Recently ornithologists have started to learn how birds know where they are and navigate during their great migrations. Near your house, it is unlikely that you would ever get lost. You know the way because you recognize landmarks; birds also use landmarks to find their way. But they also have other ways of navigating when they fly out of their neighborhood. They have several sources of information that tell them which way to go. Migrating birds can use the sun by day and the stars by night. Research shows that they can even use the earth's magnetic field (the same field detected by a compass) and that they sometimes use odor clues, and maybe even low-frequency sounds.
However birds find their way, they are rarely very late. One of the extraordinary things about migration is its precision. Birds that travel across a continent, or farther, arrive at their destination within a few days of the same time each year.
A bird's internal clock is mostly responsible for the start of migration, but weather is also a factor. Many species, such as American robins, time their arrival to match the spring thaw. Birds are sensitive to changes in atmospheric pressure. Strong favorable winds cause birds to migrate in large numbers.
