The reason for migration is not hard to understand. Take mobile animals and a seasonally fluctuating food supply, and the natural consequence is migration. Long ago, and even today, the birds that moved in the right direction survived.

Much time and effort has gone into the study of the migrations of birds. For example, by using radar in the 1950s, researchers learned that birds start their night migrations soon after dark and that their numbers are greatest between 10 p.m. and 1 a.m., with the maximum numbers of birds observed between 11 p.m. and midnight. Then comes a gradual dwindling in numbers to virtually none at dawn, when the birds descend to earth to feed and rest.

Night migrating birds are able to navigate by the stars. Day migrants get compass directions from the Sun and use landmarks such as the Mississippi River. Night migrants include some larger birds, such as the secretive rails and the American woodcock, and most small insect-eating birds such as wrens, most thrushes, kinglets, vireos, wood warblers, tanagers, orioles, and most sparrows. Hawks, eagles, vultures, cranes, and pelicans migrate by day. Also crows, jays, most blackbirds, and the eastern bluebird migrate by day. Small birds of strong flight, such as swifts, swallows and hummingbirds usually migrate by day.