Estimating the Latitudinal Origins of Migratory Birds Using Hydrogen and Sulfur Stable Isotopes in Feathers: Influence of Marine Prey Base

Hydrogen stable isotope analysis of feathers is an important tool for estimating the natal or breeding latitudes of nearctic-neotropical migratory birds. This method is based on the latitudinal variation of hydrogen stable isotope ratios in precipitation in North America (δD_p) and the inheritance of this variation in newly formed feathers (δD_f). We hypothesized that the typically strong relationship between δD_p and δD_f would be decoupled in birds that forage in marine food webs because marine waters have relatively high δD values compared to δD values for local precipitation. Birds that forage on marine prey bases should also have feathers with high δ³⁴S values, since δ³⁴S values for marine sulfate are generally higher than δ³⁴S values in terrestrial systems. To examine this potential marine effect on feather stable isotope ratios, we measured δD and δ³⁴S in the feathers of nine different species of raptors from both inland and coastal locations across North America. Feathers from coastal bird-eating raptors had consistently higher δD and δ³⁴S values than feathers from inland birds. Birds that had high δ³⁴S values also deviated strongly from the typical relationship between δD_p and δD_f. We recommend measuring both sulfur and hydrogen stable isotope ratios in feathers when some members of a migrant population could potentially forage in marine habitats. We suggest using a practical cutoff of δ³⁴S >10‰ to remove marine-foraging birds from a migrant sample when using stable isotopes of hydrogen to estimate the latitudinal origins of migrants because high δD_f values for marine-foraging birds could potentially distort estimates of origins.