IN many socially organized vertebrates, successful parental care depends on individual recognition1. Particularly with birds breeding in dense colonies there are clear signs that the basis for recognition may be mainly auditory rather than visual. In the sandwich tern (Sterna sandvicensis), for example2, the ``fish call'' used by the parent returning with fish for the young has the kind of individual differences in acoustic structure on which recognition might be expected to depend. Our observation of the behaviour suggests that the young use these calls to distinguish their own parents. There is also reason to suppose that young common terns (Sterna hirundo) can recognize their parents by call alone3, although because of practical difficulties in the field it has not hitherto been possible to provide a rigorous demonstration. Working at Scolt Head Island (Norfolk) during June, 1969, we were able to assess the ability of young common tern chicks to recognize the calls of their parents. This was done by recording the calls of individual parents when they landed at their respective nests and then testing the responsiveness of chicks to the recorded calls.