Bird migration

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Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Bird migration_sentence_0

Many species of bird migrate. Bird migration_sentence_1

Migration carries high costs in predation and mortality, including from hunting by humans, and is driven primarily by availability of food. Bird migration_sentence_2

It occurs mainly in the northern hemisphere, where birds are funneled on to specific routes by natural barriers such as the Mediterranean Sea or the Caribbean Sea. Bird migration_sentence_3

Migration of species such as storks, turtle doves, and swallows was recorded as many as 3,000 years ago by Ancient Greek authors, including Homer and Aristotle, and in the Book of Job. Bird migration_sentence_4

More recently, Johannes Leche began recording dates of arrivals of spring migrants in Finland in 1749, and modern scientific studies have used techniques including bird ringing and satellite tracking to trace migrants. Bird migration_sentence_5

Threats to migratory birds have grown with habitat destruction especially of stopover and wintering sites, as well as structures such as power lines and wind farms. Bird migration_sentence_6

The Arctic tern holds the long-distance migration record for birds, travelling between Arctic breeding grounds and the Antarctic each year. Bird migration_sentence_7

Some species of tubenoses (Procellariiformes) such as albatrosses circle the earth, flying over the southern oceans, while others such as Manx shearwaters migrate 14,000 km (8,700 mi) between their northern breeding grounds and the southern ocean. Bird migration_sentence_8

Shorter migrations are common, including altitudinal migrations on mountains such as the Andes and Himalayas. Bird migration_sentence_9

The timing of migration seems to be controlled primarily by changes in day length. Bird migration_sentence_10

Migrating birds navigate using celestial cues from the sun and stars, the earth's magnetic field, and mental maps. Bird migration_sentence_11

Historical views Bird migration_section_0

In the Pacific, traditional landfinding techniques used by Micronesians and Polynesians suggest that bird migration was observed and interpreted for more than 3000 years. Bird migration_sentence_12

In Samoan tradition, for example, Tagaloa sent his daughter Sina to Earth in the form of a bird, Tuli, to find dry land, the word tuli referring specifically to landfinding waders, often to the Pacific golden plover. Bird migration_sentence_13

Records of bird migration were also known in Europe from at least 3,000 years ago as indicated by the Ancient Greek writers Hesiod, Homer, Herodotus and Aristotle. Bird migration_sentence_14

The Bible also notes migrations, as in the Book of Job, where the inquiry is made: "Is it by your insight that the hawk hovers, spreads its wings southward?" Bird migration_sentence_15

The author of Jeremiah wrote: "Even the stork in the heavens know its seasons, and the turtle dove, the swift and the crane keep the time of their arrival." Bird migration_sentence_16

Aristotle noted that cranes traveled from the steppes of Scythia to marshes at the headwaters of the Nile. Bird migration_sentence_17

Pliny the Elder, in his Historia Naturalis, repeats Aristotle's observations. Bird migration_sentence_18

Swallow migration versus hibernation Bird migration_section_1

Aristotle, however, suggested that swallows and other birds hibernated. Bird migration_sentence_19

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. Bird migration_sentence_20

Even the "highly observant" Gilbert White, in his posthumously published 1789 The Natural History of Selborne, quoted a man's story about swallows being found in a chalk cliff collapse "while he was a schoolboy at Brighthelmstone", though the man denied being an eyewitness. Bird migration_sentence_21

However, he also writes that "as to swallows being found in a torpid state during the winter in the Isle of Wight or any part of this country, I never heard any such account worth attending to", and that if early swallows "happen to find frost and snow they immediately withdraw for a time—a circumstance this much more in favour of hiding than migration", since he doubts they would "return for a week or two to warmer latitudes". Bird migration_sentence_22

It was not until the end of the eighteenth century that migration as an explanation for the winter disappearance of birds from northern climes was accepted. Bird migration_sentence_23

Thomas Bewick's A History of British Birds (Volume 1, 1797) mentions a report from "a very intelligent master of a vessel" who, "between the islands of Menorca and Majorca, saw great numbers of Swallows flying northward", and states the situation in Britain as follows: Bird migration_sentence_24

Bewick then describes an experiment which succeeded in keeping swallows alive in Britain for several years, where they remained warm and dry through the winters. Bird migration_sentence_25

He concludes: Bird migration_sentence_26

Pfeilstörche Bird migration_section_2

In 1822, a white stork was found in the German state of Mecklenburg with an arrow made from central African hardwood, which provided some of the earliest evidence of long-distance stork migration. Bird migration_sentence_27

This bird was referred to as a Pfeilstorch, German for "Arrow stork". Bird migration_sentence_28

Since then, around 25 Pfeilstörche have been documented. Bird migration_sentence_29

General patterns of migration Bird migration_section_3

Migration is the regular seasonal movement, often north and south, undertaken by many species of birds. Bird migration_sentence_30

Bird movements include those made in response to changes in food availability, habitat, or weather. Bird migration_sentence_31

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). Bird migration_sentence_32

Migration is marked by its annual seasonality. Bird migration_sentence_33

Non-migratory birds are said to be resident or sedentary. Bird migration_sentence_34

Approximately 1800 of the world's 10,000 bird species are long-distance migrants. Bird migration_sentence_35

Many bird populations migrate long distances along a flyway. Bird migration_sentence_36

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. Bird migration_sentence_37

Of course, in the southern hemisphere the directions are reversed, but there is less land area in the far south to support long-distance migration. Bird migration_sentence_38

The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter. Bird migration_sentence_39

Also, the longer days of the northern summer provide extended time for breeding birds to feed their young. Bird migration_sentence_40

This helps diurnal birds to produce larger clutches than related non-migratory species that remain in the tropics. Bird migration_sentence_41

As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season. Bird migration_sentence_42

These advantages offset the high stress, physical exertion costs, and other risks of the migration. Bird migration_sentence_43

Predation can be heightened during migration: Eleonora's falcon Falco eleonorae, which breeds on Mediterranean islands, has a very late breeding season, coordinated with the autumn passage of southbound passerine migrants, which it feeds to its young. Bird migration_sentence_44

A similar strategy is adopted by the greater noctule bat, which preys on nocturnal passerine migrants. Bird migration_sentence_45

The higher concentrations of migrating birds at stopover sites make them prone to parasites and pathogens, which require a heightened immune response. Bird migration_sentence_46

Within a species not all populations may be migratory; this is known as "partial migration". Bird migration_sentence_47

Partial migration is very common in the southern continents; in Australia, 44% of non-passerine birds and 32% of passerine species are partially migratory. Bird migration_sentence_48

In some species, the population at higher latitudes tends to be migratory and will often winter at lower latitude. Bird migration_sentence_49

The migrating birds bypass the latitudes where other populations may be sedentary, where suitable wintering habitats may already be occupied. Bird migration_sentence_50

This is an example of leap-frog migration. Bird migration_sentence_51

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 order. Bird migration_sentence_52

Within a population, it is common for different ages and/or sexes to have different patterns of timing and distance. Bird migration_sentence_53

Female chaffinches Fringilla coelebs in Eastern Fennoscandia migrate earlier in the autumn than males do and the European tits of genera Parus and Cyanistes only migrate their first year. Bird migration_sentence_54

Most migrations begin with the birds starting off in a broad front. Bird migration_sentence_55

Often, this front narrows into one or more preferred routes termed flyways. Bird migration_sentence_56

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. Bird migration_sentence_57

The specific routes may be genetically programmed or learned to varying degrees. Bird migration_sentence_58

The routes taken on forward and return migration are often different. Bird migration_sentence_59

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. Bird migration_sentence_60

Many, if not most, birds migrate in flocks. Bird migration_sentence_61

For larger birds, flying in flocks reduces the energy cost. Bird migration_sentence_62

Geese in a V-formation may conserve 12–20% of the energy they would need to fly alone. Bird migration_sentence_63

Red knots Calidris canutus and dunlins Calidris alpina were found in radar studies to fly 5 km/h (3.1 mph) faster in flocks than when they were flying alone. Bird migration_sentence_64

Birds fly at varying altitudes during migration. Bird migration_sentence_65

An expedition to Mt. Bird migration_sentence_66 Everest found skeletons of northern pintail Anas acuta and black-tailed godwit Limosa limosa at 5,000 m (16,000 ft) on the Khumbu Glacier. Bird migration_sentence_67

Bar-headed geese Anser indicus have been recorded by GPS flying at up to 6,540 metres (21,460 ft) while crossing the Himalayas, at the same time engaging in the highest rates of climb to altitude for any bird. Bird migration_sentence_68

Anecdotal reports of them flying much higher have yet to be corroborated with any direct evidence. Bird migration_sentence_69

Seabirds fly low over water but gain altitude when crossing land, and the reverse pattern is seen in landbirds. Bird migration_sentence_70

However most bird migration is in the range of 150 to 600 m (490 to 1,970 ft). Bird migration_sentence_71

Bird strike aviation records from the United States show most collisions occur below 600 m (2,000 ft) and almost none above 1,800 m (5,900 ft). Bird migration_sentence_72

Bird migration is not limited to birds that can fly. Bird migration_sentence_73

Most species of penguin (Spheniscidae) migrate by swimming. Bird migration_sentence_74

These routes can cover over 1,000 km (620 mi). Bird migration_sentence_75

Dusky grouse Dendragapus obscurus perform altitudinal migration mostly by walking. Bird migration_sentence_76

Emus Dromaius novaehollandiae in Australia have been observed to undertake long-distance movements on foot during droughts. Bird migration_sentence_77

Nocturnal migratory behavior Bird migration_section_4

While participating in nocturnal migration, many birds give 'Nocturnal Flight Calls', which are short, contact-type calls. Bird migration_sentence_78

These calls likely serve to maintain the composition of a migrating flock, and can sometimes encode the gender of a migrating individual. Bird migration_sentence_79

They also likely serve to avoid collision in the air. Bird migration_sentence_80

Nocturnal migration can also be monitored using weather radar data, which can be used to estimate the number of birds migrating on a given night, as well as the direction of the migration. Bird migration_sentence_81

Future research in this field includes the automatic detection and identification of nocturnally calling migrant birds, which could have broad implications on species conservation and land management. Bird migration_sentence_82

Nocturnal migrants land in the morning and may feed for a few days before resuming their migration. Bird migration_sentence_83

These birds are referred to as passage migrants in the regions where they occur for a short duration between the origin and destination. Bird migration_sentence_84

Nocturnal migrants minimize depredation, avoid overheating, and can feed during the day. Bird migration_sentence_85

One cost of nocturnal migration is the loss of sleep. Bird migration_sentence_86

Migrants may be able to alter their quality of sleep to compensate for the loss. Bird migration_sentence_87

Long-distance migration Bird migration_section_5

The typical image of migration is of northern landbirds, such as swallows (Hirundinidae) and birds of prey, making long flights to the tropics. Bird migration_sentence_88

However, many Holarctic wildfowl and finch (Fringillidae) species winter in the North Temperate Zone, in regions with milder winters than their summer breeding grounds. Bird migration_sentence_89

For example, the pink-footed goose migrates from Iceland to Britain and neighbouring countries, whilst the dark-eyed junco migrates from subarctic and arctic climates to the contiguous United States and the American goldfinch from taiga to wintering grounds extending from the American South northwestward to Western Oregon. Bird migration_sentence_90

Some ducks, such as the garganey Anas querquedula, move completely or partially into the tropics. Bird migration_sentence_91

The European pied flycatcher Ficedula hypoleuca also follows this migratory trend, breeding in Asia and Europe and wintering in Africa. Bird migration_sentence_92

Migration routes and wintering grounds are both genetically and traditionally determined depending on the social system of the species. Bird migration_sentence_93

In long-lived, social species such as white storks (Ciconia ciconia), flocks are often led by the oldest members and young storks learn the route on their first journey. Bird migration_sentence_94

In short-lived species that migrate alone, such as the Eurasian blackcap Sylvia atricapilla or the yellow-billed cuckoo Coccyzus americanus, first-year migrants follow a genetically determined route that is alterable with selective breeding. Bird migration_sentence_95

Often, the migration route of a long-distance migratory bird doesn't follow a straight line between breeding and wintering grounds. Bird migration_sentence_96

Rather, it could follow a hooked or arched line, with detours around geographical barriers or towards suitable stopover habitat. Bird migration_sentence_97

For most land-birds, such barriers could consist of seas, large water bodies or high mountain ranges, a lack of stopover or feeding sites, or a lack of thermal columns (important for broad-winged birds). Bird migration_sentence_98

Additionally, many migration routes are circuitous due to evolutionary history: the breeding range of Northern wheatears Oenanthe oenanthe has expanded to cover the entire Northern Hemisphere, but the species still migrates up to 14,500 km to reach ancestral wintering grounds in sub-Saharan Africa rather than establish new wintering grounds closer to breeding areas. Bird migration_sentence_99

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 may also be a barrier to a bird that feeds in coastal waters. Bird migration_sentence_100

Detours avoiding such barriers are observed: for example, brent geese Branta bernicla 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. Bird migration_sentence_101

In waders Bird migration_section_6

A similar situation occurs with waders (called shorebirds in North America). Bird migration_sentence_102

Many species, such as dunlin Calidris alpina and western sandpiper Calidris mauri, undertake long movements from their Arctic breeding grounds to warmer locations in the same hemisphere, but others such as semipalmated sandpiper C. pusilla travel longer distances to the tropics in the Southern Hemisphere. Bird migration_sentence_103

For some species of waders, migration success depends on the availability of certain key food resources at stopover points along the migration route. Bird migration_sentence_104

This gives the migrants an opportunity to refuel for the next leg of the voyage. Bird migration_sentence_105

Some examples of important stopover locations are the Bay of Fundy and Delaware Bay. Bird migration_sentence_106

Some bar-tailed godwits Limosa lapponica have the longest known non-stop flight of any migrant, flying 11,000 km from Alaska to their New Zealand non-breeding areas. Bird migration_sentence_107

Prior to migration, 55 percent of their bodyweight is stored as fat to fuel this uninterrupted journey. Bird migration_sentence_108

In seabirds Bird migration_section_7

Seabird migration is similar in pattern to those of the waders and waterfowl. Bird migration_sentence_109

Some, such as the black guillemot Cepphus grylle and some gulls, are quite sedentary; others, such as most terns and auks breeding in the temperate northern hemisphere, move varying distances south in the northern winter. Bird migration_sentence_110

The Arctic tern Sterna paradisaea 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. Bird migration_sentence_111

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 mi). Bird migration_sentence_112

Many tubenosed birds breed in the southern hemisphere and migrate north in the southern winter. Bird migration_sentence_113

The most pelagic species, mainly in the 'tubenose' order Procellariiformes, are great wanderers, and the albatrosses of the southern oceans may circle the globe as they ride the "roaring forties" outside the breeding season. Bird migration_sentence_114

The tubenoses spread widely over large areas of open ocean, but congregate when food becomes available. Bird migration_sentence_115

Many are also among the longest-distance migrants; sooty shearwaters Puffinus griseus nesting on the Falkland Islands migrate 14,000 km (8,700 mi) between the breeding colony and the North Atlantic Ocean off Norway. Bird migration_sentence_116

Some Manx shearwaters Puffinus puffinus do this same journey in reverse. Bird migration_sentence_117

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 kilometres (5 million miles) during its over-50-year lifespan. Bird migration_sentence_118

Diurnal migration in large birds using thermals Bird migration_section_8

Some large broad-winged birds rely on thermal columns of rising hot air to enable them to soar. Bird migration_sentence_119

These include many birds of prey such as vultures, eagles, and buzzards, but also storks. Bird migration_sentence_120

These birds migrate in the daytime. Bird migration_sentence_121

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. Bird migration_sentence_122

Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points. Bird migration_sentence_123

Massive numbers of large raptors and storks pass through areas such as the Strait of Messina, Gibraltar, Falsterbo, and the Bosphorus at migration times. Bird migration_sentence_124

More common species, such as the European honey buzzard Pernis apivorus, can be counted in hundreds of thousands in autumn. Bird migration_sentence_125

Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants. Bird migration_sentence_126

This is a notable factor in the Central American migratory bottleneck. Bird migration_sentence_127

Batumi bottleneck in the Caucasus is one of the heaviest migratory funnels on earth. Bird migration_sentence_128

Avoiding flying over the Black Sea surface and across high mountains, hundreds of thousands of soaring birds funnel through an area around the city of Batumi, Georgia. Bird migration_sentence_129

Birds of prey such as honey buzzards which migrate using thermals lose only 10 to 20% of their weight during migration, which may explain why they forage less during migration than do smaller birds of prey with more active flight such as falcons, hawks and harriers. Bird migration_sentence_130

From observing the migration of eleven soaring bird species over the Strait of Gibraltar, species which did not advance their autumn migration dates were those with declining breeding populations in Europe. Bird migration_sentence_131

Short-distance and altitudinal migration Bird migration_section_9

Main article: Altitudinal migration Bird migration_sentence_132

Many long-distance migrants appear to be genetically programmed to respond to changing day length. Bird migration_sentence_133

Species that move short distances, however, may not need such a timing mechanism, instead moving in response to local weather conditions. Bird migration_sentence_134

Thus mountain and moorland breeders, such as wallcreeper Tichodroma muraria and white-throated dipper Cinclus cinclus, may move only altitudinally to escape the cold higher ground. Bird migration_sentence_135

Other species such as merlin Falco columbarius and Eurasian skylark Alauda arvensis move further, to the coast or towards the south. Bird migration_sentence_136

Species like the chaffinch are much less migratory in Britain than those of continental Europe, mostly not moving more than 5 km in their lives. Bird migration_sentence_137

Short-distance passerine migrants have two evolutionary origins. Bird migration_sentence_138

Those that have long-distance migrants in the same family, such as the common chiffchaff Phylloscopus collybita, are species of southern hemisphere origins that have progressively shortened their return migration to stay in the northern hemisphere. Bird migration_sentence_139

Species that have no long-distance migratory relatives, such as the waxwings Bombycilla, are effectively moving in response to winter weather and the loss of their usual winter food, rather than enhanced breeding opportunities. Bird migration_sentence_140

In the tropics there is little variation in the length of day throughout the year, and it is always warm enough for a food supply, but altitudinal migration occurs in some tropical birds. Bird migration_sentence_141

There is evidence that this enables the migrants to obtain more of their preferred foods such as fruits. Bird migration_sentence_142

Altitudinal migration is common on mountains worldwide, such as in the Himalayas and the Andes. Bird migration_sentence_143

Many bird species arid regions across southern Australia are nomadic; they follow water and food supply around the country in an irregular pattern, unrelated to season but related to rainfall. Bird migration_sentence_144

Several years may pass between visits to an area by a particular species. Bird migration_sentence_145

Irruptions and dispersal Bird migration_section_10

Sometimes circumstances such as a good breeding season followed by a food source failure the following year lead to irruptions in which large numbers of a species move far beyond the normal range. Bird migration_sentence_146

Bohemian waxwings Bombycilla garrulus well show this unpredictable variation in annual numbers, with five major arrivals in Britain during the nineteenth century, but 18 between the years 1937 and 2000. Bird migration_sentence_147

Red crossbills Loxia curvirostra too are irruptive, with widespread invasions across England noted in 1251, 1593, 1757, and 1791. Bird migration_sentence_148

Bird migration is primarily, but not entirely, a Northern Hemisphere phenomenon. Bird migration_sentence_149

This is because land birds in high northern latitudes, where food becomes scarce in winter, leave for areas further south (including the Southern Hemisphere) to overwinter, and because the continental landmass is much larger in the Northern Hemisphere. Bird migration_sentence_150

In contrast, among (pelagic) seabirds, species of the Southern Hemisphere are more likely to migrate. Bird migration_sentence_151

This is because there is a large area of ocean in the Southern Hemisphere, and more islands suitable for seabirds to nest. Bird migration_sentence_152

Physiology and control Bird migration_section_11

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. Bird migration_sentence_153

The ability to navigate and orient themselves during migration is a much more complex phenomenon that may include both endogenous programs as well as learning. Bird migration_sentence_154

Timing Bird migration_section_12

The primary physiological cue for migration is the changes in the day length. Bird migration_sentence_155

These changes are also related to hormonal changes in the birds. Bird migration_sentence_156

In the period before migration, many birds display higher activity or Zugunruhe (German: migratory restlessness), first described by Johann Friedrich Naumann in 1795, as well as physiological changes such as increased fat deposition. Bird migration_sentence_157

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 programs in controlling bird migrations. Bird migration_sentence_158

Caged birds display a preferential flight direction that corresponds with the migratory direction they would take in nature, changing their preferential direction at roughly the same time their wild conspecifics change course. Bird migration_sentence_159

In polygynous species with considerable sexual dimorphism, males tend to return earlier to the breeding sites than their females. Bird migration_sentence_160

This is termed protandry. Bird migration_sentence_161

Orientation and navigation Bird migration_section_13

Main article: Animal navigation Bird migration_sentence_162

Navigation is based on a variety of senses. Bird migration_sentence_163

Many birds have been shown to use a sun compass. Bird migration_sentence_164

Using the sun for direction involves the need for making compensation based on the time. Bird migration_sentence_165

Navigation has also been shown to be based on a combination of other abilities including the ability to detect magnetic fields (magnetoreception), use visual landmarks as well as olfactory cues. Bird migration_sentence_166

Long-distance migrants are believed to disperse as young birds and form attachments to potential breeding sites and to favourite wintering sites. Bird migration_sentence_167

Once the site attachment is made they show high site-fidelity, visiting the same wintering sites year after year. Bird migration_sentence_168

The ability of birds to navigate during migrations cannot be fully explained by endogenous programming, even with the help of responses to environmental cues. Bird migration_sentence_169

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. Bird migration_sentence_170

Satellite tracking of day migrating raptors such as ospreys and honey buzzards has shown that older individuals are better at making corrections for wind drift. Bird migration_sentence_171

The birds navigate through an innate biological sense resulting from evolution. Bird migration_sentence_172

Migratory birds may use two electromagnetic tools to find their destinations: one that is entirely innate and another that relies on experience. Bird migration_sentence_173

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. Bird migration_sentence_174

It does this through a radical pair mechanism whereby chemical reactions in special photo pigments sensitive to short wavelengths are affected by the field. Bird migration_sentence_175

Although this only works during daylight hours, it does not use the position of the sun in any way. Bird migration_sentence_176

At this stage the bird is in the position of a Boy Scout with a compass but no map, until it grows accustomed to the journey and can put its other capabilities to use. Bird migration_sentence_177

With experience it learns various landmarks and this "mapping" is done by magnetites in the trigeminal system, which tell the bird how strong the field is. Bird migration_sentence_178

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. Bird migration_sentence_179

There is a neural connection between the eye and "Cluster N", the part of the forebrain that is active during migrational orientation, suggesting that birds may actually be able to see the magnetic field of the earth. Bird migration_sentence_180

Vagrancy Bird migration_section_14

See also: Vagrancy (biology) § In birds Bird migration_sentence_181

Migrating birds can lose their way and appear outside their normal ranges. Bird migration_sentence_182

This 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. Bird migration_sentence_183

Certain areas, because of their location, have become famous as watchpoints for such birds. Bird migration_sentence_184

Examples are the Point Pelee National Park in Canada, and Spurn in England. Bird migration_sentence_185

Reverse migration, where the genetic programming of young birds fails to work properly, can lead to rarities turning up as vagrants thousands of kilometres out of range. Bird migration_sentence_186

Drift migration of birds blown off course by the wind can result in "falls" of large numbers of migrants at coastal sites. Bird migration_sentence_187

A related phenomenon called "abmigration" involves birds from one region joining similar birds from a different breeding region in the common winter grounds and then migrating back along with the new population. Bird migration_sentence_188

This is especially common in some waterfowl, which shift from one flyway to another. Bird migration_sentence_189

Migration conditioning Bird migration_section_15

It has been possible to teach a migration route to a flock of birds, for example in re-introduction schemes. Bird migration_sentence_190

After a trial with Canada geese Branta canadensis, microlight aircraft were used in the US to teach safe migration routes to reintroduced whooping cranes Grus americana. Bird migration_sentence_191

Adaptations Bird migration_section_16

Birds need to alter their metabolism to meet the demands of migration. Bird migration_sentence_192

The storage of energy through the accumulation of fat and the control of sleep in nocturnal migrants require special physiological adaptations. Bird migration_sentence_193

In addition, the feathers of a bird suffer from wear-and-tear and require to be moulted. Bird migration_sentence_194

The timing of this moult – usually once a year but sometimes twice – varies with some species moulting prior to moving to their winter grounds and others molting prior to returning to their breeding grounds. Bird migration_sentence_195

Apart from physiological adaptations, migration sometimes requires behavioural changes such as flying in flocks to reduce the energy used in migration or the risk of predation. Bird migration_sentence_196

Evolutionary and ecological factors Bird migration_section_17

Migration in birds is highly labile and is believed to have developed independently in many avian lineages. Bird migration_sentence_197

While it is agreed that the behavioral and physiological adaptations necessary for migration are under genetic control, some authors have argued that no genetic change is necessary for migratory behavior to develop in a sedentary species because the genetic framework for migratory behavior exists in nearly all avian lineages. Bird migration_sentence_198

This explains the rapid appearance of migratory behavior after the most recent glacial maximum. Bird migration_sentence_199

Theoretical analyses 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 to cross a long barrier flies less efficiently. Bird migration_sentence_200

However some species show circuitous migratory routes that reflect historical range expansions and are far from optimal in ecological terms. Bird migration_sentence_201

An example is the migration of continental populations of Swainson's thrush Catharus ustulatus, which fly far east across North America before turning south via Florida to reach northern South America; this route is believed to be the consequence of a range expansion that occurred about 10,000 years ago. Bird migration_sentence_202

Detours may also be caused by differential wind conditions, predation risk, or other factors. Bird migration_sentence_203

Climate change Bird migration_section_18

Large scale climatic changes are expected to have an effect on the timing of migration. Bird migration_sentence_204

Studies have shown a variety of effects including timing changes in migration, breeding as well as population declines. Bird migration_sentence_205

Many species have been expanding their range as a likely consequence of climate change. Bird migration_sentence_206

This is sometimes in the form of former vagrants becoming established or regular migrants. Bird migration_sentence_207

Currently legislation and protection mechanisms have been slow to recognise the concept of range-shifts from climate refugee birds. Bird migration_sentence_208

In September 2020, large numbers of birds were reported dying during migration through the southwestern United States. Bird migration_sentence_209

Some 100,000 flycatchers, swallows and warblers were estimated to have died between August and September. Bird migration_sentence_210

Ecological effects Bird migration_section_19

The migration of birds also aids the movement of other species, including those of ectoparasites such as ticks and lice, which in turn may carry micro-organisms including those of concern to human health. Bird migration_sentence_211

Due to the global spread of avian influenza, bird migration has been studied as a possible mechanism of disease transmission, but it has been found not to present a special risk; import of pet and domestic birds is a greater threat. Bird migration_sentence_212

Some viruses that are maintained in birds without lethal effects, such as the West Nile virus may however be spread by migrating birds. Bird migration_sentence_213

Birds may also have a role in the dispersal of propagules of plants and plankton. Bird migration_sentence_214

Some predators take advantage of the concentration of birds during migration. Bird migration_sentence_215

Greater noctule bats feed on nocturnal migrating passerines. Bird migration_sentence_216

Some birds of prey specialize on migrating waders. Bird migration_sentence_217

Study techniques Bird migration_section_20

Early studies on the timing of migration began in 1749 in Finland, with Johannes Leche of Turku collecting the dates of arrivals of spring migrants. Bird migration_sentence_218

Bird migration routes have been studied by a variety of techniques including the oldest, marking. Bird migration_sentence_219

Swans have been marked with a nick on the beak since about 1560 in England. Bird migration_sentence_220

Scientific ringing was pioneered by Hans Christian Cornelius Mortensen in 1899. Bird migration_sentence_221

Other techniques include radar and satellite tracking. Bird migration_sentence_222

The rate of bird migration over the Alps (up to a height of 150 m) was found to be highly comparable between fixed-beam radar measurements and visual bird counts, highlighting the potential use of this technique as an objective way of quantifying bird migration. Bird migration_sentence_223

Stable isotopes of hydrogen, oxygen, carbon, nitrogen, and sulphur can establish avian migratory connectivity between wintering sites and breeding grounds. Bird migration_sentence_224

Stable isotopic methods to establish migratory linkage rely on spatial isotopic differences in bird diet that are incorporated into inert tissues like feathers, or into growing tissues such as claws and muscle or blood. Bird migration_sentence_225

An approach to identify migration intensity makes use of upward pointing microphones to record the nocturnal contact calls of flocks flying overhead. Bird migration_sentence_226

These are then analyzed in a laboratory to measure time, frequency and species. Bird migration_sentence_227

An older technique developed by George Lowery and others to quantify migration involves observing the face of the full moon with a telescope and counting the silhouettes of flocks of birds as they fly at night. Bird migration_sentence_228

Orientation behaviour studies have been traditionally carried out using variants of a setup known as the Emlen funnel, which consists of a circular cage with the top covered by glass or wire-screen so that either the sky is visible or the setup is placed in a planetarium or with other controls on environmental cues. Bird migration_sentence_229

The orientation behaviour of the bird inside the cage is studied quantitatively using the distribution of marks that the bird leaves on the walls of the cage. Bird migration_sentence_230

Other approaches used in pigeon homing studies make use of the direction in which the bird vanishes on the horizon. Bird migration_sentence_231

Threats and conservation Bird migration_section_21

Main article: Bird migration perils Bird migration_sentence_232

Human activities have threatened many migratory bird species. Bird migration_sentence_233

The distances involved in bird migration mean that they often cross political boundaries of countries and conservation measures require international cooperation. Bird migration_sentence_234

Several international treaties have been signed to protect migratory species including the Migratory Bird Treaty Act of 1918 of the US. Bird migration_sentence_235

and the African-Eurasian Migratory Waterbird Agreement Bird migration_sentence_236

The concentration of birds during migration can put species at risk. Bird migration_sentence_237

Some spectacular migrants have already gone extinct; during the passenger pigeon's (Ectopistes migratorius) migration the enormous flocks were a mile (1.6 km) wide, darkening the sky and 300 miles (480 km) long, taking several days to pass. Bird migration_sentence_238

Other significant areas include stop-over sites between the wintering and breeding territories. Bird migration_sentence_239

A capture-recapture study of passerine migrants with high fidelity for breeding and wintering sites did not show similar strict association with stop-over sites. Bird migration_sentence_240

Hunting along migration routes threatens some bird species. Bird migration_sentence_241

The populations of Siberian cranes (Leucogeranus leucogeranus) that wintered in India declined due to hunting along the route, particularly in Afghanistan and Central Asia. Bird migration_sentence_242

Birds were last seen in their favourite wintering grounds in Keoladeo National Park in 2002. Bird migration_sentence_243

Structures such as power lines, wind farms and offshore oil-rigs have also been known to affect migratory birds. Bird migration_sentence_244

Other migration hazards include pollution, storms, wildfires, and habitat destruction along migration routes, denying migrants food at stopover points. Bird migration_sentence_245

For example, in the East Asian–Australasian Flyway, up to 65% of key intertidal habitat at the Yellow Sea migration bottleneck has been destroyed since the 1950s. Bird migration_sentence_246

See also Bird migration_section_22

Bird migration_unordered_list_0

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: migration.