A GEOGRAPHIC-INFORMATION-SYSTEM APPROACH TO ESTIMATING THE ORIGIN OF MIGRATORY RAPTORS IN NORTH AMERICA USING STABLE HYDROGEN ISOTOPE RATIOS IN FEATHERS

Analysis of stable hydrogen isotope ratios in feathers (δDf) is a promising method for investigating population connectivity in migratory birds. Stable hydrogen isotope ratios in precipitation (δDp) vary across North America with respect to latitude, elevation, and seasonal air-mass trajectories. A strong relationship between δDf and δDp at locations of feather growth has been documented for several bird species. Some studies have used measurements of δDf to plot the origins of migrants on maps of long-term weighted-average, growing-season North American δDp (hereafter “δDp maps”) using the observed relationship between δDf and δDp from a reference sample of known-origin birds. The accuracy of this method depends on the strength of the δDf and δDp relationship and accuracy of the δDp maps. Recently, a high-resolution (1-km2) model of North American δDp was published (Meehan et al. 2004) that accounts for the effect of elevation on δDp where previous models did not. We compared δDf measurements from a geographically diverse sample of 264 raptor feathers with δDp estimates for feather-sample locations. We documented a strong relationship between raptor δDf and δDp across North America. However, we also documented substantial regional variation in this relationship. We created a “base map” of North American raptor δDf that incorporated the regional variation described by our sample. We plotted δDf values from migrant Sharp-shinned Hawks (Accipiter striatus) captured in eastern Nevada directly on this map to demonstrate how it can be used to view the origins of migrants. Un Procedimiento Basado en Sistemas de Información Geográfica para Estimar el Origen de las Aves Rapaces Migratorias en Norte América Usando los Cocientes de Isótopos Estables de Hidrógeno Presentes en las Plumas