Most birds make regular seasonal journeys between wintering grounds and breeding, or nesting, grounds. These journeys are called migrations.
Birds of temperate regions migrate because of the diminished food supply brought on by winter and also to escape cold weather. Tropical birds migrate because of the diminished food supply brought on by the dry season.
Some species migrate at night, others during the day. Sparrows and most other small birds migrate at night, feeding during the day when they are safe from most of their natural enemies. Soaring birds migrate during the day when thermals, created by the heat of the sun, help them conserve energy. Some species, such as swifts and swallows, migrate during the day when flying insects, their main source of food, are active.
Migrating birds fly at different speeds. Large birds fly faster than small birds. Ducks and geese maintain an average speed of 40-50 miles per hour (64-80 km/h) while hawks and ravens fly at 22-28 mph (35-45 km/h). Most birds migrate at an altitude of 3,000 feet (900 m) or less. Some species of cranes and geese migrate at altitudes of 15,000-21,000 feet (4,600-6,400 m).
Birds, like other vertebrates, have an internal mechanism called a biological clock that controls biological rhythms, or daily and seasonal changes in the body that occur in response to changes in the environment. The nature and location of this time-keeping mechanism are not clearly understood. However, it is believed to trigger changes in the bird's body that stimulate the urge to migrate.
Birds use a number of methods to find their way during migration. Many use celestial navigation, a method of orienting the body to the arc of the sun, to the phase of the moon, or to the pattern of the stars. Some birds, such as pigeons, are sensitive to changes in the earth's magnetic field and to the pull of gravity.
The senses of vision, hearing, and, in some species, smell, play important roles in navigation during migration. Birds use topographical landmarks such as mountains, river valleys, and forests to orient themselves on the migration route. They are able to detect infrasounds, low-frequency sounds that travel long distances. The pounding of the ocean surf, for example, emits infra-sounds that are detected by birds. Many birds, particularly seabirds, identify their destinations by characteristic odors. Others, such as hummingbirds and pigeons, are able to determine the position of the sun on overcast days because they can detect the ultraviolet radiation it emits.
It was once believed that migrating birds from the same general area followed the same migration routes, called flyways, each year. It is now known that only certain waterfowl follow flyways; other birds take different routes each year.
The length of the annual migration varies greatly with the species. The quail, cardinal, and a few other species do not migrate at all. Meadowlarks move southward less than 100 miles (160 km). Many Canadian species, such as the junco and the snow bunting, winter in the United States. Some birds from the northern United States spend the winter around the Gulf of Mexico.The Arctic tern breeds near the North Pole and winters in the Antarctic regions.