Abstract
In some bird species, mothers can advantage the offspring of one sex either by elevating them in the laying order to promote earlier hatching or by allocating greater resources to eggs of the preferred sex. In size dimorphic species, the predictions as to which sex should benefit most from such pre-laying adjustments are ambiguous. The smaller sex would benefit from an initial size advantage to help compensate for the faster growth rate of the larger sex. However, an early advantage to offspring of the larger sex might have a greater effect on their lifetime reproductive success than an equivalent advantage to offspring of the smaller sex. We investigated these hypotheses in the polygynous brown songlark, Cinclorhamphus cruralis, which is one of the most sexually size dimorphic birds known. We conducted within-clutch comparisons and found that females hatched from larger eggs and were initially heavier (but not structurally larger) than their brothers. This may afford females an early competitive advantage, as egg volume remained correlated with chick mass until at least 5 days of age. Similarly, we found that hatch order was still positively associated with nestling mass and size when the brood was 10 days of age, but there was no clear relationship between offspring sex and hatching order. During this study, food was plentiful and there were few obvious cases of nestling starvation. When food is limited, we suggest that the greater nutrient reserves of female hatchlings could not only help compensate for their slower growth, but could also give them a survival advantage over their brothers early in the nestling period. Consequently, egg size dimorphism may be an adaptation that facilitates an early shift in brood sex-ratio towards cheaper daughters in conditions of low food availability.
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Acknowledgements
We are very grateful to all those land holders and managers throughout the Riverina who allowed us to work and stay on their properties. Particular thanks go to the Jones, McLean, Clarke and Rutledge families on whose properties most of this work was conducted. We are also grateful to Mathew Berg, Wouter van Dongen, Alison Kemp, Emile van Lieshout, Marjan van Meerloo, Cordellia Moore, Arnoud van Petersen, Justin Welbergen and Iain Woxvold for their assistance in the field, and Martijn van de Pol for his help with the re-sampling analysis. We also thank the staff at the NSW National Parks and Wildlife Service (Griffith) and the Rural Lands and Protection board (Hillston and Hay) for advice and assistance. Useful comments on the manuscript were made by Mark Elgar, Theresa Jones, Mathew Symonds and two anonymous reviewers. This research was conducted with the approval of the University of Melbourne Animal Experimentation Ethics Committee, and financially supported by a grant JK from the Australian Research Council (A19802459). LB received financial support from the Marco Polo Fund!
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Magrath, M.J.L., Brouwer, L. & Komdeur, J. Egg size and laying order in relation to offspring sex in the extreme sexually size dimorphic brown songlark, Cinclorhamphus cruralis . Behav Ecol Sociobiol 54, 240–248 (2003). https://doi.org/10.1007/s00265-003-0627-y
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DOI: https://doi.org/10.1007/s00265-003-0627-y