Causes and consequences of breeding dispersal and divorce in a blue tit, Cyanistes caeruleus, population

Abstract

In socially monogamous species with a resource-based mating system, mate choice and territory choice cannot easily be separated. We studied the occurrence and consequences of postbreeding dispersal and divorce in a population of blue tits breeding in nestboxes. Dispersal was defined relative to the spatial position of territories estimated as Dirichlet tiles. Although more than 80% of males and females changed nestbox between breeding seasons, only 3% of males and 25% of females moved to a new territory. Settlement following postbreeding dispersal was driven by habitat quality: dispersing females ended up in territories with higher clutch size expectancy. Dispersed females (both widowed and divorced) reared more fledglings than resident ones, suggesting that dispersal is adaptive. Our results suggest that even at relatively small spatial scales and in continuous habitats, postbreeding dispersal may be driven by habitat quality gradients. Across all years, 53% of surviving pairs divorced. We assessed the effects of mate change independently of territory quality, by restricting analyses to resident individuals that, after divorce, shared a part of their former territory or settled in its close vicinity. Reproductive success increased for males after divorce, but decreased for females. It did not change for reunited pairs in subsequent seasons. Paternity loss or gain through extrapair copulations, and hence overall siring success, did not differ between divorced and reunited males, either before divorce or pair re-formation, or in the year after. After divorce, males but not females paired with larger, presumably higher-quality partners. The results support the better option hypothesis and suggest that males, but not females, may benefit from divorce. However, our results are also compatible with the alternative hypothesis that divorce is driven by female–female competition, whereby a dominant female evicts the former mate.