Weather and Height of Nocturnal Migration in Eastcentral Alberta: A Radar Study

Birds migrating in still air would benefit by flying high. As the density of air decreases with height the speed of migrating birds increases (through a decrease in drag), but the power required to fly at those higher speeds also increases (Pennycuick, 1969). The advantage of flying high at high speed is the reduction of flight time. When birds are migrating in the atmosphere where calm conditions rarely prevail, the greater cruising speeds at higher altitudes also reduce interference from winds aloft, but this advantage may be offset when unfavorable winds are stronger aloft than at the surface.