Comparative flight behaviour of migrating hawks studied with tracking radar during autumn in central New York

Tracking radar with simultaneous visual observations was employed to study the flight behaviour of nine species of hawks during autumn migration, 1978–1979, in central New York. The predominant mode of flight for all species was thermal soaring and interthermal gliding. Although most species were seen in small flocks at some time, only Broad-winged Hawks (Buteo platypterus) could be considered flocking migrants, with most migrating in flocks < 40 individuals. Altitude of flight increased through the day as convective depth developed, with approximately 85% of all individuals flying below 1000 m. Climb rates of individuals soaring in thermals averaged 3 ms−1 and were greater than previously reported for larger soaring species. Short- to medium-distance migrants (Accipiter striatus, Falco sparverius) tended to fly at lower altitudes than longer distance migrants. The direction realized during thermal soaring was positively related to wind direction and was oriented to the southeast, a function of the prevailing northwest winds. Orientation strategy was considered to be a compromise between drift and complete compensation, resulting in an elliptical migratory flight path, probably shaped by prevailing northwest winds. Such a compromise promotes a faster and more energetically efficient migration.