MEPS 311:157-164 (2006)  -  doi:10.3354/meps311157

Stable isotopes indicate sex-specific and long-term individual foraging specialisation in diving seabirds

Stuart Bearhop1,*, Richard A. Phillips2, Rona McGill3, Yves Cherel4, Deborah A. Dawson5, John P. Croxall2

1Quercus, School of Biology & Biochemsistry, MBC, Queen’s University of Belfast, Lisburn Road, Belfast BT9 7BL, UK
2British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK
3Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, East Kilbride G75 0QF, UK
4CEBC-CNRS, BP 14, 79360 Villiers-en-Bois, France
5Sheffield Molecular Genetics Facility, Department of Animal & Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, UK

ABSTRACT: An important aspect of foraging ecology is the extent to which different individuals or genders within a population exploit food resources in a different manner. For diving seabirds, much of this information relates either to short-term dietary data or indirect measures such as time budgets. Moreover, dietary specialisation can be difficult to detect due to biases associated with conventional sampling techniques. We used stable isotope ratios in blood and feathers to infer trophic and habitat specialisations among 4 diving seabird taxa—the gentoo penguin Pygoscelis papua, the macaroni penguin Eudyptes chrysolophus, the South Georgian shag Phalacrocorax (atriceps) georgianus and the Kerguelen shag P. (atriceps) verrucosus. This allowed us to investigate foraging specialisation and assess whether social dominance or differences in foraging preferences explained the observed patterns. In all taxa where sexes were known we found that males foraged at a higher trophic level (δ15N values) than females, although this was not significant in macaroni penguins. We believe that this is linked to a dual foraging strategy among female macaroni penguins. For South Georgian shags, we found that sex-related dietary differences persisted for long periods (inferred from stable isotope analyses of feathers and blood). We suggest that the trophic differences are driven by differences in physiological performance, with males tending to dive deeper than females because of their larger size, and hence able to access higher trophic level prey items. Moreover, male and female shags tend to forage at different times of day; therefore, social dominance by males is unlikely to be driving the observed differences. We also recorded highly significant relationships between stable isotope signatures in blood (representing the breeding season diet) and those in feathers (mostly representing the previous non-breeding season diet) in both the South Georgian and Kerguelen shags. This strongly suggests that these 2 taxa include individuals with distinct foraging specialisation (and most probably foraging locations) that are maintained over long periods.

KEY WORDS: Penguin · Shag · Trophic level · δ13C · δ15N

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