Prey type and prey size selection in a Eurasian oystercatcher population at the edge of its distribution range

This study targeted prey type and prey size selection by a small but expanding population of the Eurasian oystercatcher (Haematopus ostralegus) at the southernmost range of its European distribution (Galicia, NW Spain). We compared consumption and availability of prey items delivered to chicks. Consumption was studied through piles of discarded shells and availability was sampled in the foraging grounds located around nesting sites on three different islets. The only two prey types present in shell piles were mussels (Mytillus galloprovincialis) and limpets (Patella sp.). Our chi-square analyses did not detect any prey type selection by oystercatchers, but the use of the Savage index with Manly’s method identified negative selection of limpets at one of the study sites. As a rule, the birds consumed limpets and mussels in proportion to their availability in their foraging grounds. At one of the study sites periwinkles (Littorina sp.) were present but not consumed. Regarding prey size, oystercatchers selected (1) mussels that were either similar to or larger than those available, and (2) limpets that were larger than those available. However, at one site out of three, the oystercatchers positively selected limpets of smaller size than those available. Hence, our results showed some heterogeneity among sites regarding prey type selection, but quite a consistent pattern of selection of larger prey. We suggest that oystercatcher foraging ecology is not explained solely by considering the balance between energy uptake and costs, but importantly by introducing additional variables such as the risk of chick predation, and current nesting as refugees on islets with no access to mainland beaches and sand dunes for breeding, where both adult and chick diet could be substantially different.