Absolute foraging rate and susceptibility to interference competition in blackbirds varies with patch conditions

Abstract

Summary Individual variation in feeding rate in the absence of competitors (absolute feeding rate) and change in feeding rate in the presence of competitors (susceptibility to interference) are crucial parameters used in individual‐based population models that predict the distribution of animals across patches. There are, however, few data on how these parameters are dependent on local foraging conditions, which will vary widely in natural systems. We tested the hypotheses that absolute feeding rate and change in feeding rate with group size were independent of patch conditions. We observed 27 European blackbirdsTurdus merulafeeding in seminatural experimental patches that were either open (where prey was easily visible) or covered with leaves (where prey was invisible until the leaves had been moved aside by the bird). Absolute feeding rate and susceptibility to interference were measured by recording feeding rates of blackbirds alone and in the presence of increasing numbers of competitors, respectively. There was significant individual variation in absolute feeding rate dependent on patch type, with some birds showing relatively large increases in open patches. There was no significant correlation between absolute feeding rate in open and leaf patches. There was significant variation in individual susceptibility to interference dependent on patch type. There was no correlation between an individual’s susceptibility to interference in open and leaf patches. Mathematical models that seek to make predictions of the distribution of animals across heterogeneous patches should incorporate measures of competitive ability over a wide range of patch conditions, or use more complex parameters that incorporate individual changes in density dependent pay‐offs dependent on patch conditions.