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

1Soaring flight developed as a result of behavioural and morphological adaptations that allow birds to reduce the metabolic cost of flight by harnessing the energy available in the atmosphere. Despite an increased attention given in the last decades to the physics and ecology that allow soaring flight, its study has been limited by the generally low spatio-temporal resolution of available atmospheric data. This constrained our ability to quantify the atmospheric conditions that allow soaring, and limited our understanding of its flexibility in different uplift conditions. While the use of updraughts such as thermals and orographic lifting are well described in the literature (albeit only quantified through atmospheric proxies), the use of others, such as gravity waves, was hypothesised but largely undocumented. Recent advancements in high-resolution atmospheric modelling, with hourly output available at the kilometer-scale grid spacing, offer new opportunities to investigate the flexibility of soaring flight in response to complex atmospheric dynamics. In this study, we used a combination of a high-resolution atmospheric analysis and high-resolution GPS tracking data to characterise the updraught sources used by golden eagles, Aquila chrysaetos, in the European Alps. We document that golden eagles in this region repeatedly use gravity waves, and that while thermals were still the main updraught source used for soaring, gravity waves were involved in at least 19% of the inspected soaring segments. In winter, when thermals were more scarce, the quasi-totality of soaring events were powered by gravity waves or orographic lifting, largely expanding the environmental energy available to soaring birds and therefore the landscape connectivity in topographically complex regions. Our results also emphasise the difficulty to distinguish between convective (thermals) and dynamic updraught sources, as these co-occur within the boundary layer over complex terrain.