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Animal migration is the relatively long-distance movement of individual animals, usually on a
seasonal A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and ...
basis. It is the most common form of migration in ecology. It is found in all major animal groups, including
birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
, mammals,
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
,
reptiles Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchoceph ...
, amphibians, insects, and
crustaceans Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean g ...
. The cause of migration may be local climate, local availability of food, the season of the year or for mating. To be counted as a true migration, and not just a local dispersal or irruption, the movement of the animals should be an annual or seasonal occurrence, or a major habitat change as part of their life. An annual event could include
Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
birds migrating south for the winter, or
wildebeest Wildebeest ( , , ), also called gnu ( or ), are antelopes of the genus ''Connochaetes'' and native to Eastern and Southern Africa. They belong to the family Bovidae, which includes true antelopes, cattle, goats, sheep, and other even-toe ...
migrating annually for seasonal grazing. A major habitat change could include young
Atlantic salmon The Atlantic salmon (''Salmo salar'') is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlantic salmon are ...
or sea lamprey leaving the river of their birth when they have reached a few inches in size. Some traditional forms of
human migration Human migration is the movement of people from one place to another with intentions of settling, permanently or temporarily, at a new location (geographic region). The movement often occurs over long distances and from one country to another (ex ...
fit this pattern. Migrations can be studied using traditional identification tags such as bird rings, or tracked directly with electronic tracking devices. Before animal migration was understood, folklore explanations were formulated for the appearance and disappearance of some species, such as that
barnacle geese The barnacle goose (''Branta leucopsis'') is a species of goose that belongs to the genus ''Branta'' of black geese, which contains species with largely black plumage, distinguishing them from the grey ''Anser'' species. Despite its superficial s ...
grew from goose barnacles.


Overview


Concepts

Migration can take very different forms in different species, and has a variety of causes. As such, there is no simple accepted definition of migration. One of the most commonly used definitions, proposed by the zoologist J. S. Kennedy is Migration encompasses four related concepts: persistent straight movement; relocation of an individual on a greater scale (in both space and time) than its normal daily activities; seasonal to-and-fro movement of a population between two areas; and movement leading to the redistribution of individuals within a population. Migration can be either
obligate {{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym ''facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organism that cannot survive without oxygen * Obligate anaerobe, an organism tha ...
, meaning individuals must migrate, or facultative, meaning individuals can "choose" to migrate or not. Within a migratory species or even within a single population, often not all individuals migrate. ''Complete migration'' is when all individuals migrate, ''partial migration'' is when some individuals migrate while others do not, and ''differential migration'' is when the difference between migratory and non-migratory individuals is based on discernible characteristics like age or sex. Irregular (non-cyclical) migrations such as irruptions can occur under pressure of famine,
overpopulation Overpopulation or overabundance is a phenomenon in which a species' population becomes larger than the carrying capacity of its environment. This may be caused by increased birth rates, lowered mortality rates, reduced predation or large scale ...
of a locality, or some more obscure influence.


Seasonal

Seasonal migration is the movement of various species from one habitat to another during the year. Resource availability changes depending on seasonal fluctuations, which influence migration patterns. Some species such as Pacific salmon migrate to reproduce; every year, they swim upstream to mate and then return to the ocean. Temperature is a driving factor of migration that is dependent on the time of year. Many species, especially birds, migrate to warmer locations during the winter to escape poor environmental conditions.


Circadian

Circadian migration is where birds utilise
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous) and responds to ...
(CR) to regulate migration in both fall and spring. In circadian migration, clocks of both circadian (daily) and circannual (annual) patterns are used to determine the birds' orientation in both time and space as they migrate from one destination to the next. This type of migration is advantageous in birds that, during the winter, remain close to the equator, and also allows the monitoring of the auditory and spatial memory of the bird's brain to remember an optimal site of migration. These birds also have timing mechanisms that provide them with the distance to their destination.


Tidal

Tidal migration is the use of tides by organisms to move periodically from one habitat to another. This type of migration is often used in order to find food or mates. Tides can carry organisms horizontally and vertically for as little as a few nanometres to even thousands of kilometres. The most common form of tidal migration is to and from the
intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species ...
during daily tidal cycles. These zones are often populated by many different species and are rich in nutrients. Organisms like crabs, nematodes, and small fish move in and out of these areas as the tides rise and fall, typically about every twelve hours. The cycle movements are associated with foraging of marine and bird species. Typically, during low tide, smaller or younger species will emerge to forage because they can survive in the shallower water and have less chance of being preyed upon. During high tide, larger species can be found due to the deeper water and nutrient upwelling from the tidal movements. Tidal migration is often facilitated by
ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth conto ...
s.


Diel

While most migratory movements occur on an annual cycle, some daily movements are also described as migration. Many aquatic animals make a
diel vertical migration Diel vertical migration (DVM), also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as copepods, living in the ocean and in lakes. The word ''diel'' comes from the Latin ''dies'' day, and means a 24- ...
, travelling a few hundred metres up and down the water column, while some jellyfish make daily horizontal migrations of a few hundred metres.


In specific groups

Different kinds of animals migrate in different ways.


In birds

Approximately 1,800 of the world's 10,000 bird species migrate long distances each year in response to the seasons. Many of these migrations are north-south, with species feeding and breeding in high northern latitudes in the summer and moving some hundreds of kilometres south for the winter. Some species extend this strategy to migrate annually between the Northern and Southern Hemispheres. The
Arctic tern The Arctic tern (''Sterna paradisaea'') is a tern in the family Laridae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe (as far south as Brittany), Asia, and North America (as far south ...
has the longest migration journey of any bird: it flies from its
Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...
breeding grounds to the
Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and othe ...
and back again each year, a distance of at least , giving it two summers every year. Bird migration is controlled primarily by day length, signalled by hormonal changes in the bird's body. On migration, birds
navigate Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
using multiple senses. Many birds use a sun compass, requiring them to compensate for the sun's changing position with time of day. Navigation involves the ability to detect magnetic fields.


In fish

Most fish species are relatively limited in their movements, remaining in a single geographical area and making short migrations to overwinter, to
spawn Spawn or spawning may refer to: * Spawn (biology), the eggs and sperm of aquatic animals Arts, entertainment, and media * Spawn (character), a fictional character in the comic series of the same name and in the associated franchise ** '' Spawn: A ...
, or to feed. A few hundred species migrate long distances, in some cases of thousands of kilometres. About 120 species of fish, including several species of
salmon Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus '' Salmo'') and North Pacific (genus '' Onco ...
, migrate between saltwater and freshwater (they are 'diadromous').
Forage fish Forage fish, also called prey fish or bait fish, are small pelagic fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the f ...
such as
herring Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Ocean ...
and capelin migrate around substantial parts of the North
Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
ocean. The capelin, for example, spawn around the southern and western coasts of Iceland; their larvae drift clockwise around Iceland, while the fish swim northwards towards
Jan Mayen Jan Mayen () is a Norwegian volcanic island in the Arctic Ocean with no permanent population. It is long (southwest-northeast) and in area, partly covered by glaciers (an area of around the Beerenberg volcano). It has two parts: larger ...
island to feed and return to Iceland parallel with Greenland's east coast. In the ' sardine run', billions of Southern African
pilchard "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the I ...
'' Sardinops sagax'' spawn in the cold waters of the Agulhas Bank and move northward along the east coast of
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...
between May and July.


In insects

Some winged insects such as locusts and certain
butterflies Butterflies are insects in the macrolepidopteran clade Rhopalocera from the order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The group comprises ...
and dragonflies with strong flight migrate long distances. Among the dragonflies, species of '' Libellula'' and '' Sympetrum'' are known for mass migration, while ''
Pantala flavescens ''Pantala flavescens'', the globe skimmer, globe wanderer or wandering glider, is a wide-ranging dragonfly of the family Libellulidae. This species and '' Pantala hymenaea'', the "spot-winged glider", are the only members of the genus ''Pantal ...
'', known as the globe skimmer or wandering glider dragonfly, makes the longest ocean crossing of any insect: between India and Africa. Exceptionally, swarms of the desert locust, '' Schistocerca gregaria'', flew westwards across the Atlantic Ocean for during October 1988, using air currents in the
Inter-Tropical Convergence Zone The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...
. In some migratory butterflies, such as the
monarch butterfly The monarch butterfly or simply monarch (''Danaus plexippus'') is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae. Other common names, depending on region, include milkweed, common tiger, wanderer, and black-veined brown. ...
and the painted lady, no individual completes the whole migration. Instead, the butterflies mate and reproduce on the journey, and successive generations continue the migration.


In mammals

Some mammals undertake exceptional migrations;
reindeer Reindeer (in North American English, known as caribou if wild and ''reindeer'' if domesticated) are deer in the genus ''Rangifer''. For the last few decades, reindeer were assigned to one species, ''Rangifer tarandus'', with about 10 subs ...
have one of the longest terrestrial migrations on the planet, reaching as much as per year in North America. However, over the course of a year, grey wolves move the most. One grey wolf covered a total cumulative annual distance of . Mass migration occurs in mammals such as the Serengeti 'great migration', an annual circular pattern of movement with some 1.7 million
wildebeest Wildebeest ( , , ), also called gnu ( or ), are antelopes of the genus ''Connochaetes'' and native to Eastern and Southern Africa. They belong to the family Bovidae, which includes true antelopes, cattle, goats, sheep, and other even-toe ...
and hundreds of thousands of other large game animals, including gazelles and zebra. More than 20 such species engage, or used to engage, in mass migrations. Of these migrations, those of the springbok,
black wildebeest The black wildebeest or white-tailed gnu (''Connochaetes gnou'') is one of the two closely related wildebeest species. It is a member of the genus '' Connochaetes'' and family Bovidae. It was first described in 1780 by Eberhard August Wilhelm ...
,
blesbok The blesbok or blesbuck (''Damaliscus pygargus phillipsi'') is a subspecies of the bontebok antelope endemic to South Africa, Eswatini and Namibia. It has a distinctive white face and forehead which inspired the name, because ''bles'' is the Afri ...
, scimitar-horned oryx, and kulan have ceased. Long-distance migrations occur in some batsnotably the mass migration of the Mexican free-tailed bat between Oregon and southern Mexico. Migration is important in
cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel th ...
ns, including whales, dolphins and porpoises; some species travel long distances between their feeding and their breeding areas. Humans are mammals, but
human migration Human migration is the movement of people from one place to another with intentions of settling, permanently or temporarily, at a new location (geographic region). The movement often occurs over long distances and from one country to another (ex ...
, as commonly defined, is when individuals often permanently change where they live, which does not fit the patterns described here. An exception is some traditional migratory patterns such as
transhumance Transhumance is a type of pastoralism or nomadism, a seasonal movement of livestock between fixed summer and winter pastures. In montane regions (''vertical transhumance''), it implies movement between higher pastures in summer and lower val ...
, in which herders and their animals move seasonally between mountains and valleys, and the seasonal movements of
nomad A nomad is a member of a community without fixed habitation who regularly moves to and from the same areas. Such groups include hunter-gatherers, pastoral nomads (owning livestock), tinkers and trader nomads. In the twentieth century, the po ...
s.


In other animals

Among the reptiles, adult sea turtles migrate long distances to breed, as do some amphibians. Hatchling sea turtles, too, emerge from underground nests, crawl down to the water, and swim offshore to reach the open sea. Juvenile
green sea turtle The green sea turtle (''Chelonia mydas''), also known as the green turtle, black (sea) turtle or Pacific green turtle, is a species of large sea turtle of the family Cheloniidae. It is the only species in the genus ''Chelonia''. Its range exten ...
s make use of
Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...
to navigate. Some crustaceans migrate, such as the largely-terrestrial Christmas Island red crab, which moves en masse each year by the millions. Like other crabs, they breathe using gills, which must remain wet, so they avoid direct sunlight, digging burrows to shelter from the sun. They mate on land near their burrows. The females incubate their eggs in their abdominal brood pouches for two weeks. Then they return to the sea to release their eggs at high tide in the moon's last quarter. The larvae spend a few weeks at sea and then return to land.


Tracking migration

Scientists gather observations of animal migration by tracking their movements. Animals were traditionally tracked with identification tags such as bird rings for later recovery. However, no information was obtained about the actual route followed between release and recovery, and only a fraction of tagged individuals were recovered. More convenient, therefore, are electronic devices such as radio-tracking collars that can be followed by radio, whether handheld, in a vehicle or aircraft, or by satellite. GPS animal tracking enables accurate positions to be broadcast at regular intervals, but the devices are inevitably heavier and more expensive than those without GPS. An alternative is the Argos Doppler tag, also called a 'Platform Transmitter Terminal' (PTT), which sends regularly to the polar-orbiting Argos satellites; using
Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who ...
, the animal's location can be estimated, relatively roughly compared to GPS, but at a lower cost and weight. A technology suitable for small birds which cannot carry the heavier devices is the geolocator which logs the light level as the bird flies, for analysis on recapture. There is scope for further development of systems able to track small animals globally. Radio-tracking tags can be fitted to insects, including dragonflies and
bees Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfam ...
.


In culture

Before animal migration was understood, various folklore and erroneous explanations were formulated to account for the disappearance or sudden arrival of birds in an area. In
Ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cu ...
,
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ...
proposed that robins turned into redstarts when summer arrived. The
barnacle goose The barnacle goose (''Branta leucopsis'') is a species of goose that belongs to the genus ''Branta'' of black geese, which contains species with largely black plumage, distinguishing them from the grey ''Anser'' species. Despite its superficial ...
was explained in European Medieval bestiaries and manuscripts as either growing like fruit on trees, or developing from goose barnacles on pieces of driftwood. Another example is the swallow, which was once thought, even by naturalists such as Gilbert White, to hibernate either underwater, buried in muddy riverbanks, or in hollow trees.


See also

*
Great American Interchange The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which lan ...


References


Further reading


General

* * Baker, R. R. (1978) ''The Evolutionary Ecology of Animal Migration''. Holmes & Meier. . * Dingle, H. (1996) ''Migration: The Biology of Life on the Move''.
Oxford University Press Oxford University Press (OUP) is the university press of the University of Oxford. It is the largest university press in the world, and its printing history dates back to the 1480s. Having been officially granted the legal right to print book ...
. . * Gauthreaux, S. A. (1980) ''Animal Migration, Orientation, and Navigation''.
Academic Press Academic Press (AP) is an academic book publisher founded in 1941. It was acquired by Harcourt, Brace & World in 1969. Reed Elsevier bought Harcourt in 2000, and Academic Press is now an imprint of Elsevier. Academic Press publishes refer ...
. . * Milner-Gulland, E. J., Fryxell, J. M., and Sinclair, A. R. E. (2011) ''Animal Migration: A Synthesis''.
Oxford University Press Oxford University Press (OUP) is the university press of the University of Oxford. It is the largest university press in the world, and its printing history dates back to the 1480s. Having been officially granted the legal right to print book ...
. . * Rankin, M. (1985) ''Migration: Mechanisms and Adaptive Significance: Contributions in Marine Science''. Marine Science Institute. . * Riede, K. (2002) ''Global Register of Migratory Species. With database and GIS maps on CD''. .


By group

* Drake, V. A. and Gatehouse, A. G. (1995) ''Insect migration: tracking resources through space and time''.
Cambridge University Press Cambridge University Press is the university press of the University of Cambridge. Granted letters patent by King Henry VIII in 1534, it is the oldest university press in the world. It is also the King's Printer. Cambridge University Pr ...
. * Elphick, J. (1995) ''The atlas of bird migration: tracing the great journeys of the world's birds''.
Random House Random House is an American book publisher and the largest general-interest paperback publisher in the world. The company has several independently managed subsidiaries around the world. It is part of Penguin Random House, which is owned by Germ ...
. * Greenberg, R. and Marra, P. P. (2005) ''Birds of Two Worlds: The Ecology and Evolution of Migration''.
Johns Hopkins University Press The Johns Hopkins University Press (also referred to as JHU Press or JHUP) is the publishing division of Johns Hopkins University. It was founded in 1878 and is the oldest continuously running university press in the United States. The press publ ...
. * Lucas, M. C. and Baras, E. (2001) ''Migration of freshwater fishes''. Blackwell Science. * MacKeown, B. A. (1984) ''Fish migration''.
Timber Press Workman Publishing Company, Inc., is an American publisher of trade books founded by Peter Workman. The company is comprised of either imprints: Workman, Workman Children’s, Workman Calendars, Artisan, Algonquin Books of Chapel Hill and Algonq ...
. * Sonnenschein, E; Berthold, P. (2003) ''Avian migration''. Springer.


For children

* Gans, R. and Mirocha, P. ''How do Birds Find their Way?''
HarperCollins HarperCollins Publishers LLC is one of the Big Five English-language publishing companies, alongside Penguin Random House, Simon & Schuster, Hachette, and Macmillan. The company is headquartered in New York City and is a subsidiary of News C ...
. (Stage 2) * Marsh, L. (2010) ''Amazing Animal Journeys''.
National Geographic Society The National Geographic Society (NGS), headquartered in Washington, D.C., United States, is one of the largest non-profit scientific and educational organizations in the world. Founded in 1888, its interests include geography, archaeology, ...
. (Level 3)


External links


Migration Basics from U.S. National Park Service



Global Register of Migratory Species
– identifies, maps and features 4,300 migratory vertebrate species
Animal migration on PubMed
MeSH ter
F01.145.113.083
{{Authority control Ethology