Stable isotopes reveal individual variation in migration strategies and habitat preferences in a suite of seabirds during the nonbreeding period
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Information on predator and prey distributions is integral to our understanding of migratory connectivity, food web dynamics and ecosystem structure. In marine systems, although large animals that return to land can be fitted with tracking devices, minimum instrument sizes preclude deployments on small seabirds that may nevertheless be highly abundant and hence major consumers. An increasingly popular approach is to use N and C stable isotope analysis of feathers sampled at colonies to provide information on distribution and trophic level for the preceding, and generally little-known, nonbreeding period. Despite the burgeoning of this research, there have been few attempts to verify such relationships. In this study, we demonstrate a clear correspondence between isotope ratios of feathers and nonbreeding distributions of seven species from South Georgia tracked using loggers. This generated a rudimentary isoscape that was used to infer the habitat preferences of eight other species ranging in size from storm petrels to albatrosses, and which could be applied, with caveats, in other studies. Differences in inferred distribution within and between species had major implications for relative exposure to anthropogenic threats, including climate change and fisheries. Although there were no differences in isotope values between sexes in any of the smaller petrels, mean stable C (δ13C), but not stable N isotope ratios (δ15N), tended to be greater in females than males of the larger, and more sexually size-dimorphic species. This indicates a difference in C source (distribution), rather than trophic level, and a correspondence between the degree of sexual size dimorphism in Procellariiformes and the level of between-sex niche segregation.
KeywordsGeolocation Isoscape Seabird Sexual segregation Tracking
We are very grateful to the many field assistants, particularly Dafydd Roberts, Ben Phalan, Isaac Forster and Robin Snape for help with device deployments and sampling, and to Vsevolod Afanasyev, John Croxall, Jim Fox, Dirk Briggs and Janet Silk for their contribution to the geolocation studies. Robert Buckland and Douglas Ross performed the molecular sex-typing (http://www.shef.ac.uk/mgf-s/birdsexing.html) at the Molecular Genetics Facility (Sheffield), supported by the Natural Environment Research Council. Thanks are also due to Catarina Henriques for sample preparation, and to Raül Ramos and Scott Shaffer for useful comments on an earlier draft of the manuscript. The isotope analysis at the Life Sciences Mass Spectrometry Facility was funded by NERC award EK50-5/02. The study complied with the current laws of the Government of South Georgia and the South Sandwich Islands.
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