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.