Movement, either seasonal or as part of a single life cycle, of certain animals, chiefly birds and fish, to distant breeding or feeding grounds.
The precise methods by which animals navigate and know where to go are still obscure. Birds have much sharper eyesight and better visual memory of ground clues than humans, but in long-distance flights appear to navigate by the Sun and stars, possibly in combination with a ‘reading’ of the Earth's magnetic field through an inbuilt ‘magnetic compass’, which is a tiny mass of tissue between the eye and brain in birds. Similar cells occur in ‘homing’ honeybees and in certain bacteria that use it to determine which way is ‘down’. Leatherback turtles use the contours of underwater mountains and valleys to navigate by. Most striking, however, is the migration of young birds that have never flown a route before and are unaccompanied by adults. It is postulated that they may inherit as part of their genetic code an overall ‘sky chart’ of their journey that is triggered into use when they become aware of how the local sky pattern, above the place in which they hatch, fits into it. Similar theories have been advanced in the case of fish, such as eels and salmon, with whom vision obviously plays a lesser role, but for whom currents and changes in the composition and temperature of the sea in particular locations may play a part – for example, in enabling salmon to return to the precise river in which they were spawned. Migration also occurs with land animals; for example, lemmings and antelope.
Species migration is the spread of the home range of a species over many years, for example the spread of the collared dove (Streptopelia decaocto) from Turkey to Britain over the period 1920–52. Any journey that takes an animal outside of its normal home range is called individual migration; when the animal does not return to its home range it is called removal migration. An example of return migration is the movement of birds that fly south for the winter and return to their home ranges in the spring. Many types of whale also make return migrations. In remigration, the return leg of the migration is completed by a subsequent generation, for example locust swarms migrate, but each part of the circuit is completed by a different generation.
Related to migration is the homing ability of pigeons, bees, and other creatures.
The elephant seal makes the longest mammalian migration, covering 21,000 km a year, travelling twice between California's Channel Islands and their feeding areas in the Pacific.
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