The Size Selection of Clams by African Black Oystercatchers and Kelp Gulls

Abstract

The selction of prey by two species of shore—dwelling birds was compared with the predictions of a simple optimal diet model. The preferences of African Black Oystercatchers (Haaematopus moquini) and Kelp Gulls (Larus dominicanus) for different size classes of the sand—burrowing bivalve Donax serra were examined. Gulls took only large clams, regardless of the availability of the different size classes. This preference is ascribed to the high energy expenditure required for gulls to open clam shells. Oystercatchers, which open bivalve shells relatively easily, took both small and large clams, even when large clams were abundant. The selection of small clams when large clams were abundant appears to have resulted the lack of discrimination of clam sizes by oystercatchers and the temporal segregation of availability of small and large clams.