Combining stable isotopes, trace elements, and distribution models to assess the geographic origins of migratory bats

The expansion of industrial-scale wind-energy facilities has not only increased the production of low-carbon emission energy but has also resulted in mortality of wildlife, including migratory bats. Management decisions can be limited by a lack of understanding of the geographic impact of bats killed at wind-energy facilities. Several studies have leveraged stable hydrogen isotope ratios (delta H-2) of bat fur to illuminate this issue but are limited in the precision of conclusion because delta H-2 values vary primarily across latitudinal and elevational bands. One approach to increase the precision of geographic assignment is to combine independent inferences about spatial location from additional biomarkers and other related information. To test this possibility, we assigned known-origin individuals of three bat species (Lasiurus borealis, L. cinereus, and Lasionycteris noctivagans) commonly killed at on-shore wind-energy facilities in North America to a probable origin using delta H-2 values, trace element concentrations, and species distribution models. We used cross-validated calibrated combined model tuning to determine the degree to which assignment probabilities improved when combining datasets. We found that combining markers typically performed better than single approaches. For Lasiurus borealis and L. cinereus, combining all three data sources outperformed any single or other combined approach. With an accuracy set at 80%, an average of 39.7% and 36.0% of each species' total geographic range was considered a potential origin, respectively; stable hydrogen alone included 51.8% and 50.6% of the total geographic area. In contrast, for Lasionycteris noctivagans, including trace elements did not increase precision and adding distribution data to delta H-2 values only improved precision by 0.6%. Thus, we found that a combination of multiple biomarkers typically, but not always, outperforms single-marker approaches and optimized combinations of different markers outperform equal weighting of each marker. From a practical perspective, delta H-2 values performed better than trace elements alone; in cases where cost is a limiting factor, the stable hydrogen should be the single biomarker used in conjunction with species distribution models. Overall, these results highlight the importance of validating methods for each species they are applied to and show that combining information from intrinsic biomarker approaches is a useful tool to document bat movements.