Migration
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What is animal migration?
Animal migration is the regular, usually seasonal, movement of all or part of an animal population to and from a specific area. This phenomenon is closely linked with the cyclic pattern of the seasons and is often annual. Migratory behavior enables animals to exploit fluctuating resources and settle in areas where survival would not be possible for animals incapable of rapid travel.
Most migrations involve horizontal travel, covering distances from a few miles to thousands of miles. Some migrations take a vertical direction, involving movement from deep water to the surface or altitudinal shifts in mountainous areas.
Migrants often return to breed in the exact locality where they were hatched or born, demonstrating their ability to orient themselves geographically.
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Why do animals migrate?
Migration is the regular, seasonal movement of animals to and from an area, often on an annual cycle, to coincide with the seasons. Animals migrate for various reasons, including warmth and the availability of food and water. Migratory behavior is most common among species at specific trophic levels where there are maximal fluctuations in both breeding and wintering areas.
Many birds, for example, nest in areas comfortable only during warmer months. As cold weather arrives, they migrate to warmer climates where they can find food, sometimes traveling thousands of miles. This allows them to exploit fluctuating resources and settle in areas where life would not be sustainable year-round.
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Which animals are known for migrating long distances?
Many animal species migrate long distances, often due to seasonal changes in food availability and climate.
Migratory birds are notable for their ability to travel swiftly over long distances. Arctic terns travel the farthest of any bird, with a migration spanning approximately 17,600 kilometers (11,000 miles) from their breeding range to their wintering grounds. Seabirds such as albatrosses also migrate across the globe.
Among terrestrial mammals, caribou in North America migrate between summer and winter habitats, following the same routes each year. In Africa, wildebeests and zebras travel long distances in response to wet and dry seasons.
Gray whales undertake one of the longest migrations of any mammal, traveling about 5,000 km (3,000 miles) each way between their summer feeding grounds in the Arctic and their winter breeding lagoons off the coast of Baja California.
Monarch butterflies are well-known migrants, with some traveling as far as 3,000 km (1,800 miles) to overwinter in specific locations.
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How do animals know where to go during migration?
Migrating animals navigate using a combination of methods, including landmarks, celestial cues, and magnetic fields.
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Landmarks Animals use topographical, ecological, and climatic landmarks such as mountain ranges, coastlines, vegetation zones, and prevailing winds.
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Celestial bearings Many species use the sun and stars for directional orientation. They possess an internal clock to compensate for the sun's movement. Nocturnal migrants use star patterns to determine their bearings.
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Magnetic fields Some animals, like sea turtles and salmon, can detect the Earth’s magnetic field and use it to navigate back to their hatching or spawning grounds.
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What challenges do animals face during migration?
During migration, animals face multiple challenges, including habitat loss and climate change, which can alter ecosystems and affect species' ability to adapt. Migratory animals may encounter man-made structures that impede their relocation to regions with better climate conditions. Changing seasonal processes can also impact migration patterns. Additionally, the availability of food, temperature, and weather conditions influence migratory activity, and a sudden period of cold weather may induce immediate departure.
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How does environmental change affect animal migration patterns?
Environmental changes significantly influence animal migration patterns. Migratory species depend on environmental cues and internal rhythms to time their movements with resource availability.
Global warming alters seasonal patterns, affecting processes like vegetation greening, egg laying, and migration timing. Rising temperatures may cause habitat loss, forcing animals to seek new ranges. Climate change, combined with deforestation and urbanization, creates obstacles for migrating animals. Light pollution also disorients nocturnal migrants, leading to collisions and population declines.
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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 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 their 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.
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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.
Survey of migratory behaviour in animals
Lower invertebrates
Many marine invertebrates travel considerable distances during certain seasons. A large proportion of them, however—particularly planktonic organisms, plant and animal aquatic drifters—do not travel deliberately but are carried by ocean currents. Planktonic organisms also travel vertically in a daily rhythm. Very small or microscopic animals remain at great depths during the day and rise at dusk, concentrating in the upper layers of water during the night. Their predators, particularly fishes, follow them in their cycle. The daily activity of pelagic birds (those living on the open sea), such as petrels and shearwaters, which feed on planktonic crustaceans and squids, follows this same rhythm.
A seasonal change of habitat, analogous to migration, is made by some Polychaeta (sandworms). Along the coast of Europe, clam worms (Nereis) live during the colder months in rock crevices and among algae. During the summer, however, they become planktonic and swim out some distance from the coast, where reproduction occurs. In the South Pacific, near Samoa and Fiji, the palolo worm (Palola siciliensis) lives among coral reefs, where it develops posterior segments filled with genital (reproductive) cells. These are cast off, and the worm rises to the surface. The phenomenon occurs regularly on the first day of the last quarter of the October–November moon.
Some of the best-known migrations among the invertebrates occur in crustaceans during the reproductive period, when some of them travel as far as 240 kilometres (150 miles). Generally in the crabs, females move into shallow coastal waters to mate and to lay their eggs. After the eggs have been laid, the females return to deep water.
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Some freshwater crabs, such as the Chinese crab (Eriocheir sinensis), after remaining for three to five years in fresh water, migrate to brackish water, where mating occurs. Females with eggs externally attached then travel to the sea and remain a few miles offshore for several months during winter. The following spring they enter shallower water near the shore. Here the eggs hatch. Young crabs spend a year in brackish water and migrate upstream the following spring, settling in fresh water and growing to maturity.
Some crabs, such as robber crabs (Birgus) and land crabs of tropical regions (Geocarcinus), have adapted to life on land. They migrate to the sea to reproduce and then return inland and are followed at a later time by the young.