Experimental evidence for interference competition in oystercatchers, Haematopus ostralegus. II. Free-living birds

Field studies of interference competition in free-living animals have relied on natural fluctuations in forager density, which are often confounded with other factors. We therefore experimentally studied interference in the wild, capitalizing on 2 cockle beds in an isolated bay that were exploited by a population of individually marked oystercatchers. We successfully increased forager density by chasing birds from one cockle bed, leaving the other cockle bed as the only nearby alternative. The density increase was most pronounced on the eastern cockle bed where food stocks were poorer and initial feeding densities were lower compared with the western cockle bed. Oystercatchers residing on this eastern bed suffered a significant decline in intake rate when bird density was experimentally increased, providing evidence of interference. “Refugee” birds, that is, the birds that were displaced from their home bed, experienced an even stronger reduction in intake rate compared with the residents and compared with their intake rate on their “home bed,” probably partly due to the fact that the refugee birds were forced to feed on an unfamiliar cockle bed. The fact that disturbance at 1 site influenced both the refugees and the local resident birds indicates that human disturbance (an important conservation issue) has an effect that extends beyond the site where the disturbance takes place. The benefits and costs of site fidelity and interference competition are likely to play an important role in understanding animal distributions and how they change in response to environmental perturbations, including human disturbance of foraging sites.