Migration Overview
On just a short walk through autumn woods, brushy fields, roadside thickets or even urban areas, you will notice birds on the move. Flocks of robins and thrushes are flitting about berry-laden trees and shrubs. Numerous blue jays are bustling in the treetops. Out of thickets float flurries of the shrill chirps of sparrows. Fall migration is ready to begin.
Not all songbirds migrate. Mingled with the migrant birds in woods and thickets are cardinals, chickadees, nuthatches and woodpeckers — all stay-at-homes. But what causes birds to make their long trip south? How do they find their way over such great distances? The changing day length, particularly the shorter days of late summer and fall, stimulates the production of a hormone that induces migratory behavior and the deposition of body fat needed for the long trip. Following this, an environmental signal such as a drop-in temperature or the arrival of a cold front tells birds it is time to head southward.
How birds find their way south is another matter as birds use a variety of mechanisms. Daytime migrants use the sun as a compass by determining their directions and position by the arc of the sun. Nighttime migrants travel by the configuration of the stars, using the position of the sunset for the initial selection of direction.
Other birds use the magnetic fields of the earth or geographic landforms to find their way southward. But science will probably never isolate the exact mechanism of migration or the hold that it has on the human spirit.