MEPS 296:107-113 (2005)  -  doi:10.3354/meps296107

Stable isotopes reveal trophic segregation by sex and age in the southern giant petrel in two different food webs

Manuela G. Forero1,*, Jacob González-Solís2,3, Keith A. Hobson4,5, José A. Donázar1,Marcelo Bertellotti6, Guillermo Blanco7, Gary R. Bortolotti5

1Department of Applied Biology, Estación Biológica de Doñana, Avenida María Luisa s/n, Pabellón del Perú, 41013 Sevilla, Mallorca, Spain
2Departament de Biología Animal (Vertebrats), Universitat de Barcelona, Avenida Diagonal 645, 08028 Barcelona, Spain
3British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK
4Canadian Wildlife Service, 115 Perimeter Road, Saskatoon, Saskatchewan S7N 0X4, Canada
5Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon SK S7N 5E2, Canada
6Centro Nacional Patagónico (CONICET), Brown 3500, U9120ACV Puerto Madryn, Chubut, Argentina
7Instituto de Investigación en Recursos Cinegéticos (CSIC-UCLM), Ronda de Toledo, s/n, 13005 Ciudad Real, Spain

ABSTRACT: We investigated trophic ecology variation among colonies as well as sex- and age-related differences in the diet of the southern giant petrel Macronectes giganteus, a long-lived seabird that is sexually dimorphic in size. We measured stable isotopes (δ13C, δ15N) in blood samples collected during breeding at Bird Island (South Georgia, Antarctica) in 1998 and at 2 colonies in the Argentinean area of Patagonia in 2000 and 2001. Individuals from South Georgia showed lower δ13C and δ15N values than those in Patagonia, as expected from the more pelagic location and the short length of the Antarctic food web. Males and females showed significant differences in the isotopic signatures at both localities. These differences agree with the sexual differences in diet found in previous studies, which showed that both sexes rely mainly on penguin and seal carrion, but females also feed extensively on marine prey, such as fish, squid and crustaceans. However, males from Patagonia showed significantly higher δ15N and δ13C values than females did, and the reverse trend was observed at South Georgia. This opposite trend is probably related to the different trophic level of carrion between locations: whereas penguins and pinnipeds in Patagonia rely mainly on fish and cephalopods, in South Georgia they rely mainly on krill. Stable isotope values of male and female chicks in Patagonia did not differ; both attained high values, similar to adult males and higher than adult females, suggesting that parents do not provision their single offspring differently in relation to sex; however, they seem to provide offspring with a higher proportion of carrion, probably of higher quality, and more abundant food, than they consume themselves. Stable isotopes at South Georgia were not affected by age of adults. We have provided new information on intraspecific segregation in the diet in a seabird species and have also underlined the importance of considering food web structure when studying intraspecific variability in trophic ecology.

KEY WORDS: Carbon · Nitrogen · Diet · Interspecific variability · Sexual segregation · Macronectes giganteus

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