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

This article presents tests of the theoretical predictions on optimalnsoaring and gliding flight of large, diurnal migrants using Pennycuick’snprogram 2 for “bird flight performance.” Predictions were comparednwith 141 observed flight paths of migrating steppe buzzards, Buteonbuteo vulpinus. Calculations of cross-country speed relative to thenair included bird’s airspeeds and sinking rates in interthermal glidingnand climbing rates in thermal circling. Steppe buzzards adjustedninterthermal gliding airspeed according to their actual climbingnrate in thermal circling. By optimizing their gliding airspeed, thenbirds maximized their cross-country performance relative to the air.nDespite this general agreement with the model, there was much scatternin the data, for the model neglects horizontal winds and updraftsnduring the gliding phase. Lower sinking rates due to updrafts duringnthe gliding phases allowed many birds to achieve higher cross-countrynspeeds than predicted. In addition, birds reacted to different windndirections and speeds: in side and opposing winds, the steppe buzzardsncompensated for wind displacement during soaring and increased theirngliding airspeed with decreasing tailwind component. Nevertheless,ncross-country speed relative to the ground, which is the importantnmeasure for a migratory bird, was still higher under following winds.nThis study shows that Pennycuick’s program 2 provides reliable predictionsnon optimal soaring and gliding behavior using realistic assumptionsnand constants in the model, but a great deal of variation aroundnthe mean is generated by factors not included in the model