Bird migration
Bird migration refers to the regular seasonal journeys of varying distances to and from a given area undertaken by many (but not all) species of birds. In contrast to more irregular patterns of movement made in response to changes in food availability, habitat, or weather, bird migration is marked by its seasonality (also return trip).
The most common pattern among the migratory birds of Europe and North America involves flying north to breed in the temperate or arctic summer and returning to wintering grounds in warmer regions to the south. However, there are other patterns of migration: in tropical regions, for example, some species migrate in response to the cycle of wet and dry seasons. In mountainous areas, like the Himalayas, vertical movements may occur - from higher breeding grounds to areas of lower altitude with more temperate weather.
The primary advantage of migration is energetic. The long days of the northern summer provide greater opportunities for breeding birds to feed their young. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season. Moreover, migratory birds have evolved to undertake long-distance flights efficiently, and they undergo physiological changes (such as an accumulation of fat stores) prior to migration that minimize the energetic cost of flight.
Migrations typically occur along established routes called flyways, with the migrating species often returning to the area of their birth to breed. The birds are guided by innate behaviors (including hormonal signals) that enable them to know when to depart and that guide to a specific location over long distances; however, they also remain flexible to environmental conditions, such as food supply and temperature, which may fluctuate yearly.
Bird migration has larger ecological implications that underscore the interconnectedness of life: migratory cycles are closely attuned to the seasonal food productivity cycles of habitats, creating a mutual gain for both the migrating species and the areas to which they travel. The birds are enabled to settle in ares where life is not tenable year-round, while the food resources of some regions would not be adequately utilized without the seasonal presence of migrating popultaions.
Bird species have evolved diverse modes of migration
The diverse patterns and modes of bird migration may be understood as evolutionary adaptations occuring over time. In fact, migration itself has conferred an evolutionary advantage to certain bird species, while not evolving in other species that remain resident or sedentary year-round. Even within a given species, not all populations may be migratory, a phenomenon termed partial migration. Partial migration is very common in the southern continents; in Australia, 32% of passerine (perching) species and 44% of non-passerine birds were foundd to be partially migratory (Chan, 2001). Moreover, within a specific population, there can be different patterns of timing and migration based on characteristics like age and sex. For example, only the female Chaffinches of Scandinavia migrate, while the males stay resident, a migratory pattern that has given rise to the name coelebs, meaning "bachelor."
add paragraph on distance? Some Alaskan Bar-tailed Godwits have the longest non-stop flight of any migrant, flying 11,000 km to their New Zealand non-breeding areas (BTO News 258: 3, 2005). Prior to migration, 55% of their bodyweight is stored fat to fuel this uninterrupted journey. The Arctic Tern has the longest-distance migration of any bird, and sees more daylight than any other, moving from its Arctic breeding grounds to the Antarctic non-breeding areas. One Arctic Tern, ringed (banded) as a chick on the Farne Islands off the British east coast, reached Melbourne, Australia in just three months from fledging, a sea journey of over 22,000 km (14,000 miles).
Other species such as Merlin and Skylark will move further to the coast or to a more southerly region. Species like the Chaffinch are not migratory in Britain, but will move south or to Ireland in very cold weather.
Short-distance passerine migrants have two evolutionary origins. Those which have long-distance migrants in the same family, such as the Chiffchaff, are species of southern hemisphere origins which have progressively shortened their return migration so that they stay in the northern hemisphere.
Those species which have no long-distance migratory relatives, such as the waxwings, are effectively moving in response to winter weather, rather than enhanced breeding opportunities.
Migrations may be diurnal (occuring during the day) or nocturnal. Many of the smaller insectivorous birds, including the warblers, hummingbirds and flycatchers are nocturnal migrants. By migrating at night, they minimize the risk of predation, and avoid the overheating that could result from the energy expended to fly such long distances. Those smaller species that migrate during the day tend to be those making movements that are relatively short and weather-driven, like the larks and finches, or that can feed on the wing, like swallows and swifts.
The altitude at which birds fly during migration also varies. In general, migratory birds fly at low altitude, with most migrations in the range of 500-2000 feet. However, an expedition to Mt. Everest found skeletons of Pintail and Black-tailed Godwit at 16,400 feet on the Khumbu Glacier (Geroudet, 1995). Bar-headed Geese have been seen flying over the highest peaks of the Himalayas above 29,000 feet even when low passes of 10,000 feet were nearby (Swan, 1970).
Migratory birds follow established routes
Migration is often concentrated along well-established routes known as flyways, which are shaped by geographical, ecological, and even metereological factors. These routes typically follow mountain ranges or coastlines, and may take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water.
The same considerations about barriers and detours that apply to long-distance land-bird migration apply to water birds, but in reverse: a large area of land without bodies of water that offer feeding sites is a barrier to a water bird. Open sea may also be a barrier to a bird that feeds in coastal waters. Detours avoiding such barriers have been observed: for example, Brent Geese migrating from the Taymyr Peninsula to the Wadden Sea travel via the White Sea coast and the Baltic Sea rather than directly across the Arctic Ocean and northern Scandinavia.
Some large broad-winged birds, for example, rely on thermal columns of rising hot air to enable them to soar. These include many birds of prey, such as vultures, eagles, and buzzards, as well as storks. Migratory species in these groups have great difficulty crossing large bodies of water, since thermals only form over land. The Mediterranean and other seas therefore present a major obstacle to soaring birds, which are forced to cross at the narrowest points. Massive numbers of large raptors and storks pass through areas such as Gibraltar, Falsterbo, and the Bosphorus at migration times. More common species, such as the Honey Buzzard, can be counted in hundreds of thousands in autumn. Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants. This is a notable factor in the Central American migratory bottleneck.
By following established routes, some species risk predation during periods of peak migration. For example, the Eleonora's Falcon, 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 (Dondini, et al, 2000; Popa-Lisseanu, et al, 2007; Ibáñez, et al, 2001).
Despite the genetic and environmental factors that guide them along specific routes, migrating birds can still lose their way. In a phenomenon known as the "spring overshoot," birds returning to their breeding areas overshoot their destination and end up further north than intended. A mechanism which can lead to great rarities turning up as vagrants thousands of kilometers out of range is reverse migration, in which the genetic programming that guides young birds fails to work properly. Drift migration of birds blown off course by the wind can result in "falls" of large numbers of migrants at coastal sites.
Patterns of migration
Many migratory European and North American species fly south in winter
The distance traveled by migratory birds of the Northern hemisphere varies widely. Some European birds, such as the insect-eating warblers, flycatchers, and wagtails, as well as swallows and storks migrate to areas of Africa south of the Sahara. North American birds, like the ruby-thrated hummingbird, which breeds in southern Canada, may travel as far south as Panama for the winter; other species, like the American robin and several species of grackles, winter in the states along the Gulf Coast.
Many northern-breeding ducks, geese and swans are also long-distance migrants, but need only to move from their Arctic breeding grounds far enough south to escape frozen waters. Most Holarctic wildfowl species remain in the Northern hemisphere, but in countries with milder climates. For example, the Pink-footed Goose migrates from Iceland to Britain and neighbouring countries.
A similar situation occurs with waders (called "shorebirds" in North America). Many species, such as Dunlin and Western Sandpiper, undertake long movements from their Arctic breeding grounds to warmer locations in the same hemisphere, but others such as Semipalmated Sandpiper travel greater distances to the tropics. Like the large and powerful wildfowl, the waders are strong fliers. This means that birds wintering in temperate regions have the capacity to make further shorter movements in the event of particularly inclement weather.
Some species of the Southern Hemisphere winter in northern areas
Bird migration is primarily, but not entirely, a Northern Hemisphere phenomenon. In the Southern Hemisphere, seasonal migration tends to be less noted due to several factors:
- The largely uninterrupted expanses of land mass and ocean tend not to funnel migrations into narrow pathways, making them less obvious to the human observer.
- For terrestrial birds, climatic regions tend to fade into one another over a long distance rather than be entirely separate: this means that rather than make long trips over unsuitable habitat to reach particular destinations, migrant species can usually travel at a relaxed pace, feeding as they go. Short of banding studies, it is often not obvious that the birds seen in any particular locality as the seasons change are in fact different members of the same species passing through, gradually working their way north or south.
Many species do in fact breed in the temperate southern hemisphere regions and winter further north in the tropics. The southern African Greater Striped Swallow, and the Australian Satin Flycatcher, Dollarbird, and Rainbow Bee-eater, for example, winter well north of their breeding range.
A few seabirds, such as Wilson's Petrel and Great Shearwater, breed in the southern hemisphere and migrate north in the southern winter.
There are two types of migrating seabirds
Seabird migration may be characterized as coastal, with species staying along the continental shelf, or pelagic, with species ranging across the open sea. The former category includes birds such as the guillemots, auks, cormorants, gannets, and gulls, which are all found along the seashore.
The most pelagic species, mainly in the "tubenose" order Procellariiformes (petrels and albatrosses), are great wanderers. The albatrosses 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 also among the longest-distance migrants; Sooty Shearwaters nesting on the Falkland Islands migrate 14,000 km (9,000 miles) between the breeding colony and the North Atlantic Ocean off Norway. Some Manx Shearwaters 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 8 million km (5 million miles) during its lifespan of over 50 years.
Birds in tropical regions migrate according to a cycle of wet and dry seasons
In the tropics, there is little variation in the length of day throughout the year, and it is always warm enough for an adequate food supply. Apart from the seasonal movements of northern hemisphere wintering species, most species are in the broadest sense resident. There are a few species, notably cuckoos, which are genuine long-distance migrants within the tropics. An example is the Lesser Cuckoo, which breeds in India and spends the non-breeding season in Africa.
Howeve, some tropical species undergo movements of varying distances depending on the rainfall. Many tropical regions have cycles of wet and dry seasons, the monsoons of India being perhaps the best-known example. An example of a bird whose distribution is rain associated is the Woodland Kingfisher of west Africa.
Bird migrations may involve vertical movements
Some migrations involvement changes in altitude from the upper breeding zones to the foothills or plains during unfavorable weather. For example, mountain and moorland breeders, such as the Wallcreeper and White-throated Dipper, may move altitudinally to escape the cold higher ground. In the high mountains, such as the Himalayas and the Andes, there are also seasonal altitudinal movements in many species, and others may undertake migrations of considerable length. The Himalayan Kashmir Flycatcher and Pied Thrush both move as far south as the highlands of Sri Lanka.
Migratory birds follow a mix of endogenous and environmental signals
The control of migration, its timing and response are genetically controlled and appear to be a primitive trait that is present even in non-migratory species of birds. The ability to navigate and orient themselves during migration is a much more complex phenomenon which may include both endogenous programs as well as learning (Helm and Gwinner, 2006).
Physiological changes prepare migratory birds for flight
The primary physiological cue for migration are the changes in the day length. These changes are also related to hormonal changes in the birds.
In the period before migration, many birds display higher activity or Zugunruhe (German: migratory restlessness) as well as physiological changes such as increased fat deposition (explain why fat might accumulate). The occurrence of Zugunruhe even in cage-raised birds with no environmental cues (e.g. shortening of day and falling temperature) has pointed to the role of circannual endogenic programming in controlling bird migrations. Caged birds display a preferential flight direction that corresponds with the migratory direction they would take in nature, even changing their preferential direction at roughly the same time their wild conspecifics change course.
Given the central importance of proteins to life, particularly the importance of strong muscles for survival, animals are designed to minimize the loss of protein from muscle during periods of starvation. When dietary proteins and carbohydrates are deficient, proteins may be broken down to synthesize glucose to supply organs, like the brain, that normally utilize glucose as a fuel. However, over a period of days, the body’s metabolism switches to the breakdown of ‘’fats’’, the storage form of fatty acids, which can be precursors for ketone bodies, an alternative fuel for the brain. This mechanism also works to the advantage of migratory birds, such as the ruby-throated hummingbird, which build up their fat stores before journeying long distances over water. The brain’s transition from glucose to ketone bodies occurs quite rapidly, so that hardly any protein in muscle is lost, enabling them to make their arduous, 2,400-kilometer flight.
Navigation is based on a variety of senses. Many birds have been shown to use a sun compass. Using the sun for direction involves the need for making compensation based on the time. Navigation has also been shown to be based on a combination of other abilities including the ability to detect magnetic fields, use visual landmarks as well as olfactory cues (Walraff, 2005).
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.
As the circannual patterns indicate, there is a strong genetic component to migration in terms of timing and route, but this may be modified by environmental influences. An interesting example where a change of migration route has occurred because of such a geographical barrier is the trend for some Blackcaps in central Europe to migrate west and winter in Britain rather than cross the Alps.
Migratory birds may use two electromagnetic tools to find their destinations: one that is entirely innate and another that relies on experience. 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 long wavelengths are affected by the field. Note that although this only works during daylight hours, it does not use the position of the sun in any way. At this stage the bird is similar to a boy scout with a compass but no map, until it grows accustomed to the journey and can put its other facilities to use. The "mapping" is done by magnetites 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 latitudes let it interpret the radical pair mechanism more accurately and let it know when it has reached its destination (Wiltschko, et al, 2006).
Bird migration is part of a larger ecological system
Whether a particular species migrates depends on a number of factors. The climate of the breeding area is important, and few species can cope with the harsh winters of inland Canada or northern Eurasia. Thus the partially migratory Blackbird Turdus merula is migratory in Scandinavia, but not in the milder climate of southern Europe. The nature of the staple food is also significant. Most specialist insect eaters outside the tropics are long-distance migrants, and have little choice but to head south in winter.
Sometimes the factors are finely balanced. The Whinchat Saxicola rubetra of Europe and the Siberian Stonechat Saxicola maura of Asia are long-distance migrants wintering in the tropics, whereas their close relative, the European Stonechat Saxicola rubicola is a resident bird in most of its range, and moves only short distances from the colder north and east. A possible factor here is that the resident species can often raise an extra brood.
Recent research suggests that long-distance passerine migrants are of South American and African, rather than northern hemisphere, evolutionary origins. They are effectively southern species coming north to breed rather than northern species going south to winter.
Theoretical analyses, summarized by Alerstam (2001), show that detours that increase flight distance by up to 20% will often be adaptive on aerodynamic grounds - a bird that loads itself with food in order 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, 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.
Historical background and modern study techniques
The earliest recorded observations of bird migration were 3000 years ago, as noted by Hesiod, Homer, Herodotus, Aristotle and others. Aristotle noted that cranes traveled from the steppes of Scythia to marshes at the headwaters of the Nile. Ply the Elder in his Historia Naturalis repeats Aristotle's observations. Aristotle however suggested that swallows and other birds hibernated. This belief persisted as late as 1878, when Elliott Coues listed the titles of no less than 182 papers dealing with the hibernation of swallows. It was not until early in the nineteenth century that migration as an explanation for the winter disappearance of birds from northern climes was accepted (Lincoln, 1979).
Bird migration has been studied by a variety of techniques of which ringing is the oldest. Color marking, use of radar, satellite tracking and stable Hydrogen and Strontium isotopes are some of the other techniques being used to study the migration of birds (Font et al, 2007).
Another approach to identify migration intensity makes use of upward pointing microphones to record the contact calls of overflying flocks. These are then analyzed in a laboratory to measure time, frequency and species (Farnsworth et al, 2004).
An older observational approach to studying the intensity of migration involves telescope observation of the face of the moon towards full moon and noting the flocks of birds as they fly at night (Liechti, 1996; Lowery, 1951).
ReferencesISBN links support NWE through referral fees
- Boland, J. M. 1990. Leapfrog migration in North American shorebirds: intra- and interspecific examples. The Condor. 92:284-290.
- Chan, K. 2001. Partial migration in Australian landbirds: a review. Emu 101(4): 281-92.
- Dondini, G. and S. Vergari. 2000. Carnivory in the greater noctule bat (Nyctalus lasiopterus) in Italy. Journal of Zoology 251: 233-6.
- Dorst, J. 1963. The Migration of Birds. Boston, MA: Houghton Mifflin.
- Eastwood, E. and G. C. Rider. 1965. Some radar measurements of the altitude of bird flight. Brit Birds 58:393-426.
- Farnsworth, A., Gauthreaux, S.A., and D. van Blaricom. 2004. A comparison of nocturnal call counts of migrating birds and reflectivity measurements on Doppler radar. Journal of Avian Biology 35:365-9.
- Font, L., Geoff, M., Nowell, D., Pearson, G., Ottley, C.J. and S.G. Willis. 2007. Sr isotope analysis of bird feathers by TIMS: a tool to trace bird migration paths and breeding sites. J Anal At Spectrom 22:513.
- Geroudet, P. 1954. Des oiseaux migrateurs trouves sur la glacier de Khumbu dans l'Himalaya. Nos Oiseaux 22:254.
- Helm, B. and E. Gwinner. 2006. Migratory Restlessness in an Equatorial Nonmigratory Bird. PLoS Biol 4(4): e110.
- Ibáñez, C., Juste, J., García-Mudarra, J. L., and P. T. Agirre-Mendi. 2001. Bat predation on nocturnally migrating birds. PNAS 98:9700-2.
- Liechti, F. 1996. Instructions to count nocturnal bird migration by watching the full moon. Schweizerische Vogelwarte, CH-6204. Sempach, Switzerland.
- Lincoln, F. C. 1979. Migration of Birds. Fish and Wildlife Service, Circular 16.
- Lowery, G.H. 1951. A quantitative study of the nocturnal migration of birds. Lawrence, KS: University of Kansas Publications.
- Popa-Lisseanu, A. G., Delgado-Huertas, A., Forero, M. G., Rodriguez, A., Arlettaz, R. and C. Ibanez. 2007. Bats' conquest of a formidable foraging niche: the myriads of nocturnally migrating songbirds. PLoS ONE 2(2): e205.
- Rattenborg, N.C., Mandt, B.H., Obermeyer, W.H., Winsauer, P.J., and R. Huber. 2004. Migratory Sleeplessness in the White-Crowned Sparrow (Zonotrichia leucophrys gambelii). PLoS Biol 2(7): e212.
- Schmaljohann, H., Liechti, L. and B. Bruderer. 2007. Songbird migration across the Sahara: the non-stop hypothesis rejected! Proc Biol Sci 274(1610):735-9.
- Swan, L. W. 1970. Goose of the Himalayas. Nat Hist 79(10): 68-75.
- Walraff, H. G. 2005. Avian Navigation: Pigeon Homing as a Paradigm. New York, NY: Springer.
- Williams, G. G. 1950. Weather and spring migration. Auk 67:52-65.
- Wiltschko, W., Munro, U., Ford, H. and R. Wiltschko. 2006. Bird navigation: what type of information does the magnetite-based receiver provide? Proc R Soc B 273: 2815-20.
Further reading
- Alerstam, T. 2001. Detours in bird migration. Journal of Theoretical Biology 209:319-31.
- Berthold, P. 2001. Bird Migration: A General Survey, 2nd ed. New York, NY: Oxford University Press. ISBN 0-19-850787-9
- Dingle, H. 1996. Migration: The Biology of Life on The Move. New York, NY: Oxford University Press.
- Weidensaul, S. 1999. Living On the Wind: Across the Hemisphere With Migratory Birds. Vancouver, BC: Douglas and McIntyre.
External links
- Trektellen.nl - Migration counts and ringing records The Netherlands, Belgium, Great Britain, France and Germany
- Canadian Migration Monitoring Network (Co-ordinates bird migration monitoring stations across Canada)
- Bird Research by Science Daily- includes several articles on bird migration
- The Nature Conservancy's Migratory Bird Program
- The Compasses of Birds - a review from the Science Creative Quarterly
- BBC Supergoose - satellite tagging of light-bellied brent geese
- Soaring with Fidel - follow the annual migration of ospreys from Cape Cod to Cuba to Venezuela
- Birder's Journal: A Morning With Migrants Nationalgeographic.com
- Discuss on bat predation
| Birds |
|---|
|
| Anatomy: Anatomy - Skeleton - Flight - Eggs - Feathers - Plumage |
| Evolution and extinction. Evolution - Archaeopteryx - Hybridisation - Late Quaternary prehistoric birds - Fossils - Taxonomy - Extinction |
| Behaviour: Singing - Intelligence - Migration - Reproduction- Brood parasites |
| Bird types: Seabirds - Shorebirds - Waterbirds - Song birds - Raptors - Poultry |
| Bird lists: Familes and orders - Lists by region |
| Birds and Humans: Ringing - Ornithology - Birdwatching - Birdfeeding - Conservation - Aviculture |
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
