INTRA-SPECIFIC AND INTER-SPECIFIC INTERACTIONS OF FORAGING AMERICAN OYSTERCATCHERS ON AN OYSTER BED

We compared the rates of intraspecific and interspecific kleptoparasit ism of foraging American oystercatchers (Haematopus palliatus) on a co mmercial oyster (Crassostrea virginica) bed during two seasons and bet ween 1979 and 1995. In 1979 most conspecific kleptoparasites were imma ture oystercatchers and victims were adults. Both intra- and inter-spe cific parasitism were more common in 1979 than in 1994 or 1995. Klepto parasitism by conspecifics was more common than by gulls (Larus argent atus, L. marinus) but was not density dependent, Gulls primarily klept oparasitized oystercatchers foraging on mussels (Geukensia demissa), w ith their longer handling times. Kleptoparasitism by gulls increased a s the number of gulls on the oyster bed increased, and the presence of gulls significantly depressed intake rates and sizes of mussels taken by oystercatchers during autumn. Oystercatchers ate smaller oysters i n autumn than in winter in both the presence and absence of gulls. The presence of conspecific and gull kleptoparasites changed the oysterca tchers' relative preference for oysters over mussels in their diet. Th e presence of gulls only partly explained the oystercatchers' lower ra tes of intake of oysters in autumn than in winter.