Females call the shots: breeding dispersal and divorce in blue tits

In this paper, we study proximate causes and consequences of breeding dispersal and divorce in a Mediterranean blue tit Cyanistes caeruleus population. We attempt to evaluate the primacy of mate and territory quality in the decision making of birds. In females, the fidelity to their old territories was dependent on how they did in the previous year. We found differences in hatching success and fledging success between females that dispersed and those that did not. In particular, dispersal distance was negatively correlated with hatching success in the preceding breeding season. Dispersed females experienced a gain in terms of hatching success and increased productivity in comparison with those that retained their former mate. Both females and males improved the quality of their breeding site after dispersal. We also found evidence that divorce is adaptive for at least one pair member because adult females (but not juveniles) enhanced their breeding performance after the breakup. Such improvement was also observed when restricting the analyses to only those individuals that did not change territory, suggesting that these benefits not only accrue through the acquisition of better territories but advantages derived from pairing with a higher rank male (mate effects per se) could also be important. Our results suggest that 1) both dispersal and divorce can be regarded as adaptive decisions, 2) females play an active role in pair-bond maintenance, and 3) the intense competition for good nesting sites drives the observed high divorce rate (63%) reinforcing the view that breakups could be a side effect derived from the female's attempt to improve their breeding place.