Sulfur isotopic discrimination factors differ among avian tissues and diets: Insights from a case study in Gentoo Penguins

Abstract

Abstract The use of stable isotopes of sulfur (δ34S) to infer avian diets, foraging habitats, and movements is relatively uncommon, resulting in a lack of information on patterns of δ34S incorporation in avian tissue. In a controlled study of Gentoo Penguins (Pygoscelis papua), we found that diet-tissue isotopic discrimination factors (Δ34Sdiet-tissue) differed among egg components and feathers synthesized from a common diet, ranging from –0.4 to –1.7‰. We also found that methodical choices such as lipid extraction and prey tissue selection influenced calculated Δ34Sdiet-tissue values. Specifically, Δ34Sdiet-tissue values were lower (i.e., more negative) when calculated using whole fish relative to fish muscle and lipid-extraction biased egg yolk, but not fish tissue, δ34S values. The Δ34Sdiet-tissue values obtained for Gentoo Penguins fed a marine fish diet were generally lower than those reported for freshwater fish consumption by Double-crested Cormorants (Phalacrocorax auritus), the only other bird species in which Δ34Sdiet-tissue has been quantified. We found support for the hypothesis that tissue Δ34Sdiet-tissue values are inversely related to dietary δ34S values in birds, similar to what has been observed in mammals. Given this relationship, the discrimination factors reported here for Gentoo Penguins may be broadly applicable to other avian species with a similar marine diet. Finally, we provide recommendations for future studies seeking to quantify Δ34Sdiet-tissue in avian tissues and guidance to allow for greater application of sulfur stable isotope analysis in ornithological research.