Altricial species rely on parental provisioning for early-life sustenance, and a larger brood or litter size leads to higher levels of competition between siblings for parental resources. Early-life stress can have severe and lifelong effects on Darwinian fitness. Indeed, it is well established that being reared in a larger brood impairs growth and fitness prospects of birds, but the mechanistic underpinnings of this effect are still largely unknown. Specifically, it is not well known to what extent the reduced growth and fitness prospects of nestlings reared in large broods is due to increased resource allocation to competition versus a per capita reduction in parental provisioning rate, or a combination of the two. We cross-fostered zebra finch (Taeniopygia castanotis) chicks into small and large broods, and recorded their growth as well as the behaviour of parents and offspring throughout the nestling period. As in previous experiments, growth rate was higher in small broods. In large broods, chicks begged more and parents fed more frequently, fully compensating for brood size differences to yield comparable per capita feeding rates. We therefore conclude that the lower growth rate for nestlings raised in large broods is likely attributable, at least in part, to increased energy expenditure on begging and limits to parental allocation efficiency, rather than solely a reduction in their feeding frequency. These patterns are consistent with significant energetic costs associated with begging and raise the interesting possibility that brood size would not have negatively affected growth in large broods if chicks had not increased their begging effort due to increased levels of competition in the nest.