Stable isotopes reveal regional heterogeneity in the pre-breeding distribution and diets of sympatrically breeding Pygoscelis spp. penguins

While the foraging ecology of the Adelie penguin Pygoscelis adeliae and gentoo penguin P. papua has been well studied, little is known on the distribution and diet of these species outside the breeding season. In the present study we used stable carbon (delta C-13) and nitrogen (delta N-15) isotope analyses of eggshells to examine the pre-breeding diets and foraging habitats of female Adelie and gentoo penguins from 23 breeding colonies along the eastern and western Antarctic Peninsula (AP), South Shetland Islands, and South Orkney Islands, in 2006. Adelie penguin eggshells from the eastern AP, South Shetland Islands, and South Orkney Islands shared similar isotopic signatures and were significantly lower in both delta C-13 and delta N-15 values than eggshells from birds breeding along the western AP. This result suggests that Adelie penguin populations that are geographically separated during the breeding season by the Adelie 'gap,' a 400 km region in the western AP devoid of breeding Adelie penguins, also inhabit geographically distinct habitats during the late winter prior to the breeding season. To a lesser degree, gentoo penguin eggshell isotope values also varied across breeding colonies, which likely reflects local scale variation in their near-shore foraging grounds. Furthermore, unlike the breeding period when krill (primarily Euphausia superba) dominates penguin diets in these regions, our findings suggest that fish and/or other high trophic-level prey species comprise a significant portion (46.8 to 62.9%) of female Adelie and gentoo penguin diets prior to breeding.