The Flight Behavior of Migrating Birds in Changing Wind Fields: Radar and Visual Analyses

Abstract

This paper examines the influence of atmospheric structure and motion (principally winds aloft) on the flight behavior and altitudinal distribution of migrating songbirds. Bird migration data that I gathered using surveillance radars operated by the United States National Weather Service and the Federal Aviation Administration and a vertically directed fixed-beam marine radar mounted on a mobile laboratory are analyzed in relation to winds aloft. Migrating birds appear to fly at altitudes where winds will minimize the cost of transport and assist movements in seasonally appropriate directions. When migratory flights occur at altitudes that are higher than usual, a significant correlation exists between the altitude of densest migration and the altitude of most favorable wind. Lower altitudes may be favored over slightly more favorable winds at much higher altitudes. Radar data on the flight behavior of migrating birds in the vicinity of frontal systems is also examined. The flight strategies of migrants (fly over the front, change the direction of flight, or land and terminate the flight) differ depending on season and the “thickness” of the front. Recent migration studies that are related to atmospheric structure and motion are summarized and related to atmospheric processes operating simultaneously at vastly different spatial and temporal scales.