Optimal foraging theory predicts that individuals should resort to kleptoparasitizing food when the net energy obtained from this behavior is greater than that gained from searching for food conventionally. We studied kleptoparasitism in the Cattle Egret (Bubulcus ibis) in southwestern Spain. Intraspecific kleptoparasitism in Cattle Egrets was rare. We recorded this behavior only when egrets foraged in large flocks and when potential victims had captured large prey items. In addition, we observed kleptoparasitism in feeding areas with high food availability. Food intake rates for both aggressor and victim, prior to the attack, were greater than that of undisturbed individuals. Victims with defensive behavior maximized their feeding rate by capturing the largest prey items and expending the least energy. Kleptoparasites' attack success depended mainly on the degree of the victim's resistance to releasing prey, which in turn was apparently determined by the victim's foraging success and condition. We conclude that intraspecific kleptoparasitism in Cattle Egrets is not an optimal foraging behavior, but rather is to establish the social status of dominant individuals within a foraging flock.
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Vol. 73 • No. 2