Prey Depletion and the Regulation of Predator Density: Oystercatchers (Haematopus Ostralegus) Feeding on Mussels (Mytilus Edulis)

Abstract

One of the many decisions facing birds attempting to maximize their food intake is the choice of where to go to feed. Certainly they should restrict their search to where food is plentiful, and concentration in the optimal parts of the feeding area has been documented for the Oystercatcher by Hulscher (1976) and Goss-Custard (1977). On the other hand, birds should avoid high feeding densities as the presence of conspecifics may in itself depress the feeding rate, a direct and immediate effect not dependent on prey depletion. This inhibitory effect has previously been demonstrated in other birds feeding on intertidal areas (Redshank, Tringa totanus, Goss-Custard, 1976; Curlew, Numenius arquata, Zwarts, in press). The outcome of these opposing tendencies expresses itself in a compromise, whereby the richest feeding areas are filled first, and as increasing numbers of birds utilize the area, marginal feeding areas come into use. This “buffer effect” is a familiar concept in discussions on the function of territoriality (Kluijver and Tinbergen, 1953; Fretwell and Lucas, 1970) and appears to play a role in the spacing of birds over the intertidal feeding area (Zwarts, 1976; Goss-Custard, 1977).