Golden eagles regularly use gravity waves to soar : new insights from high-resolution weather data

Abstract

Soaring flight allows birds to reduce the metabolic cost of flight by harnessing energy from the atmosphere. The study of soaring behaviour has been significantly constrained by the low resolution of available atmospheric data, limiting our ability to accurately describe the conditions enabling soaring and its adaptability to different updraught types. For instance, while the use of thermals and orographic lifting updraughts are well described in the literature, the role of gravity waves has remained largely unexplored. Advancements in high-resolution atmospheric modelling, with hourly output available at the kilometre-scale grid spacing, offer new opportunities to investigate the flexibility of soaring flight in response to complex atmospheric dynamics, including gravity waves. In this study, we used a combination of a high-resolution atmospheric analysis and high-resolution global positioning system tracking data to characterize the updraught sources used by golden eagles, Aquila chrysaetos, in the European Alps. Our findings reveal that golden eagles repeatedly used gravity waves, with at least 19% of the inspected soaring segments involving this updraught source. Thermals remained the primary energy source for soaring, but during winter, when thermals were more scarce, the quasi-totality of soaring events were powered by gravity waves or orographic lifting. Our results provide a new perspective on the environmental energy available to soaring birds and on landscape connectivity in topographically complex regions.