Geographic distribution of feather δ34S in Europe

Geographic distribution models of environmental stable isotopes (so called ‘isoscapes’) are widely employed in animal ecology, wildlife forensics and conservation. However, the application of isoscapes is limited to elements and regions for which the spatial patterns have been estimated. Here, we focused on the ubiquitous yet less commonly used stable sulfur isotopes ( δ 34S). To predict the European δ 34S isoscape, we used 242 feather samples from Eurasian Reed Warbler ( Acrocephalus scirpaceus ) formed at 69 European wetland sites. We quantified the relationships between sample δ 34S and environmental covariates using a random forest regression model and applied the model to predict the geographic distribution of δ 34S. We also quantified within-site variation in δ 34S and complementarity with other isotopes on both individual and isoscape levels. The predicted feather δ 34S isoscape shows only slight differences between the central and southern parts of Europe while the coastal regions were most enriched in 34S. The most important covariates of δ 34S were distance to coastline, surface elevation and atmospheric concentrations of SO2 gases. The absence of a systematic spatial pattern impedes the application of the δ 34S isoscape but high complementarity with other isoscapes advocates the combination of multiple isoscapes to increase precision of animal tracing. Feather δ 34S compositions showed considerable within-site variation with highest values in inland parts of Europe, likely attributed to wetland anaerobic conditions and redox sensitivity of sulfur. The complex European geography and topography as well as using δ 34S samples from wetlands may contribute to the absence of a systematic spatial gradient of δ 34S values in Europe. We thus encourage future studies to focus on the geographic distribution of δ 34S using tissues from diverse taxa collected in various habitats over large land masses in the world (i.e., Africa, South America or East Asia). Open Research statement Sample metadata are stored and available for reuse in the AviSample Network database (: AS00001–AS00117; [Brlík et al. 2022b][1]). The information on the isotopic composition and geographic origin of feather samples used to produce the δ 34S isoscape, predicted feather δ 34S, δ 2H and δ 13C isoscapes are publicly available from the Zenodo data repository at ([Brlík et al. 2022c][2]). ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-12 [2]: #ref-13