| Summary: | The use of stable isotopes of sulfur (δ 34 S) to infer avian diets, foraging habitats, and movements is relatively uncommon, resulting in a lack of information on patterns of δ 34 S incorporation in avian tissue. In a controlled study of Gentoo Penguins ( Pygoscelis papua ), we found that diet-tissue isotopic discrimination factors (Δ 34 S diet-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 Δ 34 S diet-tissue values. Specifically, Δ 34 S diet-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, δ 34 S values. Δ 34 S diet-tissue values obtained for Gentoo Penguins fed a marine fish diet were generally lower than those reported for freshwater-fish-consuming Double-crested Cormorants ( Phalacrocorax auritus ), the only other bird species in which Δ 34 S diet-tissue has been quantified. We found support for the hypothesis that tissue Δ 34 S diet-tissue values are inversely related to dietary δ 34 S 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 Δ 34 S diet-tissue in avian tissues and guidance to allow for greater application of sulfur stable isotope analysis in ornithological research. Funding provided by: Scientific Committee on Antarctic Research Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100000706 Award Number:
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