Ventral hammering oystercatchers tend to specialise upon attacking either the right or left valve of mussels. Both in the field and in captivity they initially select medium sized mussels. Selection against small mussels can be understood from the low profitability of small mussels, but large mussels are very profitable. The hammering oystercatchers select thin shelled mussels even though capable of opening ones with thicker shells. Once the preferred prey are depleted, they take thicker and larger mussels. Indirect evidence suggests that within a size class the profitability of thick-shelled mussels is lower. Stabbing birds open larger and thicker mussels than do hammering birds. When feeding on boiled mussels size taken is larger than when feeding on live mussels.