The Effects of Overcast Skies on the Orientation of Free-Flying Nocturnal Migrants

Abstract

Radar tracks of individual birds migrating on overcast nights were compared with a similar set of tracks obtained under clear skies. I examined the accuracy of orientation within and among tracks, flight altitude and changes in altitude, the effects of flying within clouds, and the effects of continuous overcast of several days’ duration. When birds were flying below the cloud cover there were essentially no discernible effects. Their flight was as straight, level, and fast as comparable birds flying on clear nights. Likewise, the spread in headings among birds was similar. On each of four nights when birds appeared to be flying within or between cloud layers, headings were random and the individual tracks were slightly, but significantly less straight than when birds were flying beneath the clouds. Uninterrupted overcast of several days’ duration did not result in random headings so long as the birds were flying below the cloud ceiling.