Estimating the latitudinal origins of migratory birds using hydrogen and sulfur stable isotopes in feathers: influence of marine prey base
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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 (δDp) and the inheritance of this variation in newly formed feathers (δDf). We hypothesized that the typically strong relationship between δDp and δDf 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 δDp and δDf. 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 δDf values for marine-foraging birds could potentially distort estimates of origins.