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Bird migration is a seasonal movement of birds between breeding and wintering grounds that occurs twice a year. It is typically from north to south or from south to north. Animal migration is inherently risky, due to predation and mortality.
The Arctic tern holds the long-distance migration record for birds, travelling between Arctic breeding grounds and the Antarctic each year. Some species of tubenoses, such as , circle the Earth, flying over the southern oceans, while others such as Manx shearwaters migrate between their northern breeding grounds and the southern ocean. Shorter migrations are common, while longer ones are not. The shorter migrations include altitudinal migrations on mountains, including the Andes and Himalayas.
The timing of migration seems to be controlled primarily by changes in day length. Migrating birds navigate using celestial cues from the Sun and stars, the Earth's magnetic field, and mental maps.
Writings of Ancient Greece recognized the seasonal comings and goings of birds. Aristotle recorded that cranes traveled from the steppes of Scythia to marshes at the headwaters of the Nile, an observation repeated by Pliny the Elder in his Historia Naturalis. Aristotle, however, suggested that swallows and other birds hibernated. This belief persisted as late as 1878 when Elliott Coues listed the titles of no fewer than 182 papers dealing with the hibernation of swallows. Even the "highly observant" (2025). 9780701169077, Chatto & Windus. ISBN 9780701169077 Gilbert White, in his posthumously published 1789 The Natural History of Selborne, quoted a man's story about swallows being found in a chalk cliff collapse "while he was a schoolboy at Brighthelmstone", though the man denied being an eyewitness. However, he writes that "as to swallows being found in a torpid state during the winter in the Isle of Wight or any part of this country, I never heard any such account worth attending to",White, 1898. pp. 27–28 and that if early swallows "happen to find frost and snow they immediately withdraw for a time—a circumstance this much more in favour of hiding than migration", since he doubts they would "return for a week or two to warmer latitudes".White, 1898. pp. 161–162
Only at the end of the eighteenth century was migration accepted as an explanation for the winter disappearance of birds from northern climes. Thomas Bewick's A History of British Birds (Volume 1, 1797) mentions a report from "a very intelligent master of a vessel" who, "between the islands of Menorca and Majorca, saw great numbers of Swallows flying northward",Bewick, 1797. p. xvii and states the situation in Britain as follows:
Bewick then describes an experiment that succeeded in keeping swallows alive in Britain for several years, where they remained warm and dry through the winters. He concludes:
In 1822, a white stork was found in the German state of Mecklenburg with an arrow made from central African hardwood, which provided some of the earliest evidence of long-distance stork migration. Flyer for the Rostock University Zoological Collection Der Sproessling 3 edition of the local student association's magazine containing an article about the Pfeilstorch This bird was referred to as a Pfeilstorch, German for "Arrow stork". Since then, around 25 Pfeilstörche have been documented.
Many bird populations migrate long distances along a flyway. The most common pattern involves flying north in the spring to breed in the temperate or Arctic summer and returning in the autumn to wintering grounds in warmer regions to the south. In the southern hemisphere, the directions are reversed, but there is less land area in the far south to support long-distance migration.
The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter. In addition, the longer days of the northern summer provide extended time for breeding birds to feed their young. This helps Diurnality birds to produce larger clutches than related non-migratory species that remain in the tropics. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.
These advantages offset the high stress, physical exertion costs, and other risks of migration. Predation can be heightened during migration: Eleonora's falcon Falco eleonorae, which breeds on Mediterranean islands, has a very late breeding season, coordinated with the autumn passage of southbound passerine migrants, which it feeds to its young. A similar strategy is adopted by the greater noctule bat, which preys on nocturnal passerine migrants. The higher concentrations of migrating birds at stopover sites make them prone to parasites and pathogens, which require a heightened immune response.
Within a species not all populations may be migratory; this is known as "partial migration". Partial migration is very common in the southern continents; in Australia, 44% of non-passerine birds and 32% of passerine species are partially migratory. In some species, the population at higher latitudes tends to be migratory and will often winter at lower latitude. The migrating birds bypass the latitudes where other populations may be sedentary, where suitable wintering habitats may already be occupied. This is an example of leap-frog migration. Many fully migratory species show leap-frog migration (birds that nest at higher latitudes spend the winter at lower latitudes), and many show the alternative, chain migration, where populations 'slide' more evenly north and south without reversing the order.
Within a population, it is common for different ages and/or sexes to have different patterns of timing and distance. Female Common chaffinch Fringilla coelebs in Eastern Fennoscandia migrate earlier in the autumn than males do and the European tits of genera Parus and Cyanistes only migrate in their first year.Ketterson, E. D., and V. Nolan. 1985. Intraspecific variation in avian migration: evolutionary and regulatory aspects, Pages 553–579 in M. A. Rankin, ed. Migration: mechanisms and adaptive significance, University of Texas, Austin.
Most migrations begin with the birds starting off in a broad front. Often, this front narrows into one or more preferred routes termed . These routes typically follow mountain ranges or coastlines, sometimes rivers, and may take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water. The specific routes may be genetically programmed or learned to varying degrees. The routes taken on forward and return migration are often different. A common pattern in North America is clockwise migration, where birds flying North tend to be further West, and flying South tend to shift Eastwards.
Many, if not most, birds migrate in flocks. For larger birds, flying in flocks reduces the energy cost. Geese in a V formation may conserve 12–20% of the energy they would need to fly alone. Red knots Calidris canutus and dunlins Calidris alpina were found in radar studies to fly faster in flocks than when they were flying alone.
Birds fly at varying altitudes during migration. An expedition to Mount Everest found skeletons of northern pintail Anas acuta and black-tailed godwit Limosa limosa at on the Khumbu Glacier. Bar-headed goose Anser indicus have been recorded by GPS flying at up to while crossing the Himalayas, at the same time engaging in the highest rates of climb to altitude for any bird. Anecdotal reports of them flying much higher have yet to be corroborated with any direct evidence. Seabirds fly low over water but gain altitude when crossing land, and the reverse pattern is seen in land birds. However most bird migration is in the range of . Bird strike Aviation records from the United States show most collisions occur below and almost none above .
Bird migration is not limited to birds that can fly. Most species of penguin (Spheniscidae) migrate by swimming. These routes can cover over . Dusky grouse Dendragapus obscurus perform altitudinal migration mostly by walking. Dromaius novaehollandiae in Australia have been observed to undertake long-distance movements on foot during droughts.
Nocturnal migrants land in the morning and may feed for a few days before resuming their migration. These birds are referred to as passage migrants in the regions where they occur for a short period between the origin and destination.
Nocturnal migrants minimize depredation, avoid overheating and can feed during the day. One cost of nocturnal migration is the loss of sleep. Migrants may be able to alter their quality of sleep to compensate for the loss.
Migration routes and wintering grounds are both genetically and traditionally determined depending on the social system of the species. In long-lived, social species such as (Ciconia ciconia), flocks are often led by the oldest members and young storks learn the route on their first journey. In short-lived species that migrate alone, such as the Eurasian blackcap Sylvia atricapilla or the yellow-billed cuckoo Coccyzus americanus, first-year migrants follow a genetically determined route that is alterable with selective breeding.
Many migration routes of long-distance migratory birds are circuitous due to evolutionary history: the breeding range of Northern wheatears Oenanthe oenanthe has expanded to cover the entire Northern Hemisphere, but the species still migrates up to 14,500 km to reach ancestral wintering grounds in sub-Saharan Africa rather than establish new wintering grounds closer to breeding areas.
A migration route often does not follow the most direct line between breeding and wintering grounds. Rather, it could follow a hooked or arched line, with detours around geographical barriers or towards suitable stopover habitat. For most land birds, such barriers could consist of large water bodies or high mountain ranges, a lack of stopover or feeding sites, or a lack of (important for broad-winged birds).
Conversely, in water birds, large areas of land without wetlands offering suitable feeding sites may present a barrier, and detours avoiding such barriers are observed. For example, brent geese Branta bernicla bernicla migrating between the Taymyr Peninsula and the Wadden Sea travel via low-lying coastal feeding-areas on the White Sea and the Baltic Sea rather than directly across the Arctic Ocean and the mainland.
(breeding and wintering ranges with subspecies' flyway maps; diet)
make non-stop flights of 4,000–7,000 km, lasting 60–90 h, during which they change their average cruising heights from 2,000 m (above sea level) at night to around 4,000 m during daytime.
For some species of waders, migration success depends on the availability of certain key food resources at stopover points along the migration route. This gives the migrants an opportunity to refuel for the next leg of the voyage. Some examples of important stopover locations are the Bay of Fundy and Delaware River.
Some bar-tailed godwits Limosa lapponica baueri have the longest known non-stop flight of any migrant, flying 11,000 km from Alaska to their New Zealand non-breeding areas. Prior to migration, 55 percent of their bodyweight is stored as fat to fuel this uninterrupted journey.
The most pelagic species, mainly in the 'tubenose' order Procellariiformes, are great wanderers, and the of the southern oceans may circle the globe as they ride the "Roaring Forties" outside the breeding season. The tubenoses spread widely over large areas of open ocean, but congregate when food becomes available. Many are among the longest-distance migrants; Puffinus griseus nesting on the Falkland Islands migrate between the breeding colony and the Atlantic Ocean off Norway. Some Puffinus puffinus do this same journey in reverse. As they are long-lived birds, they may cover enormous distances during their lives; one record-breaking Manx shearwater is calculated to have flown during its over-50-year lifespan.
From observing the migration of eleven soaring bird species over the Strait of Gibraltar, species which did not advance their autumn migration dates were those with declining breeding populations in Europe.
Short-distance passerine migrants have two evolutionary origins. Those that have long-distance migrants in the same family, such as the common chiffchaff Phylloscopus collybita, are species of southern hemisphere origins that have progressively shortened their return migration to stay in the northern hemisphere.Cocker, 2005. p. 378
Species that have no long-distance migratory relatives, such as the Bombycilla, are effectively moving in response to winter weather and the loss of their usual winter food, rather than enhanced breeding opportunities.Cocker, 2005. p. 326
In the tropics there is little variation in the length of day throughout the year, and it is always warm enough for a food supply, but altitudinal migration occurs in some tropical birds. There is evidence that this enables the migrants to obtain more of their preferred foods such as fruits.
Altitudinal migration is common on mountains worldwide, such as in the Himalayas and the Andes. Dusky grouse in Colorado migrate less than a kilometer away from their summer grounds to winter sites which may be higher or lower by about 400 m in altitude than the summer sites.
Many bird species in arid regions across southern Australia are nomadic; they follow water and food supply around the country in an irregular pattern, unrelated to season but related to rainfall. Several years may pass between visits to an area by a particular species.
Bird migration is primarily, but not entirely, a Northern Hemisphere phenomenon. This is because continental landmasses of the northern hemisphere are almost entirely temperate and subject to winter food shortages driving bird populations south (including the Southern Hemisphere) to overwinter; In contrast, among (pelagic) seabirds, species of the Southern Hemisphere are more likely to migrate. This is because there is a large area of ocean in the Southern Hemisphere, and more islands suitable for seabirds to nest.
Satellite tracking of 48 individual Asian houbaras ( Chlamydotis macqueenii) across multiple migrations showed that this species uses the local temperature to time their spring migration departure. Notably, departure responses to temperature varied between individuals but were individually repeatable (when tracked over multiple years). This suggests that individual use of temperature is a cue that allows for population-level adaptation to climate change. In other words, in a warming world, many migratory birds are predicted to depart earlier in the year for their summer or winter destination.
In polygynous species with considerable sexual dimorphism, males tend to return earlier to the breeding sites than their females. This is termed protandry.
Long-distance migrants are believed to disperse as young birds and form attachments to potential breeding sites and to favourite wintering sites. Once the site attachment is made they show high site-fidelity, visiting the same wintering sites year after year.
The ability of birds to navigate during migrations cannot be fully explained by endogenous programming, even with the help of responses to environmental cues. The ability to successfully perform long-distance migrations can probably only be fully explained with an accounting for the cognitive ability of the birds to recognize habitats and form mental maps. Satellite tracking of day migrating raptors such as ospreys and honey buzzards has shown that older individuals are better at making corrections for wind drift. Birds rely for navigation on a combination of innate biological senses and experience, as with the two Electromagnetism tools that they use. A young bird on its first migration flies in the correct direction according to the Earth's magnetic field, but does not know how far the journey will be. It does this through a radical pair mechanism whereby chemical reactions in special photo pigments sensitive to short wavelengths are affected by the field. Although this only works during daylight hours, it does not use the position of the Sun in any way. With experience, it learns various landmarks and this "mapping" is done by in the trigeminal system, which tell the bird how strong the field is. Because birds migrate between northern and southern regions, the magnetic field strengths at different let it interpret the radical pair mechanism more accurately and let it know when it has reached its destination. There is a neural connection between the eye and "Cluster N", the part of the forebrain that is active during migrational orientation, suggesting that birds may actually be able to see the magnetic field of the Earth.
Reverse migration, where the genetic programming of young birds fails to work properly, can lead to rarities turning up as vagrants thousands of kilometres out of range.
Drift migration of birds blown off course by the wind can result in "falls" of large numbers of migrants at coastal sites.
A related phenomenon called "abmigration" involves birds from one region joining similar birds from a different breeding region in the common winter grounds and then migrating back along with the new population. This is especially common in some waterfowl, which shift from one flyway to another.
Theoretical analyses show that detours that increase flight distance by up to 20% will often be adaptive on aerodynamics grounds – a bird that loads itself with food to cross a long barrier flies less efficiently. However some species show circuitous migratory routes that reflect historical range expansions and are far from optimal in ecological terms. An example is the migration of continental populations of Swainson's thrush Catharus ustulatus, which fly far east across North America before turning south via Florida to reach northern South America; this route is believed to be the consequence of a range expansion that occurred about 10,000 years ago. Detours may also be caused by differential wind conditions, predation risk, or other factors.
Bird migration is generally synchronised to take advantage of seasonal resources. For example, there is a strong link between seasonal migration and vegetation greenness in North America. Climate-induced shifts in the phenology of seasonal resource availability can cause mismatches between the timing of increased resource availability and important life-history events such as migration and breeding (aka phenological mismatch or phenological asynchrony). These mismatches between the timing of resource availability and when organisms need additional resources may impact species’ fitness, as described by the match-mismatch hypothesis.
In birds, individuals may use local temperature as a cue for migration. Changing temperature patterns due to climate change can result in population-level shifts in migration phenology. Such shifts in the timing of migration of hundreds of species are already detectable at the continental scale. While phenological mismatches appear to be more pronounced in long-distance migrants, certain species traits such as a generalist diet may help some species avoid more severe consequences of mismatches.
Some predators take advantage of the concentration of birds during migration. Greater noctule bats feed on nocturnal migrating passerines. Some birds of prey specialize on migrating waders.
Bird migration routes have been studied by a variety of techniques including the oldest, marking. Swans have been marked with a nick on the beak since about 1560 in England. Scientific Bird ringing was pioneered by Hans Christian Cornelius Mortensen in 1899.Spencer, R. (1985) Marking. In: Campbell. B. & Lack, E. 1985. A dictionary of birds. British Ornithologists' Union. London, pp. 338–341. Other techniques include radar and GPS satellite. The rate of bird migration over the Alps (up to a height of 150 m) was found to be highly comparable between fixed-beam radar measurements and visual bird counts, highlighting the potential use of this technique as an objective way of quantifying bird migration.
Stable isotopes of hydrogen, oxygen, carbon, nitrogen, and sulphur can establish avian migratory connectivity between wintering sites and breeding grounds. Stable isotopic methods to establish migratory linkage rely on spatial isotopic differences in bird diet that are incorporated into inert tissues like feathers, or into growing tissues such as claws and muscle or blood.
An approach to identify migration intensity makes use of upward pointing microphones to record the nocturnal contact calls of flocks flying overhead. These are then analyzed in a laboratory to measure time, frequency and species.
An older technique developed by George Lowery and others to quantify migration involves observing the face of the full moon with a telescope and counting the silhouettes of flocks of birds as they fly at night.
Orientation behaviour studies have been traditionally carried out using variants of a setup known as the Emlen funnel, which consists of a circular cage with the top covered by glass or wire-screen so that either the sky is visible or the setup is placed in a planetarium or with other controls on environmental cues. The orientation behaviour of the bird inside the cage is studied quantitatively using the distribution of marks that the bird leaves on the walls of the cage. Other approaches used in pigeon homing studies make use of the direction in which the bird vanishes on the horizon.Alerstam, 1993. p.352
]] Human activities have threatened many migratory bird species. The distances involved in bird migration mean that they often cross political boundaries of countries and conservation measures require international cooperation. Several international treaties have been signed to protect migratory species including the Migratory Bird Treaty Act of 1918 of the US. and the African-Eurasian Migratory Waterbird Agreement
The concentration of birds during migration can put species at risk. Some spectacular migrants have already gone extinct; during the passenger pigeon's ( Ectopistes migratorius) migration the enormous flocks were wide, darkening the sky, and long, taking several days to pass.
Hunting along migration routes threatens some bird species. The populations of ( Leucogeranus leucogeranus) that wintered in India declined due to hunting along the route, particularly in Afghanistan and Central Asia. Birds were last seen in their favourite wintering grounds in Keoladeo National Park in 2002. Structures such as power lines, wind farms and offshore oil-rigs have also been known to affect migratory birds. Other migration hazards include pollution, storms, wildfires, and habitat destruction along migration routes, denying migrants food at stopover points. For example, in the East Asian–Australasian Flyway, up to 65% of key intertidal habitat at the Yellow Sea migration bottleneck has been destroyed since the 1950s.
Other significant areas include stop-over sites between the wintering and breeding territories. A capture-recapture study of passerine migrants with high fidelity for breeding and wintering sites did not show similar strict association with stop-over sites. Unfortunately, many historic stopover sites have been destroyed or drastically reduced due to human agricultural development, leading to an increased risk of bird extinction, especially in the face of climate change.
Conversely, so-called "ship-assisted migration" may be a modern benefit to migrating birds by giving them a mid-ocean rest stop on ships.
Plant debris provides food sources for the birds while the newly formed wetland serves as a habitat for bird prey species such as insects and other invertebrates. In turn, bird foraging assists in breaking down plant matter. Droppings then help to fertilize the field, helping the farmers and in turn significantly decreasing their need for artificial fertilizers by at least 13 percent. Recent studies have shown that the implementation of these temporary wetlands has had significant positive impacts on bird populations, such as the White-fronted Goose, as well as various species of wading birds. The artificial nature of these temporary wetlands also greatly reduces the threat of predation from other wild animals. This practice requires extremely low investment on behalf of the farmers, and researchers believe that mutually beneficial approaches such as this are key to wildlife conservation moving forward. Economic incentives are key to getting more farmers to participate in this practice. However, issues can arise if bird populations are too high with their large amounts of droppings decreasing water quality and potentially leading to eutrophication. Increasing participation in this practice would allow migratory birds to spread out and rest on a wider variety of locations, decreasing the negative impacts of having too many birds congregated in a small area. Using this practice in areas with close proximity to natural wetlands could also greatly increase their positive impact.
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