Patterns of offspring sex-ratio in a re-establishing population of Black-winged Petrels (Pterodroma nigripennis)
Stefanie M. H. Ismar A E , Karen Baird B , Emily Favell C and Mark E. Hauber A DA School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1143, New Zealand.
B Department of Conservation, Auckland Conservancy, PO Box 474, Warkworth 0941, New Zealand.
C Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand.
D Department of Psychology, Hunter College, City University of New York, NY 10065, USA.
E Corresponding author. Email: sism007@aucklanduni.ac.nz
Emu 110(1) 104-108 https://doi.org/10.1071/MU09097
Submitted: 14 October 2009 Accepted: 13 November 2009 Published: 24 February 2010
Abstract
Natural re-colonisation events are rare so it can be important to monitor newly establishing populations to understand and characterise such events. As re-colonising populations are often freed from spatial and competitive constraints, documenting the sex-ratio of offspring may provide insights in sex-allocation theory. We studied a re-establishing population of Black-winged Petrels (Pterodroma nigripennis) on Raoul Island, in the Kermadec Group, New Zealand, where all predators have recently been removed. We measured, and took DNA samples from 20 chicks in four new colonies in 2007, when the colonies first re-established, 25 chicks from seven colonies in 2008, and 25 adults captured across both years, including seven that were caught nearby at sea, and two adults caught at a colony where no chicks were sampled. We found the developmental stage of chicks to have no differences between sexes, and recorded no sexual differences in the morphometrics of chicks or adults. We report a significantly biased sex-ratio towards male offspring in the first year of re-colonisation. In contrast, the sex-ratios of offspring in the second year of re-colonisation and of adults in both years were even. We suggest that biases in offspring sex-ratios towards the more philopatric sex may be adaptive when under release from spatial or competitive constraints in re-colonising birds. Continued monitoring of the populations of Black-winged Petrels re-establishing on Raoul Island, as well as comparable data from adjacent islets with long-established breeding colonies of Black-winged Petrels will be critical to identify the realised ecological role of variation in offspring sex-ratio and of sex-biased natal philopatry.
Additional keywords: Kermadec Islands, molecular sexing, monomorphism, offspring bias, sex-ratio theory.
Acknowledgements
We thank the Department of Conservation for landing and research permits, and the NIWA (MSV Tangaroa 2007) and the Royal New Zealand Navy (MV Canterbury 2008) for transport to and from Raoul. Funding of this research was provided by an New Zealand International Doctoral Research scholarship of Education New Zealand (to SMHI), the Department of Conservation (to KB), Richard Holdaway (to EF), and the University of Auckland Research Committee and the Faculty of Science Staff Development Fund (to MEH). Thanks to Chris Gaskin, Bree Hunter, Ben Horne, Gareth Rapley, Alison Botha, Sandra Anderson, Luis Ortiz-Catedral, Vivian Ward and many others for their help on Raoul, the Meyers, and aboard the Tangaroa for catching and handling birds and in the course of this study. Thanks to Craig Millar and Selina Patel for critical discussions and support in laboratory procedures, and to Don Dearborn, Matt Rayner, and anonymous reviewers for helpful comments.
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