Stable hydrogen isotopes identify leapfrog migration, degree of connectivity, and summer distribution of Golden Eagles in eastern North America

Knowledge of the distribution and movements of populations of migratory birds is useful for the effective conservation and management of biodiversity. However, such information is often unavailable because of the difficulty of tracking sufficient numbers of individuals. We used more easily obtained feather stable hydrogen isotope ratios (delta H-2) to predict the summer grounds of the small, threatened, and migratory population of Golden Eagles (Aquila chrysaetos) in eastern North America. We then identified summer locations and the extent of migratory connectivity for this population. We collected delta H-2 (delta H-2(f)), stable carbon isotope (delta C-13), and stable nitrogen isotope (delta N-15) data from the body feathers of 47 juvenile, subadult, and adult Golden Eagles. Values of delta C-13 and delta N-15 suggested that all but 2 birds obtained food from terrestrial-based food webs and therefore that delta H-2 data were appropriate for inferring the geographic region of molt for the majority of birds. There was relatively large interfeather variation in the delta H-2 values of subadults vs. adults, suggesting that these groups molted at different times and places. The most negative delta H-2(f) values from birds with known summering grounds exhibited (1) a negative correlation with their summering latitude, and (2) a positive correlation with amount-weighted delta H-2 values of May-August precipitation at the summer location. These data validate the use of delta H-2(f) values for inferring the summer locations of Golden Eagles of unknown origin. Likelihood-of-origin maps derived from delta H-2(f) values revealed that (1) the majority of birds spent the breeding season in central Quebec and Labrador, and (2) birds that wintered at southern latitudes, from approximately northern Alabama to southwestern Virginia, migrated about twice the distance of birds that wintered at northern latitudes, from Pennsylvania to New York. We observed a positive relationship between delta H-2(f) values and the latitude of the wintering location, which, along with the likelihood-of-origin maps, revealed moderate patterns of leapfrog migration and migratory connectivity.