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

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