MIGRATION BY FLAPPING OR SOARING - FLIGHT STRATEGIES OF MARSH-HARRIERS, MONTAGU-HARRIERS AND PALLID-HARRIERS IN SOUTHERN ISRAEL

Migratory flights of Marsh Harriers (Circus aeruginosus), Montagu's Ha rriers (Circus pygargus) and Pallid Harriers (Circus macrourus) in sou thern Israel were used to test flight theory predictions. The body siz es of these closely related species are between those of the typical l arge soaring migrants, such as eagles and storks, and the typical flap ping migrants, such as small falcons and sparrowhawks. In soaring-glid ing flight, Marsh Harriers reacted to different thermal conditions by adjusting their gliding airspeed to the actual climbing rate in therma l circling; consequently, cross-country speed was related to climbing rate. In contrast, the smaller Montagu's and Pallid Harriers did not a dopt gliding airspeeds according to thermal conditions. All harrier sp ecies regularly used flapping-gliding flight, predominately soon after sunrise and before sunset, and more often in opposing winds than in f ollowing winds. Montagu's/Pallid Harriers used flapping-gliding more f requently than Marsh Harriers. Because they alternate between differen t flight styles, harriers are more independent of environmental factor s, such as thermal activity and wind, compared to pure soaring migrant s. 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 Mont agu's and Pallid Harriers are less adapted to soaring-gliding flight a nd thus are similar to smaller flapping migrants. Optimal soaring-glid ing flight seems to be less relevant for these smaller harriers; they maximize cross-country performance by efficiently combining different flight styles.