Mussel valve discrimination and strategies used in valve discrimination by the Oystercatcher, Haematopus ostralegus

Summary 1.  In 72% of a sample of common mussels Mytilus edulis L. that had been attacked by ventrally hammering Oystercatchers, Haematopus ostralegus L., the mussel had been opened through the right valve rather than the left. 2.  In a matched sample of unopened shells, 56·5% of mussels were ventrally thinner on the right. Mussel thickness differences were unrelated to overall mussel thickness, so this proportion should apply also to opened shells even though they are on average thinner than unopened shells. 3.  Since Oystercatchers attack a higher proportion of mussels by the right valve than are actually thinner on the right, their preference cannot depend solely on detecting measurable differences in thickness. 4.  The pattern of preference can best be explained if Oystercatchers detect and attack the thinner valve in the mussel when the thickness difference between the two valves is more than a threshold of 0·036 mm, and otherwise always attack the right valve. This strategy would save 15·5% of the blows the birds would make if they attacked shells without regard to differences in valve thickness. 5.  The improvement in the overall net energy intake rate that could be achieved by such a valve thickness discrimination strategy was 3·6%.