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This article presents tests of the theoretical predictions on optimal soaring and gliding flight of large, diurnal migrant* using Pennycuick's program 2 for "bird flight performance." Predictions were compared with 141 observed flight paths of migrating steppe buzzards, Buteo butto vulpinus. Calculations of cross-country speed relative to the air included bird's airspeeds and sinking rates in interthennal gliding and climbing rates in thermal circling. Steppe buzzards adjusted interthermal gliding airspeed according to their actual climbing rate in thermal circling. By optimizing their gliding airspeed, the birds maximized their crosscountry performance relative to the air. Despite this general agreement with the model, there was much scatter in the data, for the model neglects horizontal winds and updrafts during the gliding phase. Lower sinking rates due to updrafts during the gliding phases allowed many birds to achieve higher cross-country speeds than predicted. In addition, birds reacted to different wind directions and speeds: in side and opposing winds, the steppe buzzards compensated for wind displacement during soaring and increased their gliding airspeed with decreasing tailwind component NeveKheless, cross-country speed relative to the ground, which is the important measure for a migratory bird, was still higher under following winds. This study shows that Pennycuick's program 2 provides reliable predictions on optimal soaring and gliding behavior using realistic assumptions and constants in the model, but a great deal of variation around the mean is generated by factors not included in the model.

Optimal flight behavior of soaring migrants: a case study of migrating steppe buzzards, Buteo buteo vulpinus