Food patches and foraging group size in granivorous birds

Abstract

The effect of the size of food patches on the size of foraging groups was examined in field and aviary experiments. In the field, reducing the area of a high-quality patch significantly reduced the size of heterospecific foraging groups. Outdoor aviary expriments with three patch sizes measured aggression and inter-bird distance in monospecific groups. Group size in white-throated sparrows, Zonotrichia albicollis, was reduced on smaller patches. Group size in field sparrows, Spizella pusilla, was not affected by patch size, as it had been in the field where field sparrows were exposed to aggresssive attacks by heterospecifics. Individual aggression rates did not increase with decreasing patch size in white-throated sparrows, but the overall incidence of aggression in white-throated sparrow groups did increase. Aggression in field sparrows was infrequent and unaffected by patch size. Individuals of both species fed closer together when monospecific flocks were larger. Additionally, field sparrows fed closer together on smaller patches. However, patch size did not significantly affect inter-bird distance in white-throated sparrows. Aggression may regulate group size even though aggression levels may not change relative to group size.