The importance of pair duration and biparental care to reproductive success in the monogamous Australian magpie-lark

Biparental care is common in birds, where monogamy is the predominant mating system. Australian magpie-larks (Grallina cyanoleuca) are socially monogamous, and relatively unusual among passerines in the extent to which parental care is shared. Males contributed as much or more to parental care as females, sharing nest-building, incubation, brooding and feeding of nestlings and fledglings. Biparental care was thus important to survival of offspring, and probably constrained partners to stay together throughout a breeding attempt. However, partners usually remained together longer. Pairs that had bred together in the previous season tended to lay their first clutch earlier, were more likely to fledge two broods in the season, and had higher annual reproductive success than pairs breeding together for the first time. Females benefitted from staying with a male they had bred with previously, as females in established pairs decreased their feeding rates and their partners compensated to some extent. Differences between new and established pairs may have been due to the effects of the age and experience of each partner, or to pair duration, or both. Divorce rates were low, consistent with benefits associated with staying together, but also with high costs of divorce as year-round territoriality probably limited opportunities for taking different partners.