Night fliers take compass cues from star patterns. While in flight, birds may use geographic land forms to help them orient direction and course. The outline of North American coasts, and the north-south direction of many large rivers and mountain chains, aid in migration. Homing pigeons and some migratory birds see ultraviolet and polarized light to assist their migration. Many scientists believe that migratory birds tune in to the earth s magnetic field in combination with gravity for direction.
Birds usually wait to migrate until the passage of weather fronts.
They make use of the favorable winds that follow storms to aid their flights. Most long-distance migrants have a highly developed respiratory system, hollow bones, internal air sacs, and specialized body shapes. Birds pectoral chest muscles, which power their wings, are much larger and better developed in relation to overall size than are any similar muscles in mammals. Before migration, most species build up layers of fat. All of these features enable migrating birds to fly high, fast, and for long periods of time.