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

This paper examines the influence of atmospheric structure andmotion (principally winds aloft) on the flight behavior andaltitudinal distribution of migrating songbirds. Bird migrationdata that I gathered using surveillance radars operated by theUnited States National Weather Service and the Federal AviationAdministration and a vertically directed fixed-beam marine radarmounted on a mobile laboratory are analyzed in relation to windsaloft. Migrating birds appear to fly at altitudes where windswill minimize the cost of transport and assist movements inseasonally appropriate directions. When migratory flights occurat altitudes that are higher than usual, a significant correlationexists between the altitude of densest migration and the altitudeof most favorable wind. Lower altitudes may be favored overslightly more favorable winds at much higher altitudes. Radardata on the flight behavior of migrating birds in the vicinityof frontal systems is also examined. The flight strategies ofmigrants (fly over the front, change the direction of flight,or land and terminate the flight) differ depending on seasonand the "thickness" of the front. Recent migration studies thatare related to atmospheric structure and motion are summarizedand related to atmospheric processes operating simultaneouslyat vastly different spatial and temporal scales.