Begging costs rather than food received cause brood size effect on growth in zebra finches

Altricial species rely on parental provisioning for early-life sustenance, and a larger brood 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 behavior 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 invested more time feeding which more than compensated for the difference in brood size. We therefore conclude that the lower growth rate for nestlings raised in large broods must be atleast in part-attributed to increased energy expenditure on begging rather than a reduction in the amount of food received. These results suggest 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 nest.