The Role of Weather Variables and Flight Direction in Determining the Magnitude of Nocturnal Bird Migration

Autumn nocturnal bird migration was investigated at two stations in the southeastern United States by means of Weather Surveillance Radar (WSR—57) and visual observations. Multivariate analyses were used to examined the influence of environmental variables on the night—to—night magnitude of migration. Landbirds selectively flew with the wind, regardless of its direction or speed. As a result, they frequently migrated in seasonally inappropriate directions. The volume of migration varied by several orders of magnitude from night to night, but southward flights averaged about five times larger than those in other directions. This implies that the birds employed some selection in terms of the weather conditions under which they initiated migratory flights. Multivariate analyses showed that wind direction, 24—hr change in temperature, and an index of the synoptic weather situation accounted for 5.2% of the night—to—night varaibility in the magnitude of passerine migration. Migration volume was also related to the passage of cold fronts. Migations were heavier in northerly winds, falling temperatures, and the anticyclonic weather that usually prevails shortly after the passage of a cold front. The efficiency of downwind flight is enhanced by the facts that average winds blow roughly in the migratory directions of the birds and that the birds show behavioral selection in terms of the wind directions and synoptic weather situations in which they initiate migration. Night—to—night variations in migration density can be accounted for on the basis of interactions between the circadian nature of Zugunruhe, the energetic state of the bird, and the direct influence of weather on the initiation of migratory flights.