Abstract
In the cooperatively breeding apostlebird (Struthidea cinerea, Corcoracidae) both sexes are philopatric and help to raise offspring. However, male helpers provision nestlings more often than females, an activity associated with reduced nestling starvation and enhanced fledgling production. Presuming that males are the more helpful sex, we examined the helper repayment hypothesis by testing the predictions that offspring sex ratio should be skewed toward the production of males (a) among breeding groups with relatively few helpers, and (b) in the population as a whole. The relationship between sex and hatching order was examined as a potential mechanism of biasing sex allocation. The sex ratio of all sexed offspring was male biased (57.9%; n = 171) as was the mean brood sex ratio (0.579; n = 70 broods). These biases were less pronounced in the subset of clutches/broods in which all offspring were sexed. This overall bias appeared to result from two distinct patterns of skew in the hatching order. First, mothers in small breeding groups produced significantly more males among the first-hatching pair. This is consistent with the helper repayment hypothesis given that later hatching chicks were less likely to survive, particularly in small groups. Second, almost all fourth-hatching chicks, usually the last in the brood, were male (91.7%, n = 12). This bias is difficult to interpret but demonstrates the value of examining hatching sequences when evaluating specific predictions of sex allocation theory in birds.
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Acknowledgements
We are grateful to the Ryan family for accommodation and permission to work on their property. We would also like to thank Raoul Mulder and several anonymous referees for their valuable comments on the manuscript. The research was supported by grants from The Holsworth Wildlife Research Endowment, The Norman Wettenhall Foundation, Australian Geographic, The American Museum of Natural History, The Royal Zoological Society of NSW, and Birds Australia (VicGroup). IAW was supported by an Australian Research Council Large Grant awarded to Jan Komdeur, and a stipend from the Faculty of Science, University of Melbourne. Collection of field data was greatly assisted by Eron Chapman, Rebecca McIntosh, Emma and Helen Burdekin, Eric Woxvold, Bryony Anderson, Holly Dumble, Georgina Read, John Sandow, Mark Johnson, Andrew Barnes, Telford Scully, Sonia Orchard and Chris Jarvis. Capture, sampling and monitoring of apostlebirds was conducted under the authority of licences required by Australian law.
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Woxvold, I.A., Magrath, M.J.L. Sex-biases in the hatching sequence of cooperatively breeding apostlebirds Struthidea cinerea . Evol Ecol 22, 139–151 (2008). https://doi.org/10.1007/s10682-007-9163-y
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DOI: https://doi.org/10.1007/s10682-007-9163-y