Intensity of interference affects the distribution of house sparrows, Passer domesticus, at food patches

Interference ideal free distribution (IFD) models predict animal distributions across resource patches, taking into account the effects of resource density and competitor interference on an individual's resource intake rate. Assuming that individuals behave to maximize fitness, foragers should aggregate at patches of high resource abundance when interference is weak. However, when interference is strong, foragers should choose to reduce interference by foraging at less crowded patches, even if these patches have lower resource density. Consequently, interference IFD predicts disproportionately high animal densities at rich patches when interference is low and a more uniform distribution among patches when interference is strong. To test this prediction we observed house sparrows, Passer domesticus, at a rich and a poor patch with different seed densities. We manipulated interference by altering the size of the pair of patches: two small patches (high interference) or two large patches (low interference). Birds experienced no interference at large patches where fighting was infrequent and preferred the rich patch where foraging rates increased with bird numbers. In contrast, birds aggressively interfered with each other at small patches, and interference was higher in rich patches than in poor patches. As predicted, birds increased their use of poor patches to minimize interference, resulting in an even distribution of birds across rich and poor patches. Our results show how changes in interference with resource density can make consumer distributions independent of resource distribution, even at moderate interference intensities. Manipulating interference through patch size may provide a unique opportunity to examine the effects of competitive asymmetries on patch use. (c) 2006 The Association for the Study of Animal Behaviour Published by Elsevier Ltd. All rights reserved.