The effect of handling time on interference among house sparrows foraging at different seed densities

Interference models of the ideal free distribution (IFD) assume competition among foraging animals causes intake rates to decline with increasing comp etitor density and that the strength of the decline influences forager dist ributions among food patches. However, the resulting distributions of anima ls may depend on which components of foraging success contribute to interfe rence. We examined the effect of group size (1-13 birds) on the prey encoun ter rates, handling times, and foraging rates of house sparrows, Passer dom esticus, feeding at three seed densities in a suburban backyard. House spar rows did not experience interference during search. Interference arose prim arily front foraging time lost handling seeds. Foraging rates decreased wit h increasing seed density as a consequence of increased handling times. Als o, birds experiencing significant increases in handling time with group siz e suffered most from interference. Our results suggest that animals adjust handling time to avoid costly aggressive interactions, indicating that hand ling time may be an important component of interference in some foraging sy stems. Future studies estimating interference should try to identify which components of foraging contribute to interference, paying particular attent ion to handling times for species that monitor and avoid competitors.