As has been previously indicated, the advance of spring in the northern interior is much more rapid than in the Mississippi Valley and on the Gulf coast. In other words, in the North spring comes with a rush, and, during the height of migration season in Saskatchewan, the temperature in the southern part of the Mackenzie Valley just about equals that in the Lake Superior area, 700 miles farther south. Such conditions, coupled with the diagonal course of the birds across this region of fast-moving spring, exert a great influence on migration and are probably factors in the acceleration of travel speed. However, it should be remembered that the birds are getting closer to the breeding season and may be stimulated to travel faster for this reason.
Thus it has been shown that the rate of migration varies greatly under varying circumstances. Radar investigations along the eastern coasts of the United States and England indicate spring migration is several miles per hour faster than in the fall. Also, directions of migrations in spring were much less diverse than in the fall, which suggests less time lost in passage (Tedd and Lack 1958; Nisbet and Drury 1967a). King and Farner (1963) found the same species put on more fat preparatory to migration in the spring. This would give the migrants greater energy reserves for longer flights at that season.
The factors regulating the heights of bird migration are not clear. High-altitude flight may be used to locate familiar landmarks, fly over fog or clouds, surmount physical barriers, gain advantage of a following wind, or maintain a better physiological balance. Meteorological conditions probably account for most of the high-altitude records. Wind conditions at ground level are usually quite different in direction and velocity than at points higher up.
In general, human estimates of bird heights are quite unreliable except under special conditions, and these estimates will vary with the eyesight of the observer. Lucanus (1911) found a European sparrow hawk could be distinguished at 800 feet but disappeared from sight at 2,800 feet. A rook (a European member of the crow family) could be recognized at 1,000 feet but disappeared from sight at 3,300 feet. Meinertzhagen (1955) did an interesting experiment with an inflated model of a vulture painted black; it had a wing expanse of 7 feet 10 inches. When released from an airplane at 4,700 feet, it was barely visible and invisible without binoculars at 5,800 feet. At 7,000 feet it was not picked up even when ×12 binoculars were used.