The effect of patch size and competitor number on aggression among foraging house sparrows

Abstract

We examined the effect of patch size and competitor number on aggression among house sparrows, Passer domesticus, foraging at patches of seven different sizes in a doubling series (0.014, 0.029, 0.058, 0.116, 0.230, 0.462, and 0.922 m 2 ). Contrary to our expectations, the birds did not defend an entire patch, even when it was small as 0.014 m 2 . The frequency of aggression among the birds decreased gradually with increasing patch size, in contrast to the step decline predicted by resource defense theory. Moreover, the birds fought more frequently and more intensely as competitor density increased. Both results are consistent with the predictions of a modified hawk-dove model for shared patches. Females were more aggressive and fed at a higher rate than did males. The proportion of females increased as patch size decreased, and aggression became more frequent and intense. Even when patches are shared, patch size has an important effect on the frequency and intensity of foraging competition and the size and composition of foraging groups. Key words: aggression, competitor density, hawk-dove, interference, patch size, Passer domesticus, resource defense, truncated phenotypic distribution. [Behav Ecol 15:412–418 (2004)]