Effect of Group Size and Location within the Group on the Foraging Behavior of White Ibises

Abstract

We studied adult White Ibises (Eudocimus albus) on a South Carolina salt marsh to determine the effects of social grouping on the birds' foraging behavior. White Ibises on our study site fed almost exclusively on fiddler crabs (Uca spp.). Four social categories were recognized: (1) central adults in flocks of ≥ 15 birds, (2) peripheral adults in flocks of ≥ 15 birds, (3) adults in small flocks of five or fewer birds, and (4) solitary adults (singletons). We used a paired sampling scheme to compare the behavior of central birds with the behavior of birds in the other three social groupings. Although peripheral adults did not differ significantly from central adults in number of steps, number of crabs captured, or number of capture attempts, they looked up more often and for longer periods of time than did central adults. Behavior of solitary ibises was similar to that of ibises in small flocks, but both foraged differently than central adults in large flocks. Birds in the center of large flocks took fewer steps, probed more frequently, and scanned the surroundings less often than birds in the other two social groupings; there were no differences in capture rates. Thus, White Ibises used two distinct types of foraging strategies depending on flock size and their position within the flock. Ibises in small flocks, singletons, and, to some extent, ibises on the edges of large flocks, stepped quickly to capture fiddler crabs before the crabs could retreat into burrows. Centrally-located ibises in large flocks were unable to use this foraging technique because the surrounding members of the flock created a disturbance that caused the fiddler crabs to remain in their burrows. These birds, therefore, probed into crab burrows and found their prey by tactile means. Our results support the predator-protection advantage of feeding within a flock independent of the feeding-efficiency hypotheses.