Animal Migration

Animal Migration, seasonal or periodic mass movement of animals away from and back to their natural breeding areas. Migration usually occurs before and after breeding seasons. During a breeding season, migratory animals are subjected to seasonal changes in environment and undergo physiological and anatomical changes. In its strictest sense, the term migration is not applicable to the nomadic wanderings of many animal groups constantly threatened with overpopulation, or to the change in habitat of many larval animals during various stages of metamorphosis. Thus, the periodic mass movements of the lemming and the locust are not considered truly migratory.

The best-known examples of true migration are annual bird movements, in which swarms of birds native to the Temperate and Arctic zones seek warmer regions late in the summer and in the autumn and return to their original nesting sites in the spring. A spectacular example is the Arctic tern, which breeds within eight degrees of the North Pole and winters in the Antarctic.

Such seasonal migrations also occur among some mammals, especially those that live a partially or entirely aquatic life. The Greenland seal, Pagophilus groenlandicus, makes long journeys from Greenland to Spitsbergen and Jan Mayen islands, where the young are born; and the fur seal, Callorhinus ursinus, native to many Arctic areas, always converges at a rookery in the Pribilof Islands during the breeding season. Many northern bats travel to warmer winter habitats; in northern North America, bats that inhabit hollow trees migrate southward as far as Bermuda to pass the winter. The migration of large African mammals is timed to the wet and dry seasons. Herds of animals including wildebeests, zebras, and elephants spread out in the wet season, congregating again in the dry season at water holes, having typically travelled up to 1,500 kilometres or more.

Many fish migrate from open sea to warmer coastal waters and from the north to warmer southern regions during the winter months. Salmon ascend from the ocean upstream through various rivers to spawn, and most eels pass down from the rivers and enter the sea for the same purpose. Some marine invertebrates, such as the sea urchin and the starfish, migrate from deep to shallow water during spring and early summer to spawn.

Many theories have been advanced to explain the origin of migrations and the physiological mechanisms that guide animals in migratory journeys, but no single theory has been judged wholly satisfactory by zoologists. The seasonal movements of birds and most other migrating animals are actuated by a combination of external and internal stimuli that release a physiological migratory “trigger”. Once an animal has begun a migratory journey, however, many factors may act to maintain it on a proper path.

Navigation by the Sun and stars seems to be involved in the migration of birds, and fish may be guided through the sea by minute traces of chemical odours from their ancestral rivers. According to current research, birds may possess a sensitivity to the magnetic field of the Earth and to the effect of its rotation about an axis. The combination of these two forces, being unique for various parts of the world, could thus supposedly direct a bird to the location desired. Traces of iron in the brain tissue of birds indicate a possible mechanism for this sensitivity to the magnetic field. Other theories involve the use by animals of topographic landmarks during flight, and some redundancy of navigational information may be involved. None of these theories fully explains the usually unerring maiden migrations of animals, unless it is supposed that much of the phenomenon is hereditary and that animals instinctively pursue ancient paths.

See Animal Behaviour; Biological Clocks.