Field Studies of the Orientation Cue Hierarchy of Nocturnal Songbird Migrants

Abstract

Tracking radar, surveillance radar, and visual observation of free-flying nocturnal migrants in the United States are revealing relationships among several compass cues. Whereas passerine migrants in the southeastern United States selectively flew downwind even under clear skies, downwind flight in seasonally inappropriate directions in the northeastern United States occurred only under solid overcast that prevented the birds from seeing both stars and the sun near the time of sunset. The hypothesis that a view of either stars or the sun near sunset is sufficient to allow the birds to perform “correct” orientation was supported by data from experimentally released birds.