FORAGING BEHAVIOR OF OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS SPECIALIZING ON DIFFERENT SPECIES OF PREY

Adult oystercatchers Haematopus ostralegus specializing on four prey species, the mussel Mytilus edulis, the winkle Littorina littorea, the ragworm Nereis diversicolor, and the clam Scrobicularia plana, were compared. Some clear trends from mussels to winkles to clams to worms were identified. First, mean biomass (size) of prey decreased but more prey were attacked per unit time and, since fewer attacks were aborted, more prey were actually captured per unit time. Second, handling time decreased, and birds raised their heads less often while handling a prey item and carried prey less frequently. Third, foraging density, attack rate, and avoidance rate all decreased. However, average rates of food intake did not follow the same order and were inconsistent with the expected preference for prey. The greatest intake rate was achieved by birds specializing on clams, the least preferred prey. The rate of intake of mussels, the most preferred prey, was significantly lower. Alternatives to the energy rate maximization model to explain prey preference are discussed and dismissed. The results of this study contrast with those from several studies on prey-size selection within a single prey species, which show that oystercatchers prefer prey that maximize their intake rates. We suggest that the basis for selecting a prey species and for selecting different size classes within one prey species may be quite different.