Interference Competition at Low Competitor Densities in Blackbirds Turdus merula

Abstract

1. Interference competition in a wild population of European blackbirds was examined by comparing the feeding rates of individual birds feeding alone and with the addition of a competitor within small, experimental feeding patches. 2. Blackbirds showed type II functional responses. There was a highly significant decrease in feeding rate on addition of a competing individual to a patch: average declines in feeding rates of an individual on addition of a competitor, where nearest neighbour distance was always less than about 5m, were between 16% and 43% of the feeding rate of a lone individual. 3. The decline in feeding rate was similar whether the individuals interacted aggressively or apparently ignored each other during feeding. When patch size was doubled, the effects on feeding rate of adding a single competitor remained similar. The high levels of interference recorded may therefore have been mainly a consequence of active rather than passive interference competition. 4. The results suggest that the costs to an individual blackbird of monitoring other individuals within a group in order to avoid interactions may be sufficiently high to counter any benefits of group feeding, such as vigilance benefits.