Correlates of the occurrence of inbreeding in a wild bird population

Inbreeding occurs when close relatives mate. In most populations, inbreeding is uncommon, with the result that we know little about the characteristics of those individuals that inbreed. However, such information may be invaluable for understanding the causes of inbreeding and inbreeding avoidance and the consequences of inbreeding. We investigated the characteristics of closely inbreeding individuals using 43 years of data from a wild great tit (Parus major) population in order to 1) characterize close inbreeding in terms of the family relationships involved; 2) compare inbreeding with noninbreeding individuals; and 3) determine whether close inbreeding was subsequently avoided by those that experience it. We found an uneven distribution of different types of close inbreeding, with sib mating most frequent and father--daughter matings least frequent. We did not find any evidence for clear predictors of inbreeding. By comparing the reproductive success of individuals that both inbred and outbred in their lifetimes, we demonstrate that inbreeding depression is solely attributable to the mixing of relatives' genes and is thus independent of any tendency for low-quality parental genotypes or phenotypes to inbreed. Surprisingly, mating with close kin reduced the probability of divorce in the following reproductive season, while brood or fledging failure did not predict for remating. Overall, our results suggest that inbreeding is not a response to lowered prospects and that despite the substantial fitness cost of inbreeding, its avoidance in this population may be based on general mechanisms such as dispersal rather than specific kin avoidance. Copyright 2008, Oxford University Press.