Periodic large-scale movement of animals, usually seasonal
"Irruption" redirects here. For rapid changes in population size, seeIrruptive growth.
Goose migration is an iconic migration phenomenon in parts of the Northern Hemisphere[1]
Animal migration is the relatively long-distance movement of individualanimals, usually on aseasonal basis. It is the most common form ofmigration in ecology. It is found in all major animal groups, includingbirds,mammals,fish,reptiles, amphibians,insects, andcrustaceans. 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 localdispersal 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 includeNorthern Hemisphere birds migrating south for the winter, orwildebeest migrating annually for seasonal grazing. A major habitat change could include youngAtlantic salmon or sea lamprey leaving the river of their birth when they have reached a few inches in size. Some traditional forms ofhuman migration fit this pattern.
Migrations can be studied using traditional identification tags such asbird 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 thatbarnacle geese grew fromgoose barnacles.
Migration can take very different forms in different species, and has a variety of causes.[2][3][4]As such, there is no simple accepted definition of migration.[5] One of the most commonly used definitions, proposed by the zoologist J. S. Kennedy[6] is
Migratory behavior is persistent and straightened-out movement effected by the animal's own locomotory exertions or by its active embarkation on a vehicle. It depends on some temporary inhibition of station-keeping responses, but promotes their eventual disinhibition and recurrence.[6]
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.[5] Migration can be eitherobligate, 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, anddifferential migration is when the difference between migratory and non-migratory individuals is based on discernible characteristics like age or sex.[5] Irregular (non-cyclical) migrations such as irruptions can occur under pressure of famine,overpopulation of a locality, or some more obscure influence.[7]
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.[8] 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.[9]
Circadian migration is where birds utilisecircadian rhythm (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.[10]
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.[11] The most common form of tidal migration is to and from theintertidal zone during daily tidal cycles.[11] 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 byocean currents.[12][13][14]
While most migratory movements occur on an annual cycle, some daily movements are also described as migration. Many aquatic animals make adiel vertical migration, travelling a few hundred metres up and down the water column,[15] while some jellyfish make daily horizontal migrations of a few hundred metres.[16]
Approximately 1,800 of the world's 10,000bird species migrate long distances each year in response to the seasons.[17] 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.[18] Some species extend this strategy to migrate annually between the Northern and Southern Hemispheres. TheArctic tern has the longest migration journey of any bird: it flies from itsArctic breeding grounds to theAntarctic and back again each year, a distance of at least 19,000 km (12,000 mi), giving it two summers every year.[19]
Bird migration is controlled primarily by day length, signalled by hormonal changes in the bird's body.[20] On migration, birdsnavigate using multiple senses. Many birds use a sun compass, requiring them to compensate for the sun's changing position with time of day.[21] Navigation involves the ability todetect magnetic fields.[22]
Most fish species are relatively limited in their movements, remaining in a single geographical area and making short migrations to overwinter, tospawn, 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 ofsalmon, migrate between saltwater and freshwater (they are 'diadromous').[23][24]
Forage fish such asherring andcapelin migrate around substantial parts of the NorthAtlantic 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 towardsJan Mayen island to feed and return to Iceland parallel with Greenland's east coast.[25]
An aggregation of migratoryPantala flavescens dragonflies, known as globe skimmers, inCoorg, India
Some wingedinsects such aslocusts and certainbutterflies anddragonflies with strong flight migrate long distances. Among the dragonflies, species ofLibellula andSympetrum are known for mass migration, whilePantala flavescens, known as the globe skimmer or wandering glider dragonfly, makes the longest ocean crossing of any insect: between India and Africa.[27] Exceptionally, swarms of the desert locust,Schistocerca gregaria, flew westwards across the Atlantic Ocean for 4,500 kilometres (2,800 mi) during October 1988, using air currents in theInter-Tropical Convergence Zone.[28]
In somemigratory butterflies, such as themonarch butterfly and thepainted lady, no individual completes the whole migration. Instead, the butterflies mate and reproduce on the journey, and successive generations continue the migration.[29]
Some mammals undertake exceptional migrations;reindeer have one of the longest terrestrial migrations on the planet, reaching as much as 4,868 kilometres (3,025 mi) 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 7,247 kilometres (4,503 mi).[30]
Mass migration occurs in mammals such as theSerengeti 'great migration',[31] an annual circular pattern of movement with some 1.7 millionwildebeest and hundreds of thousands of other large game animals, includinggazelles andzebra.[32][33] More than 20 such species engage, or used to engage, in mass migrations.[34] Of these migrations, those of thespringbok,black wildebeest,blesbok,scimitar-horned oryx, andkulan have ceased.[35] Long-distance migrations occur in some bats – notably the mass migration of theMexican free-tailed bat between Oregon and southern Mexico.[36] Migration is important incetaceans, including whales, dolphins and porpoises; some species travel long distances between their feeding and their breeding areas.[37]
Humans are mammals, buthuman migration, 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 astranshumance, in which herders and their animals move seasonally between mountains and valleys, and the seasonal movements ofnomads.[38][39]
Among the reptiles, adultsea 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.[40] Juvenilegreen sea turtles make use ofEarth's magnetic field to navigate.[41]
Some crustaceans migrate, such as the largely-terrestrialChristmas 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.[42][43]
A migratory butterfly, amonarch, tagged for identification
Scientists gather observations of animal migration by tracking their movements. Animals were traditionally tracked with identification tags such asbird 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 asradio-tracking collars that can be followed by radio, whether handheld, in a vehicle or aircraft, or by satellite.[44]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; usingDoppler shift, the animal's location can be estimated, relatively roughly compared to GPS, but at a lower cost and weight.[44] A technology suitable for small birds which cannot carry the heavier devices is thegeolocator whichlogs the light level as the bird flies, for analysis on recapture.[45] There is scope for further development of systems able to track small animals globally.[46]
Radio-tracking tags can be fitted to insects, includingdragonflies andbees.[47]
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. InAncient Greece,Aristotle proposed that robins turned intoredstarts when summer arrived.[48] Thebarnacle goose was explained in European Medieval bestiaries and manuscripts as either growing like fruit on trees, or developing fromgoose barnacles on pieces of driftwood.[49] Another example is theswallow, which was once thought, even bynaturalists such asGilbert White, tohibernate either underwater, buried in muddy riverbanks, or in hollow trees.[50]
^abKennedy, J. S. (1985). "Migration: Behavioral and ecological". In Rankin, M. (ed.).Migration: Mechanisms and Adaptive Significance: Contributions in Marine Science. Marine Science Institute. pp. 5–26.
^Hufnagl, M.; Temming, A.; Pohlmann, T. (2014). "The missing link: tidal-influenced activity a likely candidate to close the migration triangle in brown shrimpCrangon crangon (Crustacea, Decapoda)".Fisheries Oceanography.23 (3):242–257.Bibcode:2014FisOc..23..242H.doi:10.1111/fog.12059.
^McLaren, I. A. (1974). "Demographic strategy of vertical migration by a marine copepod".The American Naturalist.108 (959):91–102.doi:10.1086/282887.JSTOR2459738.S2CID83760473.
^Berthold, Peter; Bauer, Hans-Günther; Westhead, Valerie (2001).Bird Migration: A General Survey. Oxford: Oxford University Press.ISBN978-0-19-850787-1.
^Harden Jones, F. R.Fish Migration: strategy and tactics. pp139–166 in Aidley, 1981.
^Myers, George S. (1949). "Usage of Anadromous, Catadromous and allied terms for migratory fishes".Copeia.1949 (2):89–97.doi:10.2307/1438482.JSTOR1438482.
^Tipping, Christopher (8 May 1995)."Chapter 11: The Longest Migration".Book of Insect Records. Department of Entomology & Nematology University of Florida.Archived from the original on 24 September 2015.
^Lohmann, Kenneth J.; Lohmann, Catherine M. F.; Ehrhart, Llewellyn M.; Bagley, Dean A.; Swing, Timothy (2004). "Geomagnetic map used in sea-turtle navigation".Nature.428 (6986):909–910.doi:10.1038/428909a.PMID15118716.S2CID4329507.
^"Red Crabs". Parks Australia. 2013.Archived from the original on 3 July 2014. Retrieved19 June 2014.
^Wikelski, Martin; Kays, Roland W.; Kasdin, N. Jeremy; Thorup, Kasper; Smith, James A.; Swenson, George W. (15 January 2007). "Going wild: what a global small-animal tracking system could do for experimental biologists".Journal of Experimental Biology.210 (2). The Company of Biologists:181–186.doi:10.1242/jeb.02629.ISSN1477-9145.PMID17210955.S2CID8073226.
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