Relative competitive ability does not change over time in blackbirds

1. Individual variation in feeding rate in the absence of competitors (abso lute feeding rate) and decline in feeding rate in the presence of competito rs (susceptibility to interference) are frequently used as measures of comp etitive ability or fitness in short-term foraging studies. They are also cr ucial parameters of individual-based population models that predict the dis tribution of animals across patches. There are, however, few data on whethe r absolute feeding rate and susceptibility to interference competition rema in constant for individuals between years. 2. The hypothesis that absolute feeding rate and susceptibility to interfer ence were similar between years was tested by observing 25 European blackbi rds Turdus merula feeding in seminatural experimental patches, during Janua ry to March over a 2-4-year period 1995-98. Absolute feeding rate was measu red as the feeding rates of solitary blackbirds. Susceptibility to interfer ence was measured as the change in feeding rate of a focal bird when it fed in the presence of different numbers of competitors. 3. Individuals changed significantly in their feeding rate between years, b ut most individuals did not change in their feeding rate relative to others in the population. The absolute feeding rate of an individual was signific antly positively correlated with its feeding rate in the subsequent year. 4. There was no significant variation in susceptibility to interference for individuals between years. Only two of 27 blackbirds showed a significant change in susceptibility to interference between years. 5. Relative absolute foraging rate and susceptibility to interference compe tition were reasonably similar between years in blackbirds. Relative fitnes s measures derived from short-term measures of foraging ability may therefo re be valid over long periods. In ideal free models that incorporate indivi dual competitive ability, rather than population averages, temporal changes in competitive ability can possibly be ignored.