Difference between revisions of "Bird migration" - New World Encyclopedia

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[[Image:BrantaLeucopsisMigration.jpg|thumb|300px|Flock of [[Barnacle Goose|Barnacle Geese]] during autumn migration]]
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[[Image:BrantaLeucopsisMigration.jpg|thumb|300px|A flock of [[Barnacle Goose|Barnacle Geese]] during autumn migration.]]
  
'''Bird migration''' refers to the regular seasonal journeys of varying distances undertaken by many species of [[bird]]s. The movements of birds includes those made in response to changes in food availability, habitat or weather. These however are usually irregular and are termed variously as nomadism, invasions or irruptions. Migration is marked by its annual seasonality.  
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'''Bird migration''' refers to the regular (and often seasonal) journeys to and from a given area undertaken by all or part of a [[bird]] population. Not all bird [[species]] (or even populations within the same species) are migratory. In contrast to more irregular movements such as [[emigration]], [[nomadism]], and [[invasion]], which are made in response to changes in food availability, habitat, or weather, bird migration is marked by its cyclical pattern.
  
==General patterns==
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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, other patterns of migration have been observed: 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 lower altitudes with less exposure to harsh winter weather.
Many land birds migrate long distances. The most common pattern involves flying north to breed in the temperate or arctic summer and returning to wintering grounds in warmer regions to the south.
 
  
The primary advantage of migration is energetic. The long days of the northern summer provide greater opportunities for breeding birds to feed their young. The extended hours allow [[diurnal]] birds to produce larger clutches than related non-migratory species that remain in the tropics year-round.  As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.  
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[[Image:Rubythroathummer65.jpg|thumb|right|150px|The fat stores of the ruby-throated hummingbird increase prior to migration.]]
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The primary advantage of migration is energetic. In the Northern Hemisphere, the long days of 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. 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.  
  
These advantages offset the high stress, energetic costs, and other risks of the migration. Predation can be heightened during migration; the [[Eleonora's Falcon]] which breeds on [[Mediterranean Sea|Mediterranean]] islands has a very late breeding season, coordinated with the autumn passage of southbound [[perching bird|passerine]] migrants which it feeds to its young. A similar strategy is adopted by the [[Greater Noctule bat]], which preys on nocturnal passerine migrants.<ref>Dondini, G., Vergari, S. 2000 Carnivory in the greater noctule bat (''Nyctalus lasiopterus'') in Italy. Journal of Zoology 251: 233-236.</ref><ref>Popa-Lisseanu, A. G., Delgado-Huertas, A., Forero, M. G., Rodriguez, A., Arlettaz, R. & Ibanez, C. 2007. Bats' conquest of a formidable foraging niche: the myriads of nocturnally migrating songbirds. PLoS ONE 2(2): e205. [http://www.plosone.org/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1371%2Fjournal.pone.0000205 URL]</ref><ref>Ibáñez, C., Juste, J., García-Mudarra, J. L., Agirre-Mendi, P. T. 2001. Bat predation on nocturnally migrating birds. PNAS 98:9700-9702. [http://www.pnas.org/cgi/content/full/98/17/9700#B8 full article.]</ref>
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Migrations typically occur along established routes called "flyways." The migrating species often return to the area of their birth to breed. The birds are guided by innate behaviors (including [[hormone|hormonal]] signals) that enable them to know when to depart and that orient them toward a specific location over long distances. However, they also remain flexible to environmental conditions, such as food supply and temperature, which may fluctuate yearly.
  
Within a species not all populations may be migratory and this is termed as partial migration. Partial migration is very common in the southern continents; in Australia, 44% of non-passerine birds and 32% of passerine species were partially migratory.<ref>Chan K (2001) "Partial migration in Australian landbirds: a review" ''Emu'' '''101''' (4): 281-292)''</ref> In some species the population at higher latitudes tend to be migratory and will often winter at lower latitude past the latitudes where other populations may be sedentary and this is termed as ''leap-frog migration''.<ref>Boland, J. M. 1990. Leapfrog migration in North American shorebirds: intra- and interspecific examples. The Condor. 92:284-290 [http://elibrary.unm.edu/sora/Condor/files/issues/v092n02/p0284-p0290.pdf]</ref> Within a population, there can also be different patterns of timing and migration based on the age groups and sex. Only the female [[Chaffinch]]es in [[Scandinavia]] migrate with the males staying resident. This has given rise to its specific name ''coelebs'', a bachelor.
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Bird migration has larger [[ecology|ecological]] implications that underscore the interconnectedness of life: Migratory cycles are closely attuned to seasonal food productivity cycles, which leads to a mutual gain for both the migrating species and the ecosystems in which they participate. Migratory birds are able to settle in areas 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 populations.
  
Migration is often concentrated along well established routes known as [[flyway]]s. 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 specific routes may be [[genetic programming|genetically programmed]] or learnt to varying degrees.
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==Bird species have diverse modes of migration==
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[[Image:Arctic terns.jpg|thumb|200px|left|Arctic Terns migrate vast distances.]]
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The varied patterns and modes of bird migration may be understood as [[adaptation]]s. In fact, migration itself has conferred an advantage to only certain bird species, while not evolving in other species that remain resident, or sedentary, year-round. Whether a particular species migrates depends on a number of factors. The climate of the breeding area is important, as few species can cope with the harsh winters of inland [[Canada]] or northern [[Eurasia]]. The nature of the staple food is also significant. Most specialist insect eaters that breed outside the tropics are long-distance migrants, and have little choice but to head south in winter.  
  
The altitude at which birds fly during migration varies. An expedition to [[Mount Everest|Mt. Everest]] found skeletons of [[Northern Pintail|Pintail]] and [[Black-tailed Godwit]] at 16,400 feet on the [[Khumbu Glacier]].<ref>Geroudet, P. 1954. Des oiseaux migrateurs trouves sur la glacier de Khumbu dans l'Himalaya. Nos Oiseaux 22: 254.</ref> [[Bar-headed Goose|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.<ref>Swan, L. W. 1970. Goose of the Himalayas. Nat. Hist. 79(10): 68-75.</ref> Seabirds fly low over water but gain altitude when crossing land and the reverse pattern in seen in landbirds.<ref>Dorst, J. 1963. The migration of birds. Houghton Mifflin Co., Boston. 476 p.</ref><ref>Eastwood, E. and G. C. Rider. 1965. Some radar measurements of the altitude of bird flight. Br. Birds 58:</ref> However most bird migration is in the range of 500 to 2000 feet. Bird hit records from the United States show most collisions below 2000 feet and almost none above 6000 feet.<ref>Williams, G. G. 1950. Weather and spring migration. Auk 67: 52-65</ref>
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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 percent of [[passerine]] (perching) species and 44 percent of non-passerine birds were found 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 [[Chaffinch]]es of [[Scandinavia]] migrate, while the males stay resident, a migratory pattern that has given rise to the name ''coelebs,'' meaning "bachelor."
  
==Historical views==
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Migrations vary widely in terms of the distance traveled. Short-distance passerine migrants, such as the [[waxwing]]s, are effectively moving in response to winter weather, rather than enhanced breeding opportunities. Some [[Alaskan]] [[Bar-tailed Godwit]]s have the longest non-stop flight of any migrant, flying 11,000 kilometers (km) to their [[New Zealand]] non-breeding areas. Prior to migration, 55 percent 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 wintering areas. One Arctic Tern, [[Bird ringing|ringed]] (banded) as a chick on the [[Farne Islands]] off the [[Great Britain|British]] east coast, reached [[Melbourne]], [[Australia]] in just three months from fledging, a sea journey of over 22,000km (14,000 miles).
The earliest recorded observations of bird migration were 3000 years ago, as noted by [[Hesiod]], [[Homer]], [[Herodotus]], [[Aristotle]] and others. The bible also notes migrations, as in the Book of Job (39:26), where the inquiry is made: "Doth the hawk fly by Thy wisdom and stretch her wings toward the south?" The author of Jeremiah (8:7) wrote: "The stork in the heavens knoweth her appointed time; and the turtledove, and the crane, and the swallow, observe the time of their coming."
 
  
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.<ref>Lincoln, F. C. 1979 Migration of Birds. Fish and Wildlife Service. Circular 16. [http://www.archive.org/details/migrationofbirds00lincrich] </ref>
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[[image:Cedar Waxwing-27527-1.jpg|right|200px|thumb|The Cedar Waxwing is a short-distance migrant.]]
  
==Long-distance migration==
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Migrations may be diurnal (occurring during the day) or nocturnal. Many of the smaller insectivorous birds, including the [[warbler]]s, [[hummingbird]]s, and [[flycatcher]]s, 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 [[lark]]s and [[finch]]es, or that can feed on the wing, like [[swallow]]s and [[swift]]s.
[[image:SwainsonThrush23.jpg|right|thumb|Swainson's Thrush]]
 
[[Image:Npintail09a.jpg|right|thumb|Northern Pintail]]
 
The typical image of migration is of northern landbirds such as [[swallow]]s and birds of prey making long flights to the tropics. Many northern-breeding [[duck]]s, [[goose|geese]] and [[swan]]s 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 [[Great Britain|Britain]] and neighbouring countries. Migratory routes and wintering grounds are traditional and learned by young during their first migration with their parents. Some ducks, such as the [[Garganey]], move completely or partially into the tropics.
 
  
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 are observed: for example, [[Brent Goose|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]].
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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 [[Mount Everest|Mt. Everest]] found skeletons of [[Northern Pintail|Pintail]] and [[Black-tailed Godwit]] at 16,400 feet on the [[Khumbu Glacier]] (Geroudet 1995). [[Bar-headed Goose|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).
  
[[image:BartailedGodwit24.jpg|right|thumb|Bar-tailed Godwit]]
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==Migratory birds follow established routes==
A similar situation occurs with [[wader]]s (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 huge 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.
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Migration often is concentrated along well-established routes known as flyways, which are shaped by geographical, ecological, and even meteorological factors. Flyways typically follow mountain ranges or coastlines, and may take advantage of updrafts and other wind patterns, or avoid geographical barriers, such as (in the case of land birds) large stretches of open water.
  
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|Delaware Bay]].
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Theoretical analyses, summarized by Alerstam (2001), show that detours that increase flight distance by up to 20 percent will often be adaptive on [[aerodynamics|aerodynamic]] grounds&mdash;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.
  
Some [[Alaska]]n [[Bar-tailed Godwit]]s 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.
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[[Image:Vulture 19o05.jpg|left|thumb|A Griffon Vulture soaring. The migratory routes of vultures and many other birds of prey take advantage of ecological features like thermal columns to power their flight.]]
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Some large broad-winged birds rely on [[thermal|thermal columns]] of rising hot air to enable them to soar. These include many [[bird of prey|birds of prey]], such as [[vulture]]s, [[eagle]]s, and [[buzzard]]s, as well as [[stork]]s. Migratory species in these groups have great difficulty crossing large bodies of water, since thermals form over land only. 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 [[Bird of prey|raptor]]s and storks pass through areas such as [[Gibraltar]], [[Falsterbo]], and the [[Bosphorus]] at migration times.  
  
[[Image:Arctic terns.jpg|thumb|right|Arctic Terns]]
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By following established routes, some species risk predation during periods of peak migration. For example, the [[Eleonora's Falcon]], which breeds on [[Mediterranean Sea|Mediterranean]] islands, has a very late breeding season, coordinated with the autumn passage of southbound [[perching bird|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).
[[Seabird]] migration is similar in patter to those of the waders and waterfowl. Some, such as the [[Black Guillemot]] and some [[gull]]s, are quite sedentary; others, such as most [[tern]]s and [[auk]]s breeding in the temperate northern hemisphere, move varying distances south in winter. 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, [[Bird ringing|ringed]] (banded) as a chick on the [[Farne Islands]] off the [[Great Britain|British]] east coast, reached [[Melbourne]], [[Australia]] in just three months from fledging, a sea journey of over 22,000 km (14,000 miles).   A few seabirds, such as [[Wilson's Petrel]] and [[Great Shearwater]], breed in the southern hemisphere and migrate north in the southern winter. Seabirds have the additional advantage of being able to feed during migration over open waters.
 
  
The most pelagic species, mainly in the 'tubenose' order [[Procellariiformes]], are great wanderers, and the [[albatross]]es 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 Shearwater]]s nesting on the [[Falkland Islands]] migrate 14,000 km (9,000 miles) between the breeding colony and the [[Atlantic Ocean|North Atlantic Ocean]] off [[Norway]]. Some [[Manx Shearwater]]s 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 over-50 year lifespan.
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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. "Drift migrations" of birds blown off course by the wind can result in "falls" of large numbers of migrants at coastal sites.
  
[[Image:Vulture 19o05.jpg|right|thumb|Griffon Vulture soaring]]
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==Patterns of migration==
Some large broad-winged birds rely on [[thermal|thermal columns]] of rising hot air to enable them to soar. These include many [[bird of prey|birds of prey]] such as [[vulture]]s, [[eagle]]s, and [[buzzard]]s, but also [[stork]]s. These birds migrate in the daytime. Migratory species in these groups have great difficulty crossing large bodies of water, since thermals only form over land, and these birds cannot maintain active flight for long distances. 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 [[Bird of prey|raptor]]s 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 America]]n migratory bottleneck.
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===Many migratory European and North American species fly south in winter===
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[[Image:Npintail09a.jpg|left|thumb|The Northern Pintail breeds in the northern areas of [[Europe]] and [[Asia]] and across most of [[Canada]], [[Alaska]] and the mid-western [[United States]]; it winters as far south as the Equator.]]
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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 [[stork]]s, migrate to areas of [[Africa]] south of the Sahara. North American birds, like the ruby-throated [[hummingbird]], which breeds in southern [[Canada]], may travel as far south as [[Panama]] for the winter; others, like the American robin and several species of grackles, winter in the states along the Gulf Coast.  
  
[[image:Rubythroathummer65.jpg|thumb|right|Ruby-throated Hummingbird]]
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Many northern-breeding [[duck]]s, [[goose|geese]], and [[swan]]s 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 [[Great Britain|Britain]] and neighboring countries.
Many of the smaller insectivorous birds including the [[warbler]]s, [[hummingbird]]s and [[flycatcher]]s migrate large distances, usually at night. They land in the morning and may feed for a few days before resuming their migration. The birds are referred to as passage migrants in the regions where they occur for short durations between the origin and destination.<ref>Schmaljohann, Heiko, Felix Liechti and Bruno Bruderer: Songbird migration across the Sahara: the non-stop hypothesis rejected! Proc Biol Sci. 2007 Mar 7;274(1610):735-739 [http://www.journals.royalsoc.ac.uk/openurl.asp?genre=article&id=doi:10.1098/rspb.2006.0011 "online first"] DOI: 10.1098/rspb.2006.0011</ref>
 
  
By migrating at night, nocturnal migrants minimize predation, and avoid overheating that could result from the energy expended to fly such long distances. 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.<ref>Rattenborg, N.C., Mandt, B.H., Obermeyer, W.H., Winsauer, P.J., Huber, R.(2004) Migratory Sleeplessness in the White-Crowned Sparrow (''Zonotrichia leucophrys gambelii''). PLoS Biol 2(7): e212 [http://dx.doi.org/10.1371/journal.pbio.0020212]</ref>
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A similar situation occurs with [[wader]]s (called "shorebirds" in North America). Many species, such as the [[Dunlin]] and [[Western Sandpiper]], undertake long movements from their Arctic breeding grounds to warmer locations in the same hemisphere, while others, such as the [[Semipalmated Sandpiper]], travel greater distances to the tropics.
  
==Short-distance migration==
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===Some Southern species winter in northern areas===
[[image:Cedar Waxwing-27527-1.jpg|right|thumb|Cedar Waxwing]]
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[[image:Rainbowbeeeater.jpg|right|thumb|150px|The Australian Rainbow Bee-eater winters north of its breeding range.]]
Many of the long-distance migrants in the previous section are effectively genetically programmed to respond to changing lengths of days. However, many species move shorter distances, but may do so only in response to harsh weather conditions.  
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Although bird migrations in the Southern Hemisphere are less well-observed than Northern ones (in part because the largely uninterrupted expanses of land mass and ocean tend not to funnel migrations into narrow pathways), many species do in fact breed in the temperate regions of the Southern Hemisphere and winter further north in the tropics. The southern [[Africa]]n [[Greater Striped Swallow]], the [[Australia]]n  [[Satin Flycatcher]], [[Dollarbird]], and [[Rainbow Bee-eater]], for example, winter well north of their breeding range. A few seabirds, such as [[Wilson's Petrel]]s and [[Great Shearwater]]s, breed in the Southern Hemisphere and migrate north in the southern winter.
  
Thus mountain and moorland breeders, such as [[Wallcreeper]] and [[White-throated Dipper]], may move only altitudinally to escape the cold higher ground. Other species such as [[Merlin (bird)|Merlin]] and [[Skylark]] will move further to the coast or to a more southerly region. Species like the [[Chaffinch]] are not migratory in [[Great Britain|Britain]], but will move south or to [[Ireland]] in very cold weather.  
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===Two types of migrating seabirds===
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[[Seabird]] migration may be characterized as "coastal," with species following along the continental shelf, or "pelagic," with species ranging across the open sea. The former category includes birds such as the [[guillemot]]s, [[auk]]s, [[cormorant]]s, [[gannet]]s, and [[gull]]s, which are all found along the seashore.
  
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.
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The most pelagic species, mainly in the "tubenose" order [[Procellariiformes]] (petrels and albatrosses), are great wanderers. The [[albatross]]es of the southern oceans may circle the globe as they ride the "roaring forties" outside the breeding season. Many are also among the longest-distance migrants; [[Sooty Shearwater]]s nesting on the [[Falkland Islands]] migrate 14,000km (9,000 miles) between the breeding colony and the [[Atlantic Ocean|North Atlantic Ocean]] off [[Norway]]. 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 kilometers (5 million miles) during its lifespan of over 50 years.
  
Those species which have no long-distance migratory relatives, such as the [[waxwing]]s, are effectively moving in response to winter weather, rather than enhanced breeding opportunities.
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===Tropical migration: Wet and dry seasons===
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[[image:WoodlandKingfisher.jpg|right|175px|thumb|The Woodland Kingfisher migrates into the equatorial zone during the dry season.]]
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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 [[cuckoo]]s, 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.
  
[[image:WoodlandKingfisher.jpg|right|thumb|Woodland Kingfisher]]
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However, some tropical species undergo movements of varying distances depending on rainfall. Many tropical regions have cycles of wet and dry seasons, the [[monsoon]]s 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]].
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. However many species undergo movements of varying distances depending on the rainfall.  
 
  
Many tropical regions have wet and dry seasons, the [[monsoon]]s 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]].
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===Vertical movements===
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Some migrations involve changes in altitude, as species move vertically from higher 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 [[Himalayas]] and [[Andes]], there are also seasonal vertical 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]].
  
There are a few species, notably [[cuckoo]]s, 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.
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===Pantanal: Example of region of southern, northern, and vertical movements===
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The [[Pantanal]], a semitropical region contained within the Upper Paraguay River Basin of [[Brazil]], [[Paraguay]], and [[Bolivia]], and the world's largest wetland system, is an important migratory bird stopover point and wintering ground. It is used by birds from three major migratory flyways&mdash;bringing ospreys from the Nearctic latitudes to the north, woodstorks from the Argentine pampas to the south, and flycatchers from the Andes to the west (Eckstrom 1996). Included in the bird fauna of the Pantanal are such North American migratory birds as the upland sandpiper ''(Bartramia longicauda)'', the American golden plover ''(Pluvialis dominica)'' and the black-necked stilt ''(Himantopus himantopus)'' (Swarts 2000).
  
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]].
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==Signals==
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The timing and response of migration are in large part genetically controlled. In contrast, the ability of migratory birds to navigate and orient themselves during migration is a much more complex phenomenon that may include both endogenous (internal) programs as well as learned behavior (Helm and Gwinner 2006).
  
==Irruptions and dispersal==
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===Physiological changes prepare migratory birds for flight===
Sometimes circumstances such as a good breeding season followed by a food source failure the following lead to irruptions, in which large numbers of a species move far beyond the normal range. [[Bohemian Waxwing]] and [[Common Crossbill]]s are two species which show this unpredictable variation in annual numbers.
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The primary environmental cue for migration is change in day length, which is related to hormonal changes in migratory birds. The [[pituitary gland]] (an [[endocrine system|endocrine gland]] that controls the release of hormonal stimuli) prepares birds for flight by initiating physiological changes. However, certain ecological conditions, such as changes in temperature and weather conditions, are necessary to trigger flight.
  
The temperate zones of the southern continents have extensive arid areas, particularly in Australia and western southern Africa, and weather-driven movements are common is not always predictable. A couple of weeks of heavy rain in one part or another of the usually dry centre of Australia, for example, causes dramatic plant and invertebrate growth, attracting birds from all directions. This can happen at any time of year, and, in any given area, may not happen again for a decade or more, depending on the frequency of  
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In the period before migration, many birds display higher activity known as ''zugunruhe,'' a German term meaning "migratory restlessness." 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 [[endogenous|endogenous]] programming in controlling bird migrations.
[[El Niño]] and [[La Niña]] periods.  
 
  
[[image:Rainbowbeeeater.jpg|right|thumb|Rainbow Bee-eater]]
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Birds preparing for migration also undergo metabolic changes such as increased fat deposition, which enables long-distance migrants, such as the ruby-throated hummingbird, to conserve muscle protein, enabling them to make their arduous, 2,400 kilometer flight.
Bird migration is primarily, but not entirely, a Northern Hemisphere phenomenon. In the Southern Hemisphere, seasonal migration tends to be much less obvious. There are several reasons for this.
 
  
First, the largely uninterrupted expanses of land mass or ocean tend not to funnel migrations into narrow and obvious pathways, making them less obvious to the human observer. Second, at least 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.
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===Orientation and navigation during flight draw on multiple senses===
 +
The navigational abilities of migratory birds has been shown to be based on a combination of abilities, such as detecting magnetic fields, using visual landmarks, and sensing olfactory cues (Wallraff 2005). Many birds have been demonstrated to have a "compass sense;" i.e., they are able to fly in a particular constant direction, regardless of their release point. An internal clock mechanism enables birds to use the sun as a point of orientation, determining the angle of the sun above the horizon. Nocturnal migrants may also use the stars to get their bearings.
  
Many species do in fact breed in the temperate southern hemisphere regions and winter further north in the tropics. The southern [[Africa]]n [[Greater Striped Swallow]], and the [[Australia]]n  [[Satin Flycatcher]], [[Dollarbird]], and [[Rainbow Bee-eater]]for example, winters well north of their breeding range.
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However, 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 [[gene]]tic component to migration in terms of timing and route, but this may be modified by environmental influences.
  
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==Historical background and modern study techniques==
Those smaller species that migrate during the day tend to be those making movements that are relatively short and weather-driven, like the [[lark]]s and [[finch]]es, or that can feed on the wing, like [[swallow]]s and [[swift]]s.—>
+
Although bird migrations have been observed for thousands of years, it was not until the early nineteenth century that migration was accepted as an explanation for the winter disappearance of birds from northern climes (Lincoln 1979).
  
==Physiology and control==
+
Bird migration has been studied using a variety of techniques, of which [[Bird ringing|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 today to study the migration of birds (Font et al. 2007). To identify migration intensity, one contemporary approach makes use of upward pointing microphones to record the contact calls of overflying flocks; these calls are then analyzed in a laboratory to measure time, frequency, and species (Farnsworth et al. 2004).
 
 
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.<ref>Helm B, Gwinner E (2006) Migratory Restlessness in an Equatorial Nonmigratory Bird. PLoS Biol 4(4): e110 doi:10.1371/journal.pbio.0040110 [http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040110]</ref>
 
 
 
===Timing===
 
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 language|German]]: migratory restlessness) as well as physiological changes such as increased fat deposition. 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 [[endogenous|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.
 
 
 
===Orientation and navigation===
 
 
 
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.<ref name=walraffpigeon>Walraff, H. G. 2005. Avian Navigation: Pigeon Homing as a Paradigm. Springer.</ref>
 
 
 
The ability of birds to navigate during migrations can not 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 [[gene]]tic 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 [[Blackcap]]s in central Europe to migrate west and winter in [[Great Britain|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 [[biological pigment|photo pigment]]s sensitive to [[Color#Physics of color|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 [[magnetite]]s 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 [[latitude]]s let it interpret the radical pair mechanism more accurately and let it know when it has reached its destination.<ref>Wiltschko, W., U. Munro, H. Ford & R. Wiltschko. (2006) "Bird navigation: what type of information does the magnetite-based receiver provide?" ''[[Proc. R. Soc. B]]''. '''273''': 2815-20.</ref>
 
 
 
===Vagrancy===
 
Migrating birds can lose their way and occur outside their normal ranges. These can be due to flying past their destinations as in the "spring overshoot" in which birds returning to their breeding areas overshoot and end up further north than intended.  A mechanism which can lead to great rarities turning up as vagrants thousands of kilometres out of range is [[reverse migration]], where the genetic programming of young birds fails to work properly. Certain areas, because of their location, have become famous as watchpoints for migrating birds. Examples are the [[Point Pelee National Park]] in Canada, and [[Spurn]] in [[England]]. [[Drift migration]] of birds blown off course by the wind can result in "falls" of large numbers of migrants at coastal sites.
 
 
 
===Migration conditioning===
 
It has been possible to teach a new migration route to a flock of birds, for example in re-introduction schemes. After a trial with [[Canada Goose|Canada Geese]], [[microlight]] aircraft were used in the US to teach safe migration routes to reintroduced [[Whooping Crane]]s [http://www.operationmigration.org/index.html].
 
 
 
Such an effect can also happen naturally, as exemplified by the increasing tendency of Blackcaps to winter in Britain as described above.
 
 
 
==Evolutionary and ecological factors==
 
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 America]]n and [[Africa]]n, rather than [[northern hemisphere]], [[evolution]]ary 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 [[aerodynamics|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.
 
 
 
==Study techniques==
 
 
 
Bird migration has been studied by a variety of techniques of which [[Bird ringing|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.<ref>{{cite journal|author=Laura Font, Geoff M. Nowell, D. Graham Pearson, Chris J. Ottley and Stephen G. Willis|title=Sr isotope analysis of bird feathers by TIMS: a tool to trace bird migration paths and breeding sites|journal=J. Anal. At. Spectrom.|year=2007|volume=22|page=513|id=DOI: 10.1039/b616328a}}</ref>
 
 
 
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.<ref>Farnsworth, A., Gauthreaux, S.A., and van Blaricom, D. 2004. A comparison of nocturnal call counts of migrating birds and reflectivity measurements on Doppler radar.  Journal of Avian Biology 35:365-369. [http://www.andrewfarnsworth.com/PDFs/JAB3180.pdf]</ref>
 
 
 
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.<ref>Liechti, F. (1996) Instructions to count nocturnal bird migration by watching the full moon. Schweizerische Vogelwarte, CH-6204 Sempach, Switzerland.</ref><ref>Lowery, G.H. (1951) A quantitative study of the nocturnal migration of birds. University of Kansas Publications, Museum of Natural History 3, 361-472</ref>
 
  
 
==References==
 
==References==
{{reflist|2}}
+
*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: Houghton Mifflin.
 +
*Eastwood, E., and G. C. Rider. 1965. Some radar measurements of the altitude of bird flight. ''Brit Birds'' 58: 393-426.
 +
*Eckstrom, C. K. 1996. A wilderness of water: The Pantanal. ''Audubon'' 98(2): 54-65.
 +
*Farnsworth, A., S. A. Gauthreaux, and D. van Blaricom. 2004. [http://www.andrewfarnsworth.com/PDFs/JAB3180.pdf A comparison of nocturnal call counts of migrating birds and reflectivity measurements on Doppler radar.] ''Journal of Avian Biology'' 35: 365-9. Retrieved August 16, 2007.
 +
*Font, L., M. Geoff, D. Nowell, G. Pearson, C. J. Ottley, 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. [http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040110 Migratory restlessness in an Equatorial nonmigratory bird.] ''PLoS Biol'' 4(4): e110. Retrieved August 16, 2007.
 +
*Ibáñez, C., J. Juste, J. L. García-Mudarra, and P. T. Agirre-Mendi. 2001. Bat predation on nocturnally migrating birds. ''PNAS'' 98: 9700-9702.
 +
*Liechti, F. 1996. Instructions to count nocturnal bird migration by watching the full moon. ''Schweizerische Vogelwarte'' CH-6204. Sempach, Switzerland.
 +
*Lincoln, F. C. 1979. [http://www.archive.org/details/migrationofbirds00lincrich Migration of birds.] ''Fish and Wildlife Service, Circular 16.'' Retrieved August 16, 2007.
 +
*Lowery, G.H. 1951. ''A Quantitative Study of the Nocturnal Migration of Birds''. Lawrence, KS: University of Kansas Publications.
 +
*Popa-Lisseanu, A. G., A. Delgado-Huertas, M. G. Forero, A. Rodriguez, R. Arlettaz, and C. Ibanez. 2007. [http://www.plosone.org/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1371%2Fjournal.pone.0000205 Bats' conquest of a formidable foraging niche: The myriads of nocturnally migrating songbirds.] ''PLoS ONE'' 2(2): e205. Retrieved August 16, 2007.
 +
*Rattenborg, N. C., B. H. Mandt, W. H., Obermeyer, P. J. Winsauer, and R. Huber. 2004. [http://dx.doi.org/10.1371/journal.pbio.0020212 Migratory sleeplessness in the white-crowned sparrow (''Zonotrichia leucophrys gambelii'').] ''PLoS Biol'' 2(7): e212. Retrieved August 16, 2007. 
 +
*Schmaljohann, H., L. Liechti, and B. Bruderer. 2007. [http://www.journals.royalsoc.ac.uk/openurl.asp?genre=article&id=doi:10.1098/rspb.2006.0011 Songbird migration across the Sahara: The non-stop hypothesis rejected!] ''Proc Biol Sci'' 274(1610): 735-9. Retrieved August 16, 2007. 
 +
*Swan, L. W. 1970. Goose of the Himalayas. ''Nat Hist'' 79(10): 68-75. Retrieved August 16, 2007.
 +
* Swarts, F. A. 2000. The Pantanal in the 21st Century: For the planet's largest wetland, an uncertain future. In F. A. Swarts (ed.) ''The Pantanal''. St. Paul, MN: Paragon House. ISBN 1557787913
 +
*Wallraff, H. G. 2005. ''Avian Navigation: Pigeon Homing as a Paradigm''. New York, NY: Springer. ISBN 3540223851
 +
*Williams, G. G. 1950. Weather and spring migration. ''Auk'' 67: 52-65.
 +
*Wiltschko, W., U. Munro, H. Ford, 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==
 
==Further reading==
*Alerstam, T. (2001).  Detours in bird migration.  ''Journal of Theoretical Biology'', 209, 319-331.
+
*Alerstam, T. 2001.  Detours in bird migration.  ''Journal of Theoretical Biology'' 209: 319-331.
*Berthold, Peter (2001) Bird Migration: A General Survey. Second Edition. Oxford University Press. ISBN 0-19-850787-9
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*Berthold, P. 2001. ''Bird Migration: A General Survey'', 2nd ed. New York: Oxford University Press. ISBN 0198507879
*Dingle, Hugh. Migration: The Biology of Life on The Move. Oxford Univ. Press, 1996.
+
*Dingle, H. 1996. ''Migration: The Biology of Life on The Move''. New York: Oxford University Press. ISBN 0195089626
*Weidensaul, Scott. Living On the Wind: Across the Hemisphere With Migratory Birds. Douglas & McIntyre, 1999.
+
*Weidensaul, S. 1999. ''Living On the Wind: Across the Hemisphere With Migratory Birds''. Vancouver, BC: Douglas and McIntyre. ISBN 0865475431
  
 
==External links==
 
==External links==
*[http://www.trektellen.nl/default.asp?taal=2 Trektellen.nl ] - Migration counts and ringing records The Netherlands, Belgium, Great Britain, France and Germany
+
All links retrieved February 6, 2013.
*[http://www.bsc-eoc.org/national/cmmn.html Canadian Migration Monitoring Network (Co-ordinates bird migration monitoring stations across Canada)]
+
*[http://www.bsc-eoc.org/national/cmmn.html Canadian Migration Monitoring Network (Co-ordinates bird migration monitoring stations across Canada)].  
*[http://www.sciencedaily.com/news/plants_animals/birds/ Bird Research by Science Daily]- includes several articles on bird migration
+
*[http://www.nature.org/initiatives/programs/birds/ The Nature Conservancy's Migratory Bird Program].
*[http://www.nature.org/initiatives/programs/birds/ The Nature Conservancy's Migratory Bird Program]
+
* [http://www.scq.ubc.ca/?p=173 The Compasses of Birds] - a review from the Science Creative Quarterly.
* [http://www.scq.ubc.ca/?p=173 The Compasses of Birds] - a review from the Science Creative Quarterly
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* [http://www.bbc.co.uk/nature/animals/birds/supergoose/index.shtml BBC Supergoose] - satellite tagging of light-bellied brent geese.
* [http://www.bbc.co.uk/nature/animals/birds/supergoose/index.shtml BBC Supergoose] - satellite tagging of light-bellied brent geese
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* [http://www.ospreyworld.com/soaring/index.html Soaring with Fidel] - follow the annual migration of ospreys from Cape Cod to Cuba to Venezuela.
* [http://www.ospreyworld.com/soaring/index.html Soaring with Fidel] - follow the annual migration of ospreys from Cape Cod to Cuba to Venezuela
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* [http://news.nationalgeographic.com/news/2002/09/0926_020927_birdmigration.html Birder's Journal: A Morning With Migrants] Nationalgeographic.com.  
* [http://news.nationalgeographic.com/news/2002/09/0926_020927_birdmigration.html Birder's Journal: A Morning With Migrants] Nationalgeographic.com
 
* [http://www.swild.ch/publi/Bontadina_FunEco2003.pdf Discuss on bat predation]
 
  
 
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{{Footer Birds}}
  
[[Category:Life sciences]]
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[[Category:Life sciences]][[Category:Birds]]
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Revision as of 15:57, 6 February 2013


A flock of Barnacle Geese during autumn migration.

Bird migration refers to the regular (and often seasonal) journeys to and from a given area undertaken by all or part of a bird population. Not all bird species (or even populations within the same species) are migratory. In contrast to more irregular movements such as emigration, nomadism, and invasion, which are made in response to changes in food availability, habitat, or weather, bird migration is marked by its cyclical pattern.

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, other patterns of migration have been observed: 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 lower altitudes with less exposure to harsh winter weather.

The fat stores of the ruby-throated hummingbird increase prior to migration.

The primary advantage of migration is energetic. In the Northern Hemisphere, the long days of 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. 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." The migrating species often return 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 orient them toward 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 seasonal food productivity cycles, which leads to a mutual gain for both the migrating species and the ecosystems in which they participate. Migratory birds are able to settle in areas 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 populations.

Bird species have diverse modes of migration

Arctic Terns migrate vast distances.

The varied patterns and modes of bird migration may be understood as adaptations. In fact, migration itself has conferred an advantage to only certain bird species, while not evolving in other species that remain resident, or sedentary, year-round. Whether a particular species migrates depends on a number of factors. The climate of the breeding area is important, as few species can cope with the harsh winters of inland Canada or northern Eurasia. The nature of the staple food is also significant. Most specialist insect eaters that breed outside the tropics are long-distance migrants, and have little choice but to head south in winter.

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 percent of passerine (perching) species and 44 percent of non-passerine birds were found 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."

Migrations vary widely in terms of the distance traveled. Short-distance passerine migrants, such as the waxwings, are effectively moving in response to winter weather, rather than enhanced breeding opportunities. Some Alaskan Bar-tailed Godwits have the longest non-stop flight of any migrant, flying 11,000 kilometers (km) to their New Zealand non-breeding areas. Prior to migration, 55 percent 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 wintering 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 22,000km (14,000 miles).

The Cedar Waxwing is a short-distance migrant.

Migrations may be diurnal (occurring 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 often is concentrated along well-established routes known as flyways, which are shaped by geographical, ecological, and even meteorological factors. Flyways typically follow mountain ranges or coastlines, and may take advantage of updrafts and other wind patterns, or avoid geographical barriers, such as (in the case of land birds) large stretches of open water.

Theoretical analyses, summarized by Alerstam (2001), show that detours that increase flight distance by up to 20 percent 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.

A Griffon Vulture soaring. The migratory routes of vultures and many other birds of prey take advantage of ecological features like thermal columns to power their flight.

Some large broad-winged birds 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 form over land only. 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.

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. "Drift migrations" 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

File:Npintail09a.jpg
The Northern Pintail breeds in the northern areas of Europe and Asia and across most of Canada, Alaska and the mid-western United States; it winters as far south as the Equator.

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-throated hummingbird, which breeds in southern Canada, may travel as far south as Panama for the winter; others, 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 neighboring countries.

A similar situation occurs with waders (called "shorebirds" in North America). Many species, such as the Dunlin and Western Sandpiper, undertake long movements from their Arctic breeding grounds to warmer locations in the same hemisphere, while others, such as the Semipalmated Sandpiper, travel greater distances to the tropics.

Some Southern species winter in northern areas

The Australian Rainbow Bee-eater winters north of its breeding range.

Although bird migrations in the Southern Hemisphere are less well-observed than Northern ones (in part because the largely uninterrupted expanses of land mass and ocean tend not to funnel migrations into narrow pathways), many species do in fact breed in the temperate regions of the Southern Hemisphere and winter further north in the tropics. The southern African Greater Striped Swallow, 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 Petrels and Great Shearwaters, breed in the Southern Hemisphere and migrate north in the southern winter.

Two types of migrating seabirds

Seabird migration may be characterized as "coastal," with species following 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. Many are also among the longest-distance migrants; Sooty Shearwaters nesting on the Falkland Islands migrate 14,000km (9,000 miles) between the breeding colony and the North Atlantic Ocean off Norway. 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 kilometers (5 million miles) during its lifespan of over 50 years.

Tropical migration: Wet and dry seasons

The Woodland Kingfisher migrates into the equatorial zone during the dry season.

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.

However, some tropical species undergo movements of varying distances depending on 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.

Vertical movements

Some migrations involve changes in altitude, as species move vertically from higher 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 Himalayas and Andes, there are also seasonal vertical 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.

Pantanal: Example of region of southern, northern, and vertical movements

The Pantanal, a semitropical region contained within the Upper Paraguay River Basin of Brazil, Paraguay, and Bolivia, and the world's largest wetland system, is an important migratory bird stopover point and wintering ground. It is used by birds from three major migratory flyways—bringing ospreys from the Nearctic latitudes to the north, woodstorks from the Argentine pampas to the south, and flycatchers from the Andes to the west (Eckstrom 1996). Included in the bird fauna of the Pantanal are such North American migratory birds as the upland sandpiper (Bartramia longicauda), the American golden plover (Pluvialis dominica) and the black-necked stilt (Himantopus himantopus) (Swarts 2000).

Signals

The timing and response of migration are in large part genetically controlled. In contrast, the ability of migratory birds to navigate and orient themselves during migration is a much more complex phenomenon that may include both endogenous (internal) programs as well as learned behavior (Helm and Gwinner 2006).

Physiological changes prepare migratory birds for flight

The primary environmental cue for migration is change in day length, which is related to hormonal changes in migratory birds. The pituitary gland (an endocrine gland that controls the release of hormonal stimuli) prepares birds for flight by initiating physiological changes. However, certain ecological conditions, such as changes in temperature and weather conditions, are necessary to trigger flight.

In the period before migration, many birds display higher activity known as zugunruhe, a German term meaning "migratory restlessness." 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 endogenous programming in controlling bird migrations.

Birds preparing for migration also undergo metabolic changes such as increased fat deposition, which enables long-distance migrants, such as the ruby-throated hummingbird, to conserve muscle protein, enabling them to make their arduous, 2,400 kilometer flight.

Orientation and navigation during flight draw on multiple senses

The navigational abilities of migratory birds has been shown to be based on a combination of abilities, such as detecting magnetic fields, using visual landmarks, and sensing olfactory cues (Wallraff 2005). Many birds have been demonstrated to have a "compass sense;" i.e., they are able to fly in a particular constant direction, regardless of their release point. An internal clock mechanism enables birds to use the sun as a point of orientation, determining the angle of the sun above the horizon. Nocturnal migrants may also use the stars to get their bearings.

However, 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.

Historical background and modern study techniques

Although bird migrations have been observed for thousands of years, it was not until the early nineteenth century that migration was accepted as an explanation for the winter disappearance of birds from northern climes (Lincoln 1979).

Bird migration has been studied using 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 today to study the migration of birds (Font et al. 2007). To identify migration intensity, one contemporary approach makes use of upward pointing microphones to record the contact calls of overflying flocks; these calls are then analyzed in a laboratory to measure time, frequency, and species (Farnsworth et al. 2004).

References
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  • 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: Houghton Mifflin.
  • Eastwood, E., and G. C. Rider. 1965. Some radar measurements of the altitude of bird flight. Brit Birds 58: 393-426.
  • Eckstrom, C. K. 1996. A wilderness of water: The Pantanal. Audubon 98(2): 54-65.
  • Farnsworth, A., S. A. Gauthreaux, 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. Retrieved August 16, 2007.
  • Font, L., M. Geoff, D. Nowell, G. Pearson, C. J. Ottley, 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. Retrieved August 16, 2007.
  • Ibáñez, C., J. Juste, J. L. García-Mudarra, and P. T. Agirre-Mendi. 2001. Bat predation on nocturnally migrating birds. PNAS 98: 9700-9702.
  • 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. Retrieved August 16, 2007.
  • Lowery, G.H. 1951. A Quantitative Study of the Nocturnal Migration of Birds. Lawrence, KS: University of Kansas Publications.
  • Popa-Lisseanu, A. G., A. Delgado-Huertas, M. G. Forero, A. Rodriguez, R. Arlettaz, and C. Ibanez. 2007. Bats' conquest of a formidable foraging niche: The myriads of nocturnally migrating songbirds. PLoS ONE 2(2): e205. Retrieved August 16, 2007.
  • Rattenborg, N. C., B. H. Mandt, W. H., Obermeyer, P. J. Winsauer, and R. Huber. 2004. Migratory sleeplessness in the white-crowned sparrow (Zonotrichia leucophrys gambelii). PLoS Biol 2(7): e212. Retrieved August 16, 2007.
  • Schmaljohann, H., L. Liechti, and B. Bruderer. 2007. Songbird migration across the Sahara: The non-stop hypothesis rejected! Proc Biol Sci 274(1610): 735-9. Retrieved August 16, 2007.
  • Swan, L. W. 1970. Goose of the Himalayas. Nat Hist 79(10): 68-75. Retrieved August 16, 2007.
  • Swarts, F. A. 2000. The Pantanal in the 21st Century: For the planet's largest wetland, an uncertain future. In F. A. Swarts (ed.) The Pantanal. St. Paul, MN: Paragon House. ISBN 1557787913
  • Wallraff, H. G. 2005. Avian Navigation: Pigeon Homing as a Paradigm. New York, NY: Springer. ISBN 3540223851
  • Williams, G. G. 1950. Weather and spring migration. Auk 67: 52-65.
  • Wiltschko, W., U. Munro, H. Ford, 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: 319-331.
  • Berthold, P. 2001. Bird Migration: A General Survey, 2nd ed. New York: Oxford University Press. ISBN 0198507879
  • Dingle, H. 1996. Migration: The Biology of Life on The Move. New York: Oxford University Press. ISBN 0195089626
  • Weidensaul, S. 1999. Living On the Wind: Across the Hemisphere With Migratory Birds. Vancouver, BC: Douglas and McIntyre. ISBN 0865475431

External links

All links retrieved February 6, 2013.

Birds
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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
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Birds and Humans: Ringing - Ornithology - Birdwatching - Birdfeeding - Conservation - Aviculture

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