Sea-crossing behaviour of two migratory eagles in relation to weather conditions

During migration, typical soaring species such as eagles and vultures primarily rely on soaring flight to minimise energy expenditure, exploiting thermal currents over land while avoiding prolonged water crossings. This study investigates the flight behaviour of the Booted Eagle (Hieraaetus pennatus) and the Short-toed Snake Eagle (Circaetus gallicus) as they approach the European coastline of the Strait of Gibraltar, a critical migration bottleneck spanning at least 14 km, during their pre-breeding migration. Visual observations were conducted from two watchpoints on the European side of the Strait to assess migration magnitude and record the birds’ flight altitudes upon reaching the coastline. Both species experienced wind drift when flying in crosswinds. Under calm conditions or light easterly winds, they tended to converge at the centre of the Strait, whereas westerly winds caused a shift in their migratory flow towards the eastern sector. Short-toed Snake Eagles were observed flying at lower altitudes when facing headwinds or strong crosswinds, increasing their risk of drowning. In contrast, Booted Eagles were less affected, likely due to their lower energetic cost of flapping flight, which helped mitigate drowning risks. Notably, crosswind intensity did not significantly influence the flight altitude of Booted Eagles. These findings highlight interspecific differences in response to challenging wind conditions during water crossings and provide insight into the flight strategies adopted by raptors in the Strait of Gibraltar.