Flight altitudes and the use of uplifts by adult lesser spotted eagles, Clanga pomarina, during postbreeding migration

We analysed 13 postbreeding migration tracks from five adult lesser spotted eagles using high-frequency GPS data. We focused on flight altitude and the use of thermals and orographic uplifts across diverse terrains from northern Germany to Lake Tanganyika. Climbing phases were associated with lower, and gliding phases with higher, ground speeds. The eagles exhibited obligate diurnal migration, with altitudes and speeds peaking around noon, consistent with solar radiation patterns. Mean flight altitudes averaged 655 m above ground, although altitudes up to 4224 m above sea level were recorded, where higher ground speeds suggested compensatory adjustments to thin air. We found that flat landscapes led to higher flight altitudes, shorter daily migration distances and more directional changes, indicative of thermal soaring. Conversely, orographic uplifts were utilized over slopes, where the eagles flew at lower altitudes and covered longer daily distances. However, our models revealed a significant three-way interaction between terrain slope and latitude (including the quadratic term), showing that this ‘topographical acceleration’ was most effective at high and low latitudes, where steeper slopes resulted in the greatest daily distances and directional continuity. The eagles traversed collision risk zones, particularly with modern wind turbines, at least twice daily. This risk varied with geographical region, as they flew significantly closer to the ground at the northern and southern edges of their migratory route. Low-altitude flights over slopes increased vulnerability to collisions, predominantly in the first and last hours of a migration day and during irregular flight days, where 42% of the fixes were below 250 m. We provide crucial insights into the complex flight strategies of an obligate soaring raptor and its interactions with diverse landscapes. This knowledge is important for developing effective conservation measures along the vast migratory flyways of lesser spotted eagles and other soaring raptors.