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Bird migration
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==General patterns== [[File:101111 Maison 007.jpg |thumb |upright |A flock of [[common starling]]s assembling in Geneva, Switzerland, before migrating southwards]] [[File: Waders in flight Roebuck Bay.jpg |thumb |Migrating [[wader]]s in [[Roebuck Bay]], Western Australia]] [[File:Flock of cranes feeding on harvested fields in Northern Germany (I).jpg|thumb|In the autumn, gigantic numbers of [[common crane]]s gather in the northern federal German states of [[Mecklenburg-Vorpommern]] and [[Brandenburg]] in order to rest and prepare themselves for the long journey south.]] Migration is the regular seasonal movement, often north and south, undertaken by many species of birds. Migration is marked by its annual seasonality and movement between breeding and non-breeding areas.<ref name=Berthold>{{cite book |title=Bird Migration: A General Survey |first=Peter |last=Berthold |author2=Bauer, Hans-Günther |author3=Westhead, Valerie |year=2001 |isbn=978-0-19-850787-1 |publisher=Oxford University Press |location=Oxford}}</ref> Nonmigratory bird movements include those made in response to environmental changes including in food availability, habitat, or weather. Sometimes, journeys are not termed "true migration" because they are irregular (nomadism, invasions, irruptions) or in only one direction (dispersal, movement of young away from natal area). Non-migratory birds are said to be resident or sedentary. Approximately 1,800 of the world's 10,000 bird species are long-distance migrants.<ref name=Seker>{{cite journal |title=Conservation ecology: area trumps mobility in fragment bird extinctions |last=Sekercioglu |first=C. H. |journal=Current Biology |year=2007 |volume=17 |issue=8 |pages=283–286 |doi=10.1016/j.cub.2007.04.045 |pmid=17437705 |doi-access=free|bibcode=2007CBio...17..909S }}</ref><ref name=Rolland>{{cite journal |title=Settling down of seasonal migrants promotes bird diversification |last=Rolland |first=J. |others=Jiguet, F.; Jønsson, K.A.; Condamine, F.L.; Morlon, H. |journal=Proceedings of the Royal Society B |year=2014 |volume=281 |issue=1784 |pages=20140473 |doi=10.1098/rspb.2014.0473 |pmid=24759866 |pmc=4043101}}</ref> Many bird populations migrate long distances along a flyway. The most common pattern involves flying north in the spring to breed in the temperate or [[Arctic]] summer and returning in the autumn to wintering grounds in warmer regions to the south. In the southern hemisphere, the directions are reversed, but there is less land area in the far south to support long-distance migration.<ref name="newton"/> The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter.<ref>{{cite web |url=http://www.hummingbirds.net/migration.html |title=Migration Basics |publisher=Hummingbirds.net |access-date=10 April 2014}}</ref> In addition, the longer days of the northern summer provide extended time for [[breeding in the wild|breeding]] birds to feed their young. This helps [[Diurnality|diurnal]] birds to produce larger [[Clutch (eggs)|clutches]] than related non-migratory species that remain in the tropics. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.<ref>{{cite web |url=https://www.researchgate.net/publication/237048440 |title=Environmental Impact Assessment of the National Large Solar Telescope Project and its Ecological Impact in Merak Area |date=February 2011 |access-date=10 April 2014 |last=Ramachandra |first=T. V. |pages=71 |display-authors=etal}}</ref> These advantages offset the high stress, physical exertion costs, and other risks of migration. Predation can be heightened during migration: [[Eleonora's falcon]] ''Falco eleonorae'', which breeds on [[Mediterranean Sea|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.<ref>{{cite journal |last=Dondini |first=G. |author2=Vergari, S. |year=2000 |title=Carnivory in the greater noctule bat (''Nyctalus lasiopterus'') in Italy |journal=Journal of Zoology |volume=251 |pages=233–236 |doi=10.1111/j.1469-7998.2000.tb00606.x |issue=2}}</ref><ref name=bats>{{cite journal |last=Popa-Lisseanu |first=A. G. |author2=Delgado-Huertas, A. |author3=Forero, M. G. |author4=Rodriguez, A. |author5=Arlettaz, R. |author6=Ibanez, C. |year=2007 |title=Bats' Conquest of a Formidable Foraging Niche: The Myriads of Nocturnally Migrating Songbirds |journal=PLOS ONE |volume=2 |issue=2 |page=e205 |doi=10.1371/journal.pone.0000205 |pmid=17299585 |pmc=1784064 |bibcode=2007PLoSO...2..205P |editor1-last=Rands |editor1-first=Sean|doi-access=free }}</ref><ref>{{cite journal |last=Ibáñez |first=C. |author2=Juste, J. |author3=García-Mudarra, J. L. |author4=Agirre-Mendi, P. T. |year=2001 |title=Bat predation on nocturnally migrating birds |journal=PNAS |volume=98 |pages=9700–9702 |doi=10.1073/pnas.171140598 |pmid=11493689 |issue=17 |pmc=55515|doi-access=free }}</ref> The higher concentrations of migrating birds at stopover sites make them prone to parasites and pathogens, which require a heightened immune response.<ref name=newton /> Within a species not all populations may be migratory; this is known as "partial migration". Partial migration is very common in the southern continents; in Australia, 44% of non-passerine birds and 32% of passerine species are partially migratory.<ref>{{cite journal |last=Chan |first=K. |year=2001 |title=Partial migration in Australian land birds: a review |journal=Emu |volume=101 |issue=4 |pages=281–292 |doi=10.1071/MU00034 |bibcode=2001EmuAO.101..281C |s2cid=82259620}}</ref> In some species, the population at higher latitudes tends to be migratory and will often winter at lower latitude. The migrating birds bypass the latitudes where other populations may be sedentary, where suitable wintering habitats may already be occupied. This is an example of ''leap-frog migration''.<ref>{{cite journal |last=Boland |first=J. M. |year=1990 |title=Leapfrog migration in North American shorebirds: intra- and interspecific examples |journal=The Condor |volume=92 |pages=284–290 |url=http://sora.unm.edu/sites/default/files/journals/condor/v092n02/p0284-p0290.pdf |doi=10.2307/1368226 |issue=2 |jstor=1368226}}</ref> Many fully migratory species show leap-frog migration (birds that nest at higher latitudes spend the winter at lower latitudes), and many show the alternative, chain migration, where populations 'slide' more evenly north and south without reversing the order.<ref>{{cite book |title=Bird Migration: A General Survey |publisher=Oxford University Press |author=Berthold, Peter |year=2001 |page=67}}</ref> Within a population, it is common for different ages and/or sexes to have different patterns of timing and distance. Female [[Common chaffinch|chaffinches]] ''Fringilla coelebs'' in Eastern Fennoscandia migrate earlier in the autumn than males do<ref>{{cite journal |url=http://www.zin.ru/journals/AEB/pdf/Panov_2011_19_AEB.pdf |title=Overlap between moult and autumn migration in passerines in northern taiga zone of Eastern Fennoscandia |last=Panov |first=Ilya N. |journal=Avian Ecology and Behaviour |year=2011 |volume=19 |pages=33–64}}</ref> and the European tits of genera ''[[Parus]]'' and ''[[Cyanistes]]'' only migrate in their first year.<ref>Ketterson, E. D., and V. Nolan. 1985. Intraspecific variation in avian migration: evolutionary and regulatory aspects, Pages 553–579 ''in'' M. A. Rankin, ed. Migration: mechanisms and adaptive significance, University of Texas, Austin.</ref> Most migrations begin with the birds starting off in a broad front. Often, this front narrows into one or more preferred routes termed [[flyway]]s. These routes typically follow mountain ranges or coastlines, sometimes rivers, and may take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water. The specific routes may be genetically programmed or learned to varying degrees. The routes taken on forward and return migration are often different.<ref name=newton>{{cite book |last=Newton |first=I. |year=2008 |title=The Migration Ecology of Birds |publisher=Elsevier |isbn=978-0-12-517367-4}}</ref> A common pattern in North America is clockwise migration, where birds flying North tend to be further West, and flying South tend to shift Eastwards. Many, if not most, birds migrate in flocks. For larger birds, flying in flocks reduces the energy cost. Geese in a [[V formation]] may conserve 12–20% of the energy they would need to fly alone.<ref>{{cite journal |last=Hummel |first=D. |author2=Beukenberg, M. |year=1989 |title=Aerodynamische Interferenzeffekte beim Formationsflug von Vogeln |journal=J. Ornithol. |volume=130 |issue=1 |pages=15–24 |doi=10.1007/BF01647158 |s2cid=823269}}</ref><ref>{{cite journal |author1=Cutts, C. J. |author2=J R Speakman |name-list-style=amp |year=1994 |title=Energy savings in formation flight of Pink-footed Geese |journal=Journal of Experimental Biology |volume=189 |pages=251–261 |url=http://jeb.biologists.org/cgi/reprint/189/1/251.pdf |pmid=9317742 |issue=1|doi=10.1242/jeb.189.1.251 |bibcode=1994JExpB.189..251C }}</ref> Red knots ''Calidris canutus'' and dunlins ''Calidris alpina'' were found in radar studies to fly {{convert|5|km/h|kn|round=0.5|abbr=on}} faster in flocks than when they were flying alone.<ref name=newton /> [[File: Northern Pintail.jpg |right |thumb |[[Northern pintail]] skeletons have been found high in the Himalayas]] Birds fly at varying altitudes during migration. An expedition to [[Mount Everest|Mt. Everest]] found skeletons of [[northern pintail]] ''Anas acuta'' and [[black-tailed godwit]] ''Limosa limosa'' at {{convert|5000|m|ft|abbr=on}} on the [[Khumbu Glacier]].<ref>{{cite journal |last=Geroudet |first=P. |year=1954 |title=Des oiseaux migrateurs trouvés sur la glacier de Khumbu dans l'Himalaya |journal=Nos Oiseaux |volume=22 |page=254}}</ref> [[Bar-headed goose|Bar-headed geese]] ''Anser indicus'' have been recorded by GPS flying at up to {{convert|6540|m|ft|abbr=on}} while crossing the Himalayas, at the same time engaging in the highest rates of climb to altitude for any bird. Anecdotal reports of them flying much higher have yet to be corroborated with any direct evidence.<ref>{{cite journal |author=Swan, L. W. |year=1970 |title=Goose of the Himalayas |journal=Nat. Hist. |volume=79 |issue=10 |pages=68–75}}</ref> Seabirds fly low over water but gain altitude when crossing land, and the reverse pattern is seen in land birds.<ref>{{cite book |last=Dorst |first=J. |year=1963 |title=The migration of birds. |publisher=Houghton Mifflin Co., Boston. |page=476}}</ref><ref>{{cite journal |last=Eastwood |first=E. |author2=Rider, G. C. |name-list-style=amp |year=1965 |title=Some radar measurements of the altitude of bird flight |journal=British Birds |volume=58 |pages=393–426}}</ref> However most bird migration is in the range of {{convert|150|to(-)|600|m|ft|sigfig=2|abbr=on}}. [[Bird strike]] Aviation records from the United States show most collisions occur below {{convert|600|m|ft|sigfig=2|abbr=on}} and almost none above {{convert|1800|m|ft|sigfig=2|abbr=on}}.<ref>{{cite journal |last=Williams |first=G. G. |year=1950 |title=Weather and spring migration |journal=Auk |volume=67 |issue=1 |pages=52–65 |doi=10.2307/4080769 |jstor=4080769|doi-access=free }}</ref> Bird migration is not limited to birds that can fly. Most species of [[penguin]] (Spheniscidae) migrate by swimming. These routes can cover over {{convert|1000|km|nmi|round=50|abbr=on}}. [[Dusky grouse]] ''Dendragapus obscurus'' perform altitudinal migration mostly by walking. [[Emu]]s ''Dromaius novaehollandiae'' in [[Australia]] have been observed to undertake long-distance movements on foot during droughts.<ref name=newton />
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