Relative competitive ability changes with competitor density: evidence from feeding blackbirds.

How individual competitive ability varies with competitor density in systems where interference competition occurs has been studied in few systems, despite its importance in determining the fundamental predictions of influential phenotype-limited ideal free distribution models. I measured individual variation in competitive ability in wild wintering blackbirds, Turdus merula. Variation in feeding rate of single birds, and the decline in their feeding rate in the presence of competitors, were measured in experimental patches. Individuals varied significantly both in their feeding rate when alone and in the decline in that feeding rate in the presence of competitors. Individuals that had high absolute feeding rates did not tend to have smaller declines in feeding rate in the presence of competitors. The relative ranking among birds in their feeding rate was strongly dependent on competitor density. This result has important implications for the expected distributions of competitors between patches. (c) 1998 The Association for the Study of Animal Behaviour.