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 a re unsuccessful breeders as a pair, although their individual characte ristics 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 o f the birds in search of a 'higher quality' partner (or territory). Th e ages of partners of great skuas, Catharacta skua, breeding on Foula (Shetland) were poorly correlated (r(48)=0.28), 22% of the partners di ffering in age by 10 years or more. Mate fidelity in this population w as high, with an annual divorce rate of only 6.4% (N=20/311). Death wa s responsible for-three times more interruptions of partnerships than divorces. Three types of divorce were detected. (1) In six cases, fema les deserted their partners, leaving them temporarily unpaired. (2) On e female was recorded being forced to desert her partner by a usurper. (3) One female deserted her partner after they lost their breeding te rritory to another pair. Mate changes had a direct cost, because new p airs (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 t han after becoming widowers. There was no difference between the sexes in the probability of re-pairing after a mate died. The results sugge st that divorces that are initiated by birds within the pair are bette r explained by the 'better option hypothesis' than the 'incompatibilit y hypothesis'. (C) 1997 The Association for the Study of Animal Behavi our.