Animal migration
Animal migration is the relatively longdistance movement of individual animals, usually on a seasonal basis.It is the most common form of migration in ecology.It is found in all major animal groups, including birds，mammals, fish, reptiles, amphibians, insects, and crustaceans.Different kinds of animal migrate in different ways.
·In birds
Approximately 1,800 of the world's 10,000 bird species migrate long distances each year in response to the seasons.Many of these migrations are north—south, with species feeding and breeding in high northern latitudes in the summer, and moving some hundreds of kilometres south for the winter.Some species extend this strategy to migrate annually between the Northern and Southern Hemispheres.The Arctic tern is famous for its migration; it flies from its Arctic breeding grounds to the Antarctic and back again each year, a distance of at least 19,000 km (12,000 miles), giving it two summers every year.
·In fish
Most fish species are relatively limited in their movements, remaining in a single geographical area and making short migrations for wintering, to spawn, or to feed.A few hundred species migrate long distances, in some cases of thousands of kilometres.About 120 species of fish, including several species of salmon, migrate between saltwater and freshwater.
