Seasonal changes in size selection and intake rate of Oystercatchers Haematopus ostralegus feeding on the bivalves Mytilus edulis and Cerastoderma edule

During the course of spring, Cockles Cerastoderma edule and Mussels Mytilus edulis grow in size, while the condition, as measured by the biomass content of shells of a given size, also increases. Condition temporarily drops when the larger individuals spawn. This study investigates the effects of these seasonal changes on the intake rate and the prey choice of Oystercatchers. Although profitability (biomass gained per unit time spent handling) was lower when the bivalves were in poor condition, large Cockles and Mussels were always the most profitable. It was therefore remarkable that these large prey were dropped from the diet in spring and early summer. Whereas condition of the molluscs was highest in August, intake rates of Oystercatchers peaked by the end of May, early June, when many adult birds had nests and may have been pressed for time. However, since the intake rate of subadult birds followed a similar seasonal pattern, it seems unlikely that time stress alone caused the adult birds to feed faster. It therefore seems that prey choice is 'suboptimal' in terms of rate maximization when the daily demand for food is minimal and feeding conditions are maximally good. This leads to the suggestion that the prey choice of Oystercatchers in late spring/early summer is more heavily influenced by nonenergy criteria like, for instance, the risk of parasitism, than at other times of the year.