Recently, radar has aided greatly in determining differences in the altitude of bird flight. Nocturnal migrants appear to fly slightly higher, on the average, than diurnal migrants, but daytime flights may be influenced more by cloud cover (Lack 1960a; Eastwood and Rider 1965). Bellrose (1971) found little difference in the altitudinal distribution of small nocturnal migrants under clear or overcast skies. Many night migrating birds are killed each year by striking lighthouses, television towers or other man-made illuminated obstructions, but this does not furnish proof that low altitudes are generally used during nocturnal flight because these accidents occur chiefly in foggy weather. Under such conditions, migrating birds seem to be attracted to and confused by lights. Seabirds, such as loons, eiders, and scoters, generally fly very low over the water but gain altitude when land is crossed. The reverse is true for landbirds (Dorst 1963; Bergman and Donner 1964; Eastwood and Rider 1965). There may be a seasonal difference in the altitude of migration, but the evidence is conflicting. Radar echoes studied by Bellrose and Graber in Illinois (1963) showed fall migrants flew higher than spring migrants. They speculated this difference was related to the winds during the fall being more favorable for southerly migration at higher altitudes, while winds at these altitudes in the spring would be less favorable for northerly migration. Eastwood and Rider (1965) studied seasonal migration patterns in England and found the reverse to be true. They suggested one reason for this seasonal difference was that flocks of fall migrants included many young birds whose flight capabilities are inferior to adults and consequently are unable to achieve the higher altitudes in the fall.
During the height of northward movement in spring, the woods and thickets may suddenly be filled with several species of wood warblers, thrushes, sparrows, flycatchers, and other birds. It is natural to conclude they traveled together and arrived simultaneously. Probably they did, but such combined migration is by no means the rule for all species.
As a group, the wood warblers probably travel more in mixed companies than do any other single family of North American birds. In spring and fall, the flocks are likely to be made up of the adults and young of several species. Sometimes swallows, sparrows, blackbirds, and some of the shorebirds also migrate in mixed flocks. In the fall, great flocks of blackbirds frequently sweep south across the Plains States, with common grackles, red-winged blackbirds, yellow-headed blackbirds, and Brewer's blackbirds included in the same flock.