Divorce and asynchronous arrival in common terns, Sterna hirundo

We investigated which of three hypotheses (better option, incompatibility o r asynchronous arrival) best explains divorce in the common tern. One partn er did not return the next year in 18.5% of 150 pairs. Among the 106 pairs in which both mates returned, the divorce rate was 18.9%. We found no signi ficant differences in: breeding performance or condition in relation to the probability of divorce; quality of previous mates and new mates,mean age i n relation to pair bond status; breeding success before and after divorce n or did this differ from breeding success of reunited pairs. Hence the bette r option and incompatibility hypotheses were not supported. However, divorc e was more likely in pairs in which mates arrived asynchronously on the bre eding grounds, supporting the asynchronous arrival hypothesis. Median arriv al asynchrony for divorced pairs was 7.5 days and for reunited pairs 2 days ; mates arriving more than 16 days apart always split up. About 20% of divo rced birds lost breeding status in the year of divorce, probably as a conse quence of their late arrival. Our results suggest that terns search for a n ew mate as soon as they arrive on the breeding grounds and that mates remai n faithful to each other to avoid the costs of searching for a new partner. Thus, synchrony in arrival facilitates pair bond maintenance rather than a synchrony promoting divorce, since divorce appears to be a side-effect of a synchrony and not an active decision. (C) 1999 The Association for the Stud y of Animal Behaviour.