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
thoughout 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.