Many birds make long journeys every year, at the same time each year. This annual, seasonal movement is known as migration.
Commonly, birds fly north when it is spring in the northern hemisphere, and then fly south when it is autumn in the northern hemisphere. They often breed when they are in the north.
Migrating allows birds to obtain a greater amount of daylight and a greater supply of food.
The flight path of a migratory bird is known as a flyway.
Migratory birds often fly long distances - across continents and over oceans.
The arctic tern has the longest known migration route of any animal. It breeds in the arctic and spends part of the year around Antarctica, traveling a total of about 44,000 miles each year.
There are risks associated with migration. These include the costs of extreme physical exertion, dangerous weather conditions, lack of food and water, and danger of attack from predators.
Before they migrate, birds eat more food than usual. They store the energy from this food as fat, and then retrieve this energy during their long journey.
Many large birds, such as geese, fly in a V-formation, which allows them to save energy by reducing the amount of wind resistance faced by individual birds.
In birds, migration is triggered by hormonal changes as well as by changes in the length of the day.
Migrating birds have the ability to detect magnetic fields, which helps them to navigate. This ability is known as magnetoception or magnetoreception. They also use the position of the sun and the stars, visual landmarks and smells to find their way.