Nonrigorous Foraging by Robbing Egrets

Abstract

The premise that a species will maximize its net energy intake over the short term was tested by studying the robbing behavior of the Great Egret. In wading—bird feeding aggregations of the southern Florida Everglades, the 5 most common species robbed other birds, and this behavior consumed 10% of the feeding time of the Great Egret. When an egret robbed other birds, it got larger prey, but it took longer, caught fewer prey, and expended more energy than it did feeding in more typical fashion by slow walking or standing. Nonrobbing behavior had a gross energy intake per time 3.1 x greater than robbing. The cost—benefit ratio of robbing was 70% larger than for standing feeding. The combined used of both standing feeding and robbing resulted in a cost—benefit ratio 10% larger than for standing feeding alone. The use of a nonoptimal behavior is interpreted to suggest that energy return need not always be optimized over the short term. Minimizing cost—benefit ratios, optimizing time budgets, or maximizing net energy gain may occur only intermittently during an animal's annual cycle. The existence of a less—than—optimal behavior within the egret's foraging repertoire suggests that, under circumstances other than those studied, it might contribute to foraging optimization. It is predicted that an animal should attempt to capture each prey that it encounters when the occurrence of a prey item is a relatively rare and random event and if the potential penalty were small. Under these conditions, which would occur during periods of nonrigorous foraging, an animal can use a nonoptimal behavior.