INTERFERENCE COMPETITION AT LOW COMPETITOR DENSITIES IN BLACKBIRDS TURDUS-MERULA

1. Interference competition in a wild population of European blackbird s was examined by comparing the feeding rates of individual birds feed ing alone and with the addition of a competitor within small, experime ntal feeding patches. 2. Blackbirds showed type II functional response s. There was a highly significant decrease in feeding rate on addition of a competing individual to a patch: average declines in feeding rat es of an individual on addition of a competitor, where nearest neighbo ur distance was always less than about 5 m, were between 16% and 43% o f the feeding rate of a lone individual. 3. The decline in feeding rat e was similar whether the individuals interacted aggressively or appar ently ignored each other during feeding. When patch size was doubled, the effects on feeding rate of adding a single competitor remained sim ilar. The high levels of interference recorded may therefore have been mainly a consequence of active rather than passive interference compe tition. 4. The results suggest that the costs to an individual blackbi rd of monitoring other individuals within a group in order to avoid in teractions may be sufficiently high to counter any benefits of group f eeding, such as vigilance benefits.