A trade-off between energy intake and exposure to parasites in oystercatchers feeding on a bivalve mollusc

Models of animal dispersion between habitat patches that differ in resource density assume that animals maximize their fitness by maximizing the rate at which they consume resources. How valid is this assumption? Studies on w ading birds have been central to the application of dispersion models to pr edator-prey systems. However, these birds do not always attempt to maximize their rate of energy intake, implying that maximization involves costs as well as benefits. Overwintering oystercatchers feeding on cockles in the Bu rry Inlet, South Wales, do not consume the larger more energetically profit able cockles even though consuming these prey would increase their rate of energy intake. This paper tests the hypothesis that maximizing energy intak e involves a trade-off with exposure to helminth parasites. Cockles are imp ortant intermediate hosts for helminth parasites, for which oystercatchers are the definitive host. The helminth intensity of cockles increased signif icantly with cockle size. A functional response model was used to examine h ow size selection by the birds influenced energy intake and the ingestion r ate of parasites. To maximize energy intake birds should selectively consum e the larger size classes, but to minimize the ingestion rate of parasites they should consume the smallest size classes. In the wild, birds selective ly consumed intermediate size classes, which could represent a compromise b etween these conflicting demands. The implications for animal dispersion mo dels are discussed.