Assessing the movements and diets of golden eagles in eastern North America using stable isotope and telemetry data

Background/Question/Methods

Golden eagles (Aquila chrysaetos) are apex predators and thought to have a large effect on the population dynamics of their prey, with follow-on effects on ecosystem-level processes. In eastern North America there is a small population of golden eagles (ca. 1,000-3,000 individuals) whose basic ecology, including migratory pathways, linkages between breeding and wintering areas, and food sources, remains largely unknown. To provide an initial assessment of the trophic relationships and summering locations of eastern golden eagles, we measured δ¹³C, δ¹⁵N, and δD of multiple (3-5) samples taken along the length of single body feathers from ~42 individuals captured during winter in the eastern United States between 2006 and 2012. At capture the majority of these birds were telemetered, providing a unique opportunity to evaluate the ability of feather δD data to record the geographic region-of-origin of golden eagles and evaluate linkages between diet and geographic locality.

Results/Conclusions

Intra-feather variation in isotope values was relatively small (average standard deviation of 0.5‰ for δ¹³C, 0.6‰ for δ¹⁵N, and 9.4‰ for δD), suggesting that birds did not typically shift diets or move large distances during feather formation. Average values from individual feathers range between -23.6 and -17.3‰ for δ¹³C, 5.3 and 13.2‰ for δ¹⁵N, and -120 and -21.8‰ for δD. The range of δ¹³C and δ¹⁵N values, along with their positive correlation, suggests that golden eagles have diverse diets of both marine and terrestrial prey. However, 35 of the eagles have δ¹³C values <-20‰, suggesting that most birds have primarily terrestrial diets with C₃ plants at the base of the food chain. For birds with primarily terrestrial diets, δD values suggest a summering distribution in northern Quebec and Newfoundland and Labrador, which is qualitatively consistent with bird summer locations as recorded by telemetry. These results suggest that (1) δD accurately records the summering locations of golden eagles in northern high latitudes and (2) eastern golden eagles occupy the apex of regional food chains and the species has flexible summer-time foraging strategies with greater dietary contributions from terrestrial (e.g. hares, caribou/moose carrion) than marine (e.g. waterfowl, fish) prey.