Kin do not always help: testing multiple hypotheses on nest feeding in a cooperatively breeding bird

In cooperatively breeding species, group members may derive multiple benefits from helping to raise other individuals’ offspring, yet not all individuals do so. In this study, we tested several hypotheses to explain why group members feed offspring of breeding placid greenbuls (Phyllastrephus placidus). In accordance with the kin selection hypothesis, all helpers were first-order kin of the breeding female and the presence of helpers was associated with increased survival of the breeding pair. However, the propensity to help varied widely among group members, as 46% of group members related to the breeding female did not feed nestlings. Sex, relatedness to the breeding male, and group size did not explain additional variation in helping propensity or effort, however, younger helpers fed offspring more often than older ones. Overall, our results show mixed support for predictions of the group augmentation hypothesis, while predictions of the pay-to-stay hypothesis and skills hypothesis were not supported. We suggest that costs associated with providing food to nestlings, and benefits of other types of helping behavior (e.g. anti-predator behavior), may jointly explain why members of breeding groups often refrain from helping at the nest.