Prey abundance and the strength of interference in a foraging shorebird

Interference is an important component of food competition but is often dif ficult to detect and measure in natural animal populations. Although interf erence has been shown to occur between oystercatchers Mytilus edulis L., fe eding on mussels Mytilus edulis L., four previous studies have not detected interference between oystercatchers feeding on cockles Cerastoderma edule L. In contrast, this study detected interference between cockle-feeding oys tercatchers in the Bale de Somme, France. Prey stealing (kleptoparasitism), one of the main causes of interference between mussel-feeders, also occurr ed between oystercatchers in the Bale de Somme. The kleptoparasitism rate w as related to the natural variation in the food supply, tending to be highe r when cockles were rare. Feeding rate was negatively related to competitor density, so providing evidence for interference, but, as in mussel-feeders , only above a threshold density of about 50-100 birds ha(-1). The strength of interference at a fixed competitor density was related to the cockle fo od supply, usually being greater when cockles were rare. Previous studies p robably failed to detect interference between cockle-feeders because compet itor densities were too low, or cockles were too abundant, or because they were not conducted during late winter when interference is most intense. Th e study shows that natural variation in the food supply can influence the s trength of interference within an animal population and provides support fo r those behaviour-based interference models which predict that the strength of interference will be greatest when competitor densities are high and pr ey scarce.