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Science 17 August 2007:
Vol. 317. no. 5840, pp. 941 - 944
DOI: 10.1126/science.1146037
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Reports

Reduced Egg Investment Can Conceal Helper Effects in Cooperatively Breeding Birds

A. F. Russell,1,2*{dagger} N. E. Langmore,3 A. Cockburn,3,4 L. B. Astheimer,5 R. M. Kilner6*

Cooperative breeding systems are characterized by nonbreeding helpers that assist breeders in offspring care. However, the benefits to offspring of being fed by parents and helpers in cooperatively breeding birds can be difficult to detect. We offer experimental evidence that helper effects can be obscured by an undocumented maternal tactic. In superb fairy-wrens (Malurus cyaneus), mothers breeding in the presence of helpers lay smaller eggs of lower nutritional content that produce lighter chicks, as compared with those laying eggs in the absence of helpers. Helpers compensate fully for such reductions in investment and allow mothers to benefit through increased survival to the next breeding season. We suggest that failure to consider maternal egg-investment strategies can lead to underestimation of the force of selection acting on helping in avian cooperative breeders.

1 Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.
2 Centre for the Integrative Study of Animal Behaviour, Macquarie University, Sydney NSW 2109, Australia.
3 School of Botany and Zoology, Australian National University, Canberra ACT 0200, Australia.
4 Percy Fitzpatrick Institute of African Ornithology, Department of Science and Technology/National Research Foundation Centre of Excellence, University of Cape Town, Rondebosch 7701, South Africa.
5 School of Health Sciences, University of Wollongong, Wollongong New South Wales 2522, Australia.
6 Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK.

* These authors contributed equally to this work.

{dagger} To whom correspondence should be addressed. E-mail: a.f.russell{at}sheffield.ac.uk

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