Following an instinct many thousands of years old, millions of ducks and geese fly south every autumn to overwinter in the southern United States, Mexico, and Central and South America, then return in spring to the lakes and marshes of North America, driven by a biological urge to nest. Scientists believe birds navigate for thousands of miles each way using the sun and stars, visual landmarks, the Earth’s magnetic field, and gravity, and as they migrate, waterfowl follow general routes called flyways. Natural rest stops are important stopover points because they provide resting and feeding areas along the difficult journey, and wildlife refuges and natural parks provide seasonal homes and essential support during the annual migration.