Long-term brood division and exclusive parental care in a cooperatively breeding passerine

Abstract

In some species of birds, individual adults feed different subsets of the brood after the young have left the nest. However, few studies have shown that such ‘brood division’ represents long-term separation into subfamilies, rather than short-term biases in care, and the function of brood division is unknown. Most functional hypotheses assume that brood division is an adult adaptation to increase reproductive success; others suggest that fledglings are responsible for brood division, or that brood division is the outcome of conflicts of interest within families. We tested for long-term brood division in the white-browed scrubwren, Sericornis frontalis, a cooperatively breeding passerine, and assessed functional hypotheses, using behavioural, morphological and molecular evidence. We found that 71% of scrubwren broods became divided in the second week after fledging, and remained divided for the following 4–5 weeks of care. Furthermore, most individual young were fed almost exclusively by a single adult, even in undivided broods and broods of one. Brood division did not arise through adults caring for young of a specific sex, nor did genetic paternity explain division. Tests of other hypotheses were indirect, relying for example on testing mechanisms rather than predictions. We rejected the hypotheses that brood division was primarily an adaptation to reduce the mean or variance in predation, or that it was likely to increase the rate at which young were provisioned. Fledglings probably had an active role in maintaining brood division, with fledglings that had been biggest in the nest tending to be fed by adults that had fed nestlings at the greatest rate. This suggests that the most dominant fledgling may sequester the care of the best feeder. We also found evidence that brood division might be an adult adaptation to reduce the effects of sibling competition, or that it might arise through conflicts of interest among adults. We further suggest that social specialization, resulting from learning about a particular individual, might bring additional advantages to long-term brood division. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.