Experimental evidence for interference competition in oystercatchers, Haematopus ostraelegus. I. Captive birds

Interference competition, the immediately reversible decrease in per capita foraging success with increasing forager density, has important implications for the distribution of foragers. Theoretical models predict the strength of interference at different prey densities for birds differing in dominance. Observational studies have been used to validate the theoretical predictions, but there is reason to believe that these nonexperimental studies suffer from confounding factors. We therefore manipulated forager density of oystercatchers Haematopus ostralegus foraging on live cockles Cerastoderma edule (low density: 1 bird per 50 m2 and high density: 2 birds per 50 m2) in a unique experimental facility closely mimicking natural feeding conditions. In the high-density situation, the intake rate was on average reduced by 36% compared with the interference-free intake rate. However, this effect depended on status with intake rate of subordinates being more strongly reduced than intake rate of dominants (-45% vs. -25%). We could not investigate all possible mechanisms, but we observed that birds actively avoided each other, possibly to avoid kleptoparasitism. Our experiment shows that the decline in intake rate with increasing density of conspecifics is at least partly due to direct interactions between birds and possibly also to indirect interactions via prey depression but not to an unidentified confounding factor that covaries with intake rate and bird density, as may have been the case in nonexperimental field studies