migration

Map showing animal migratory patterns.

Reindeer Rangifer tarandus migrate in herds over vast distances between their summer and winter feeding grounds. They are adapted to both tundra and woodland environments.

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.

migration

Some causes of migration. Factors that tend to repel people from an area, such as high unemployment and poor housing, are called push factors. Factors that tend to attract people to an area, such as a wide range of facilities or well-paid jobs, are called pull factors.

Movement of population away from the home region, either from one country to another (international migration) or from one part of a country to another (internal migration). Migrations may be temporary (for example, holidaymakers), seasonal (transhumance), or permanent (people moving to cities to find employment). For people, migration is often a permanent (or long-term) movement, which involves the break-up of a person's residential and social environment. People leave areas due to push factors (negative factors such as overcrowding and lack of employment) and are drawn to areas by pull factors (such as better housing, better jobs, and improved facilities). Barriers such as cost, language, politics, and knowledge also influence migration.

Migration has both positive and negative effects on the home region (source). It can lower the birth rate or increase the average age of the population, ease unemployment or cause a shortage of workers, cause an influx of new skills that are brought back or a loss of the more skilled and highly educated. At the destination, migration can introduce new foods and cultures or cause racial tensions, solve labour shortages or increase the stress on social services.

If immigration exceeds emigration, the population of an area will rise beyond the natural increase. Forced migration occurs due to war, religious persecution, famine, slavery, politics, or natural disasters, creating refugees.