Western bluebird parents preferentially feed hungrier nestlings in a design that balances location in the nest

Abstract

Parents may feed nestlings based on positional cues resulting from sibling competition for favored positions at the front of a nest cavity, or they may assess begging intensity regardless of location within the nest. We independently manipulated proximity to the nest entrance and hunger level to determine whether western bluebird (Sialia mexicana) parents favor hungrier nestlings or instead feed nestlings closest to the nest entrance. We used a balanced design to vary hungry and fed nestlings between the back and front of the nest box. We deprived half of the nestlings in each brood of food for an hour, and then, each nestling was constrained using Plexiglas dividers to a quadrant in the nest box. We videotaped inside the box to assess begging intensity and food delivery to individual nestlings. Hungry nestlings begged more intensely and received more food than fed nestlings, regardless of their proximity to the nest entrance. However, when delivering the first feed to nestlings, parents favored front nestlings over hungry nestlings, possibly due to a bias resulting from the manipulation, which involved replacing and rearranging nestlings after a 1-h period. Overall, parents did not favor nestlings in the front of the nest over those in the back. Parents therefore are able to distinguish hungrier nestlings and preferentially feed them under favorable environmental conditions, when brood reduction is uncommon. When parents choose which offspring to feed, they may assess begging signals, which can include movements and vocalizations that vary in intensity. Alternatively, cavity-nesting species may use simple rules such as favoring offspring that have pushed to the front of the nest. We conducted an experiment to determine whether western bluebird (Sialia mexicana) parents would preferentially feed hungry nestlings over fed nestlings when Plexiglas dividers prevented competition and equalized the frequency with which hungry and fed nestlings were in the front versus back of the nest. After depriving half of each brood of food for an hour, we measured begging intensity and food delivery. Parents preferentially fed hungrier nestlings, which begged more intensely, regardless of their position in the nest, indicating that parents are able to distinguish between hungry and fed nestlings.