Breeding synchronization facilitates extrapair mating for inbreeding avoidance

Extrapair (EP) mating can enable females to reduce the negative effects of inbreeding. However, opportunities for EP mating are often ecologically or demographically constrained, and it is unclear whether and how females can overcome these constraints. Here, we show that fitness costs from inbreeding in purple-crowned fairy-wrens (Malurus coronatus) are severe (≥30% reduction in hatchability of eggs in incestuous pairs). Although EP paternity was low for unrelated pairs (3% of broods), and negatively affected by low breeding density, 46% of broods produced by incestuously paired females contained EP young that were almost always sired by a direct neighbor. Moreover, females in incestuous pairs initiated egg laying more synchronously with direct neighbors than females in an unrelated pair, and more synchronous broods were more likely to contain EP young. Because male sperm stores are highest around their own partner’s fertile period, our results imply that incestuously paired females achieve high levels of EP paternity by adjusting the timing of egg laying to match peak fertility of neighboring males. This example of a dramatic increase in EP paternity due to subtle behavioral adjustments illustrates that negative effects of inbreeding exert strong selection for female strategies to facilitate EP mating, and this may contribute to unexplained variation in EP paternity.