nuclei and semicircles may enhance their effectiveness in driving or herding clumped fish prey. Foraging strategies could be arranged along a continuum based on degree of coordination, ranging from mobile individuals, then uncoordinated aggregations, through increasing degrees of coordination in following, nuclei, and semicircles. Along this continuum, prey size and capture rats were greatest for the more highly coordinated strategies, while less coordinated strategies appeared to be involved primarily in searching. Switching among strategies fit along the same continuum, with a tendency to switch from less to more coordinated strategies when prey were located and to return to less coordinated search when capture rates declined. Video analysis of captures within large nuclei and observations of positional shifts among foragers in nuclei and following flocks indicated that all individuals within a coordinated group potentially benefited from the presence of others, supporting the view that coordinated foraging strategies in this species are examples of true cooperative foraging. The range of strategies, and interplay among them, appear to provide the American white pelican with a highly effective group foraging system for harvesting mobile, clumped fish prey.