SOME ECOLOGICAL ASPECTS OF THE FEEDING BEHAVIOUR OF THE OYSTERCATCHER HAEMATOPUS OSTRALEGUS ON THE EDIBLE MUSSEL MYTILUS EDULIS

Summary Oystercatchers have two methods of opening mussels, both neatly adapted to the structure of the prey. To open mussels exposed by the tide, the bird hammers a hole along the ventral margin of the shell, whereas to open mussels under water, the bird drives its bill into the gape of the valves to cut through the posterior adductor muscle. Large and strong‐shelled mussels can be opened by stabbing but not by hammering. Feeding methods of Oystercatchers vary from mussel‐bed to mussel‐bed. These variations in behaviour are attributable to differences in the strength of the shells of the prey and to the firmness of their attachment to the substrate. Diversity of ecological conditions on the mussel‐beds causes an apparent size selection of prey by Oystercatchera.