Seasonal changes in the size selection of mussels, Mytilus edulis, by oystercatchers, Haematopus ostralegus: an optimality approach

Field observations were made of the size selection of mussels by oystercatchers throughout the year. Birds that opened mussels by hammering through the shell selected thin-shelled individuals and fed disproportionately on intermediate size classes for most months of the year. Profitability increased monotonically with mussel length for dorsal hammerers, but peaked at an intermediate size for ventral hammerers. In spring, all oystercatchers switched to feeding on small mussels and this coincided with weight loss in large mussels associated with spawning. An optimal diet model, which in addition to handling time included both the probabilistic costs of making selection errors and encounter rates with thin-shelled mussels, was a good predictor of the birds' diet selection for most months of the year, but did not explain why hammerers ate such small mussels in spring. These results are discussed in relation to some of the assumptions implicit in the model.