Relative competitive ability does not change over time in blackbirds

Abstract

1. Individual variation in feeding rate in the absence of competitors (absolute feeding rate) and decline in feeding rate in the presence of competitors (susceptibility to interference) are frequently used as measures of competitive ability or fitness in short-term foraging studies. They are also crucial parameters of individual-based population models that predict the distribution of animals across patches. There are, however, few data on whether absolute feeding rate and susceptibility to interference competition remain constant for individuals between years. 2. The hypothesis that absolute feeding rate and susceptibility to interference were similar between years was tested by observing 25 European blackbirds Turdus merula feeding in seminatural experimental patches, during January to March over a 2-4-year period 1995-98. Absolute feeding rate was measured as the feeding rates of solitary blackbirds. Susceptibility to interference 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, but most individuals did not change in their feeding rate relative to others in the population. The absolute feeding rate of an individual was significantly 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 competition were reasonably similar between years in blackbirds. Relative fitness measures derived from short-term measures of foraging ability may therefore be valid over long periods. In ideal free models that incorporate individual competitive ability, rather than population averages, temporal changes in competitive ability can possibly be ignored.