Partnerships and mechanisms of divorce in the great skua.

There are two main theories attempting to explain divorce in birds as an adaptive decision for at least one of the members of the pair. The 'incompatibility hypothesis' states that birds separate because they are unsuccessful breeders as a pair, although their individual characteristics are not negative per se. Divorce comes about as an initiative of both individuals to search for a more compatible mate. The 'better option hypothesis' suggests that divorce is usually initiated by one of the birds in search of a 'higher quality' partner (or territory). The ages of partners of great skuas, Catharacta skuabreeding on Foula (Shetland) were poorly correlated (r48=0.28), 22% of the partners differing in age by 10 years or more. Mate fidelity in this population was high, with an annual divorce rate of only 6.4% (N=20/311). Death was responsible for three times more interruptions of partnerships than divorces. Three types of divorce were detected. (1) In six cases, females deserted their partners, leaving them temporarily unpaired. (2) One female was recorded being forced to desert her partner by a usurper. (3) One female deserted her partner after they lost their breeding territory to another pair. Mate changes had a direct cost, because new pairs (of experienced birds) laid later in the season and reared fewer chicks. After mate loss, 26% (N=61) of the individuals did not breed, while faithful pairs always laid eggs (N=222). Males, but not females, had a higher probability of remaining unpaired after being divorced than after becoming widowers. There was no difference between the sexes in the probability of re-pairing after a mate died. The results suggest that divorces that are initiated by birds within the pair are better explained by the 'better option hypothesis' than the 'incompatibility hypothesis'.Copyright 1997 The Association for the Study of Animal Behaviour