High divorce rates in Corsican blue tits: how to choose a better option ina harsh environment

We investigate which hypothesis, the "better mate hypothesis" or the "bette r territory hypothesis" best explains the unusually high divorce rate (59%) in a population of blue tits (Parus caeruleus) living in a sclerophyllous habitat characterised by severe environmental constraints (trophic, parasit ic, climatic) on the island of Corsica, France. Using data from the breedin g seasons 1985-1998 and from a brood size experiment (1990-1993) we examine d the causes of divorce and their consequences on breeding performance, mat e assortment and territory choice. Breeding performance had no significant effect on whether birds re-united or divorced in the next breeding season. Re-uniting pairs did better than divorced females and the latter improved t heir breeding performance compared to prior to divorce, but this was mainly due to age and territory effects. There were no differences in male perfor mance depending on whether they re-united or divorced. The age combination of pairs did not differ between re-uniting and divorcing pairs, but mate as sortment changed after divorce with males re-mating more often with older p artners than females. Manipulation of brood size showed a trend for birds w ith enlarged broods to divorce more. Pairs responded significantly to terri tory quality by divorcing more often in poor than in good breeding sites. B oth faithful pairs and male divorcees had shorter breeding dispersal distan ces than female divorcees. Divorce rates were determined by the large diffe rences in quality among breeding sites. Males, whatever their status, usual ly retained their previous territory whereas divorced females moved signifi cantly longer distances and improved their breeding site. Moving to a bette r territory after divorce benefits only females which appear to be the choo sing sex in the decision to divorce. This study strongly supports the "habi tat mediated hypothesis" and we suggest that the large observed intraspecif ic variation in the magnitude of divorce rates in many species of birds is mostly determined by habitat characteristics.