Divorce and its consequences in the Common Blackbird Turdus merula

An urban population of Common Blackbirds Turdus merula was studied between 1995 and 1998 in the Botanic Garden in Bonn, Germany. The number of breedin g pairs varied from 32 to 39 per breeding season. Mean divorce rate (pair s eparation with remating of at least one of the partners) between two breedi ng seasons was 19.3% and 5.1% within breeding seasons. The number of fledgl ings produced per breeding season, of pairs which subsequently divorced, wa s significantly lower than that of pairs that stayed together. Females, but not males, produced more fledglings in the season after divorce than with their former mates the year before, and as many as females of the same age in stable pair bonds. The increase in reproductive success was thus unlikel y to be due to age effects. Nestling mass correlated positively with offspr ing survival to the following year. Females had heavier nestlings after div orce than in their former pair bond. There was no difference in nestling ma ss for males before and after divorce. The data were used to test the predi ctions of several hypotheses to explain divorce. Because divorce was more l ikely after low reproductive success in the previous year, and only females benefited, the findings best support the 'better option' hypothesis.