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

Lay Summary center dot Stable isotope analysis of sulfur can provide new discoveries on birds' diets and migration patterns if we gain a better understanding of how isotope values in bird tissues are related to those in their diets. center dot We measured the difference in sulfur stable isotope values between Gentoo Penguins diets and their eggs and feathers in a controlled study. center dot We found that the offset in sulfur isotope values from diets differed among tissues and is influenced by the choices researchers make when selecting and analyzing their samples. center dot The difference in sulfur stable isotope values between diets and bird tissues is related to the isotope values of the diet itself, which can help researchers studying other birds in the absence of similar controlled studies. center dot We provide recommendations for future research and guidance to promote the increased use of sulfur stable isotope analysis in bird studies. The use of stable isotopes of sulfur (delta S-34) to infer avian diets, foraging habitats, and movements is relatively uncommon, resulting in a lack of information on patterns of delta S-34 incorporation in avian tissue. In a controlled study of Gentoo Penguins (Pygoscelis papua), we found that diet-tissue isotopic discrimination factors (Delta S-34(diet-tissue)) differed among egg components and feathers synthesized from a common diet, ranging from -0.4 to -1.7 parts per thousand. We also found that methodical choices such as lipid extraction and prey tissue selection influenced calculated Delta S-34(diet-tissue) values. Specifically, Delta S-34(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, delta S-34 values. The Delta S-34(diet-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 Delta S-34(diet-tissue) has been quantified. We found support for the hypothesis that tissue Delta S-34(diet-tissue) values are inversely related to dietary delta S-34 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 Delta S-34(diet-tissue) in avian tissues and guidance to allow for greater application of sulfur stable isotope analysis in ornithological research.