The amount of food delivered by parents to their chicks is affected by various life history traits as well as environmental and social factors, and this investment ultimately determines the current and future fitness of parents and their offspring. We studied parental provisioning behaviour in the Vinous-throated Parrotbill Paradoxornis webbianus, a species with an unusual social system that is characterised by flock-living, weak territoriality and variable nesting dispersion. Parental provisioning rate had a positive influence on chick mass gain, suggesting that provisioning rate is an effective measure of parental investment in this species. Males and females fed nestlings at approximately the same rate, and no other carers were observed at nests. Parents coordinated provisioning rates so that they mostly fed chicks synchronously. However, the extent to which parents coordinated provisioning was associated with their social environment, synchrony being positively related to local breeding density and negatively to nearest neighbour distance. The rate at which parents provisioned nestlings showed the same relationships with social measures, being greatest at higher density and when neighbours were closer. Visit rate was also related to chick age, but not to brood size, brood sex ratio, extra pair paternity, laying date, temperature, parents' body characters, time of day or year. We conclude that a breeding pairs' social environment plays an important role in determining parental investment, probably through its effects on the opportunities that parents have for foraging with conspecifics.

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