Cooperative breeding, where some individuals help to raise offspring that are not their own, is a relatively rare social system in birds. We studied the breeding biology of a declining cooperative breeder, the grey-crowned babbler Pomatostomus temporalis, with the aim of isolating the social factors that affect its reproductive success. Most breeding pairs were assisted by philopatric offspring, although pairs could breed successfully without helpers. Females laid up to four clutches (usually three eggs per clutch) per season. Male (but not female) helpers increased the number of young fledged from individual nests and the likelihood of re-nesting, resulting in higher seasonal fledgling production. Helper effects on brood size and fledgling production were greater in the second year of the study, which was also characterized by higher nest failure. This suggests that helpers enhance reproduction more in poor conditions. Our study demonstrates the interacting effects of social and ecological factors on reproductive success, and that retention of offspring is not always beneficial for the breeders in cooperative species.