Divorce in cooperatively breeding long-tailed tits: a consequence of inbreeding avoidance?

The decision of whether to divorce a breeding partner between reproductive attempts can significantly affect individual fitness. In this paper, we rep ort that 63% of surviving pairs of long-tailed tits Aegithalos caudatus div orced between years. We examine three likely explanations for the high divo rce rate in this cooperative breeder. The 'better option' hypothesis predic ts that divorce and re-pairing increases an individual's reproductive succe ss. However, divorcees did not secure better partners or more helpers and t here was no improvement in their reproductive success following divorce. Th e 'inbreeding avoidance' hypothesis predicts that females should disperse f rom their family group to avoid breeding with philopatric sons. The observe d pattern of divorce was consistent with this hypothesis because, in contra st to the usual avian pattern, divorce was typical for successful pairs (81 %) and less frequent in unsuccessful pairs (36-43%). The 'forced divorce' h ypothesis predicts that divorce increases as the number of competitors incr eases. The pattern of divorce among failed breeders was consistent with thi s hypothesis, but it fails to explain the overall occurrence of divorce bec ause divorcees rarely re-paired with their partners' closest competitors. W e discuss long-tailed tits' unique association between divorce and reproduc tive success in the context of dispersal strategies for inbreeding avoidanc e.