Isotopic discrimination between food and blood and feathers of captive penguins: Implications for dietary studies

Using measurements of naturally occurring stable isotopes to reconstruct diets or source of feeding requires quantifying iso-topic discrimination factors or the relationships between iso-tope ratios in food and in consumer tissues. Diet-tissue dis-crimination factors of carbon (13C/12C, or d13C) and nitrogen (15N/14N, or d15N) isotopes in whole blood and feathers, rep-resenting noninvasive sampling techniques, were examined us-ing three species of captive penguins (king Aptenodytes pata-gonicus, gentoo Pygoscelis papua, and rockhopper Eudyptes chrysocome penguins) fed known diets. King and rockhopper penguins raised on a constant diet of herring and capelin, re-spectively, had tissues enriched in 15N compared to fish, with discrimination factors being higher in feathers than in blood. These data, together with previous works, allowed us to cal-culate average discrimination factors for 15N between whole lipid-free prey and blood and feathers of piscivorous birds; they amount to 2.7 ‰ and 4.2‰, respectively. Both fish species were segregated by their d13C and d15N values, and importantly, lipid-free fish muscle tissue was consistently depleted in 13C and enriched in 15N compared to whole lipid-free fish. This finding has important implications because previous studies usually base dietary reconstructions on muscle of prey rather than on whole prey items consumed by the predator. We teste