In ethology, the regular, usually seasonal, movement of all or part of an animal population to and from a given area. Familiar migrants include many birds; hoofed animals, especially in East Africa and in the Arctic tundra; bats; whales and porpoises; seals; and fishes, such as salmon. Migration can be contrasted with emigration, which involves a change in location not necessarily followed by a return journey; invasion or interruption, both of which involve the appearance and subsequent disappearance of great numbers of animals at irregular times and locations; and range expansion, which tends to enlarge the distribution of a species, particularly its breeding area. The migration cycle is often annual and thus closely linked with the cyclic pattern of the seasons. The migration of most birds and mammals and many of the fishes are on a yearly cycle. In many cases (e.g., salmon and eels) animals with a relatively long life-span return to the place of birth in order to reproduce and eventually die. In other cases, as in certain invertebrates, where the animal has a relatively brief life-span and reproduces rapidly, migrations may not occur in every generation. The daily movements of certain fishes and invertebrates have also been called migrations because of their regular occurrence. This type of movement, however, is not to be confused with migration in the strict sense. Most migrations involve horizontal travel. The distance traversed may be a few miles or several thousands of miles. Some migrations take a vertical direction and involve no appreciable horizontal movement. Certain aquatic animals, for example, move from deep water to the surface according to the season. Certain birds, mammals, and insects migrate altitudinally in mountainous areas, going from the upper zones, where they breed, to the foothills or plains during seasons when the weather is severe and unfavourable. Such vertical travels involve essentially the same type of environmental change as horizontal, or latitudinal, migrations over long distances.
Source: Encyclopaedia Britannica 2002