Parents and helpers compensate for experimental changes in the provisioning effort of others in the Arabian babbler.

Biparental investment in birds is implemented through parents making adaptive compensatory adjustments according to the work rate of their collaborator. Predictions for such evolutionarily stable cooperation in parental nestling feeding, and the behavioural mechanisms involved, can also be applied to systems with helpers-at-the-nest. Using the cooperatively breeding Arabian babbler, Turdoides squamiceps, we experimentally increased the provisioning rate of one or two subordinate birds in each group by giving these individuals extra food. Compared with control days before and after, the experimental treatment caused 'fed' birds to increase and 'unfed' birds to decrease their nest visit rates significantly, with no significant changes in load size. Thus, both parents and helpers in this system showed individual compensatory adjustments in their nestling-feeding effort according to the work rates of collaborators. There were no significant increases in total group visits during experimental treatments; therefore this study did not provide evidence for the predicted incomplete compensatory response. These results for helping-at-the-nest are consistent with empirical studies on parental care, and further suggest a central role for nestling begging in any cooperative provisioning of young in the nest. Copyright 1999 The Association for the Study of Animal Behaviour.