The feeding success of cattle egrets in flocks

The feeding rates of grouped (<1.5 m from conspecifics) and solo (>5 m from conspecifics) cattle egrets (Bubulcus ibis) in loose flocks away from cows were compared, to test the hypothesis that grouped cattle egrets benefit from feeding on prey flushed inadvertently by nearby conspecifics. The flock feeding rates were also compared to those of grouped and solo egrets near cows, to determine the effects of flock membership on feeding rates. Birds in flocks captured prey faster than those with cows, and tended to capture larger prey, but field observations and captive experiments failed to show that the feeding success of flock members was enhanced by the hypothesized ‘beater’ effect. Increases in prey density, however, always resulted in higher feeding rates, so some cattle egret groups may form in response to local concentrations of prey. Prey size may also play a role in group formation, because birds in the field tended to feed at greater distances from their neighbours when larger prey were captured, regardless of prey density. When small groups did form among cattle egrets feeding on relatively large prey, group members occasionally captured prey items that had been discovered by nearby conspecifics. This behaviour was not observed among birds in dense aggregations, which fed on small, highly abundant prey. These data indicate that there is a potential cost associated with feeding too near others unless the prey are relatively small and abundant.