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 adapti ve compensatory adjustments according to the work rate of their collaborato r. Predictions for such evolutionarily stable cooperation in parental nestl ing feeding, and the behavioural mechanisms involved, can also be applied t o systems with helpers-at-the-nest. Using the cooperatively breeding Arabia n babbler, Turdoides squamiceps, we experimentally increased the provisioni ng rate of one or two subordinate birds in each group by giving these indiv iduals extra food. Compared with control days before and after, the experim ental treatment caused 'fed' birds to increase and 'unfed' birds to decreas e their nest visit rates significantly, with no significant changes in load size. Thus, both parents and helpers in this system showed individual comp ensatory 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 provid e evidence for the predicted incomplete compensatory response. These result s for helping-at-the-nest are consistent with empirical studies on parental care, and further suggest a central. role for nestling begging in any coop erative provisioning of young in the nest. (C) 1999 The Association for the Study of Animal Behaviour.