Migration by Flapping or Soaring: Flight Strategies of Marsh, Montagu's and Pallid Harriers in Southern Israel

Abstract

Migratory flights of Marsh Harriers (Circus aeruginosus), Montagu's Harriers (Circus pygargus) and Pallid Harriers (Circus macrourus) in southern Israel were used to test flight theory predictions. The body sizes of these closely related species are between those of the typical large soaring migrants, such as eagles and storks, and the typical flapping migrants, such as small falcons and sparrowhawks. In soaring-gliding flight, Marsh Harriers reacted to different thermal conditions by adjusting their gliding airspeed to the actual climbing rate in thermal circling; consequently, cross-country speed was related to climbing rate. In contrast, the smaller Montagu's and Pallid Harriers did not adopt gliding airspeeds according to thermal conditions. All harrier species regularly used flapping-gliding flight, predominately soon after sunrise and before sunset, and more often in opposing winds than in following winds. Montagu's/Pallid Harriers used flapping-gliding more frequently than Marsh Harriers. Because they alternate between different flight styles, harriers are more independent of environmental factors, such as thermal activity and wind, compared to pure soaring migrants. This allows harriers to migrate under unfavorable thermal and wind conditions. Marsh Harriers are similar to typical soaring migrants in maximizing cross-country speed in soaring-gliding flight, whereas Montagu's and Pallid Harriers are less adapted to soaring-gliding flight and thus are similar to smaller flapping migrants. Optimal soaring-gliding flight seems to be less relevant for these smaller harriers; they maximize cross-country performance by efficiently combining different flight styles.