Soaring migration of Steppe Eagles Aquila nipalensis in southern Israel: flight behaviour under various wind and thermal conditions

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 strong thermal convection and in opposing winds. Under certain conditions the usual circling in thermals and inter-thermal straight line gliding, suggesting large scale updrafts. Cross-country speeds were considerably gliding tactic was combined with soaring in 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. Cross-country enhanced if birds used this combined flight tactic, since it provides gainof altitude as well as progress in the direction of migration. speed was ground, 50% even below 400 m. A comparison with Steppe Buzzard Buteo buteo vulpinus revealed similar flight characteristics. and Honey Buzzard Pernis apivorus, observed under the same conditions, However, Steppe Eagles reached higher cross- 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 speed was country speeds, reflecting a higher selectivity for optimal flight conditions combined with the use of updrafts while soaring in straight line gliding.