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 optimal soaring and gliding flight of large, diurnal migrants using Pennycuick 's program 2 for ''bird flight performance.'' Predictions were compare d with 141 observed flight paths of migrating steppe buzzards, Buteo b uteo vulpinus. Calculations of cross-country speed relative to the air included bird's airspeeds and sinking rates in interthermal gliding a nd climbing rates in thermal circling. Steppe buzzards adjusted intert hermal gliding airspeed according to their actual climbing rate in the rmal circling. By optimizing their gliding airspeed, the birds maximiz ed their crosscountry performance relative to the air. Despite this ge neral agreement with the model, there was much scatter in the data, fo r the model neglects horizontal winds and updrafts during the gliding phase. Lower sinking rates due to updrafts during the gliding phases a llowed many birds to achieve higher cross-country speeds than predicte d. 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. Nevertheless, cross-country speed relat ive to the ground, which is the important measure for a migratory bird , was still higher under following winds. This study shows that Pennyc uick's program 2 provides reliable predictions on optimal soaring and gliding behavior using realistic assumptions and constants in the mode l, but a great deal of variation around the mean is generated by facto rs not included in the model.