Radar tracking reveals influence of crosswinds and topography on migratory behavior of European honey buzzards

Variables affecting the choice of migrating European honey buzzards to fly over a flat highland or to outflank it by flying over steep slopes and deep valleys were tested. Radar located at a Mediterranean bottleneck enabled collection of raptor tracks. Variables included the effects of weather, flock size, time of day, and ground speed of buzzards. It was found that raptors passed over the highland when wind speed (mostly lateral) increased as a result of partial drift, whereas buzzards tended to outflank the highland during midday and afternoon, probably when stronger thermals generated on the valley slopes were available. These results suggest that, in this case study, European honey buzzards modulate the compensation of wind drift at different times of the day, showing a highly plastic behavior during migration.