Soaring Migration of Steppe Eagles Aquila nipalensis in Southern Israel: Flight behaviour under Various Wind and Thermal Conditions

Abstract

Raptor migration was studied at two locations in southern Israel in spring 1992. Tracking radars provided exact data on individual flight paths throughout the day, whereby emphasis was on recording entire 'inter-thermal gliding' and 'thermal circling' phases. Steppe Eagles Aquila nipalensis adjusted their flight tactics to the actual wind and thermal conditions: They increased their gliding airspeed under strong thermal convection and in opposing winds. Under certain conditions the usual circling in thermals and inter-thermal gliding tactic was combined with soaring in straight line gliding, suggesting large scale updrafts. Cross-country speeds were considerably enhanced if birds used this combined flight tactic, since it provides gain of altitude as well as progress in the direction of migration. Cross-country speed was positively related to climbing rate in thermal circling. Over the whole diurnal cycle, the mean climbing rate in thermal circling was 1.9 m/s. The highest climbing rates, occasionally up to 5 m/s, were reached around noon. Mean cross-country speed was 12.4 m/s, mean gliding airspeed 15.6 m/s. The upper limit of migration was at about 1600 m above ground level, but 90% of the Steppe Eagles flew below 1000 m above ground, 50% even below 400 m. A comparison with Steppe Buzzard Buteo buteo vulpinus and Honey Buzzard Pernis apivorus, observed under the same conditions, revealed similar flight characteristics. However, Steppe Eagles reached higher crosscountry speeds, reflecting a higher selectivity for optimal flight conditions combined with the use of updrafts while soaring in straight line gliding.