Parents preferentially feed larger offspring in asynchronously hatched broods irrespective of scramble competition

Abstract

Brood reduction is a frequent outcome in asynchronously hatching birds. In these species, first-hatched nestlings get a disproportionately larger share of the food delivered by parents, often leading the youngest nestling to starve to death soon after hatching. However, we still do not know the proximate mechanisms of such brood reduction. Starvation of the smallest nestling may be the outcome of scramble competition among nestmates (due to brood size hierarchies) or, alternatively, be caused by parents that ignore the persistent begging from their smallest nestling. To determine whether parents or scramble competition among siblings induces brood reduction, we used a cross-fostering approach to experimentally create asynchronous Eurasian blackbird, Turdus merula, broods. Then, we assessed food allocation by parents in two different situations: when sibling competition was allowed and when competition was prevented by physically separating nestlings with wooden barriers placed within the nest. We found that smaller nestlings experimentally introduced into blackbird nests received less food than their larger nestmates regardless of whether scramble competition among nestlings was allowed or not. Males preferentially fed larger nestlings regardless of the competitive context, while females showed no feeding differences. Our results suggest that nestlings may exert limited control over parental food allocation.