Most scientist agree that the changing day length is a huge cue for migration. As days get shorter, migrating birds become anxious and restless. Pretty soon they are like a wind-up toy wound too tight. Usually, a changing weather pattern such as a cold front will be the final trigger that sends them on there way. Once they start flying, birds use many special and unique ways to navigate to their wintering grounds.
Indigo bunting use the stars to find their way while other use the sun to steer.
Some birds simply use land marks or geographical sites. It is even guessed that some pigeons use the earth's magnetic field to find their way home. Regardless of how or why, this interesting natural phenomenon is not fully understood.