Brood size and begging intensity in nestling birds

Theoretical models suggest that sibling competition should select for conspicuous begging signals. If so, begging intensity might be expected to increase with the number of competitiors. The purpose of our study was to examine the relationship between begging intensity and brood size using nestling tree swallows (Tachycineta bicolor) as our model. Over 2 years, we videotaped begging behavior in unmanipulated broods of different sizes. We found that begging intensity increased with brood size. The average weight of nestlings in each brood did not vary with brood size, but feeding rate per nestling decreased with brood size, suggesting that nestlings in larger broods begged more intensively, possibly because they were hungrier. We also conducted an experiment to examine the effect of nest mates on begging in different-sized broods. We found that nestlings with similar weights, previous competitive environments, and food deprivation begged more intensively in large broods than in small broods. Overall, our study indicates that begging intensity increases with brood size in tree swallows. This relationship may result from interactions among brood mates rather than from lower feeding rates to individual nestlings in larger broods.