MIGRATION

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 the salmon. Most migration is horizontal, but the patterns of migration vary widely among animal groups. Marine invertebrates such as plankton are simply carried by ocean currents; certain species travel only during certain seasons, while others combine the two modes. Crustaceans, including those that normally live on land, migrate to reproduce in seawater. Insects migrate to breed, feed, or hibernate. Migratory fish may be classified according to whether they live and migrate entirely in the sea (e.g., herring), live in the sea and migrate to freshwater to spawn (salmon), or live in freshwater and breed in the sea (certain eels). Most reptiles and amphibians are unable to travel great distances and thus do not migrate; many species of frogs and toads, however, make annual migration-like movements to breeding ponds and lakes, and sea turtles make prodigious migrations to reach their breeding grounds. Migration is most highly developed among birds, many species of which migrate annually between their breeding grounds in higher latitudes and wintering grounds in lower latitudes. Northern Hemisphere birds tend to go south for the winter, and tropical birds migrate according to the cycle of wet and dry seasons. Pelagic, or open-sea, birds travel much greater distances than coastal birds, which stick close to land though they may change altitude. During migration, even the most individualistic birds become gregarious; some, like ducks and cranes that fly in the well-known V-shape pattern, are extremely cohesive. It is not fully understood how migrating animals find their way over long and complex routes. A compass sense has been demonstrated in birds, which navigate by the position of the Sun, stars, and the Earth's magnetic field, similar mechanisms are used by some insects, crustaceans, and fish. Migrating land animals, on the other hand, may depend on scented trails and landmarks over short distances. Migration is part of the life cycle of an animal. It is controlled by the pituitary gland in a cyclical pattern related to seasonal changes in daylight, but it may be modified by environmental conditions, such as sudden cold weather, which will stimulate early departure. Migration has great ecological significance; it results in the optimal use of food surpluses and enables fast-moving animals to live for a time in otherwise unsuitable regions. 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. Additional reading Works that cover many aspects of animal migratory behaviour include Franois Bourlire, The Natural History of Mammals, 3rd ed. rev. (1964; originally published in French, 1951), a review of mammal biology with a chapter on migration; Robert T. Orr, Animals in Migration (1970), a review of migration patterns among animals; and Talbot H. Waterman, Animal Navigation (1989), a splendid introduction to the subject of animal navigation for the lay reader. Texts dealing specifically with bird migration are A. Landsborough Thomson, Bird Migration, 3rd ed. (1949), a classic book, still useful; Jean Dorst, The Migrations of Birds (1962; originally published in French, 1956), a review of all aspects; Donald R. Griffin, Bird Migration (1964, reissued 1974), a useful summary with a good chapter on orientation; G.V.T. Matthews, Bird Navigation, 2nd ed. (1968), an original attempt to explain bird orientation; R. Robin Baker, Bird Navigation: The Solution of a Mystery? (1984), a well-written introduction to bird orientation and navigation for scientifically literate readers; and Joanna Burger and Bori L. Olla (eds.), Shorebirds: Migration and Foraging Behavior (1984), a collection of reviews directed at professionals in ethology, ornithology, and vertebrate zoology. Works examining the migration of other animals include classic texts by Carrington B. Williams, The Migration of Butterflies (1930), still of value; and by Glover Morrill Allen, Bats (1939, reissued 1967), on bat biology with a chapter on migration; and the more recent works by F.R. Harden Jones, Fish Migration (1968), a standard book, with many classic references; and by E.J. Slijper, Whales, 2nd ed. (1979; originally published in Dutch, 1958), a review of whale biology, including a chapter on migration.