Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Many species of bird migrate. Migration carries high costs in predation and mortality, including from hunting by humans, and is driven primarily by availability of food. 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.
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. 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. 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.
The Arctic tern holds the long-distance migration record for birds, travelling between Arctic breeding grounds and the Antarctic each year. Some species of tubenoses (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. Shorter migrations are common, including altitudinal migrations on mountains such as the Andes and Himalayas.
The timing of migration seems to be controlled primarily by changes in day length. Migrating birds navigate using celestial cues from the sun and stars, the earth's magnetic field, and mental maps.
Records of bird migration were known from at least 3,000 years ago as indicated by the Ancient Greek writers Hesiod, Homer, Herodotus and Aristotle. 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?" The author of Jeremiah wrote: "Even the stork in the heavens knows its seasons, and the turtle dove, the swift and the crane keep the time of their arrival."
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. 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. 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".
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. 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:
Swallows frequently roost at night, after they begin to congregate, by the sides of rivers and pools, from which circumstance it has been erroneously supposed that they retire into the water.— Bewick
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. He concludes:
These experiments have since been amply confirmed by ... M. Natterer, of Vienna ... and the result clearly proves, what is in fact now admitted on all hands, that Swallows do not in any material instance differ from other birds in their nature and propensities [for life in the air]; but that they leave us when this country can no longer furnish them with a supply of their proper and natural food ...— Bewick
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. This bird was referred to as a Pfeilstorch, German for "Arrow stork". Since then, around 25 Pfeilstörche have been documented.
Migration is the regular seasonal movement, often north and south, undertaken by many species of birds. Bird movements include those made in response to changes 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). Migration is marked by its annual seasonality. Non-migratory birds are said to be resident or sedentary. Approximately 1800 of the world's 10,000 bird species are long-distance migrants.
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. 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.
The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter. Also, the longer days of the northern summer provide extended time for breeding birds to feed their young. This helps diurnal birds to produce larger clutches than related non-migratory species that remain in the tropics. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.
These advantages offset the high stress, physical exertion costs, and other risks of the migration. Predation can be heightened during migration: Eleonora's falcon Falco eleonorae, which breeds on Mediterranean islands, has a very late breeding season, coordinated with the autumn passage of southbound passerine migrants, which it feeds to its young. A similar strategy is adopted by the greater noctule bat, which preys on nocturnal passerine migrants. The higher concentrations of migrating birds at stopover sites make them prone to parasites and pathogens, which require a heightened immune response.
Within a species not all populations may be migratory; this is known as "partial migration". Partial migration is very common in the southern continents; in Australia, 44% of non-passerine birds and 32% of passerine species are partially migratory. In some species, the population at higher latitudes tends to be migratory and will often winter at lower latitude. The migrating birds bypass the latitudes where other populations may be sedentary, where suitable wintering habitats may already be occupied. This is an example of leap-frog migration. Many fully migratory species show leap-frog migration (birds that nest at higher latitudes spend the winter at lower latitudes), and many show the alternative, chain migration, where populations 'slide' more evenly north and south without reversing order.
Within a population, it is common for different ages and/or sexes to have different patterns of timing and distance. 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.
Most migrations begin with the birds starting off in a broad front. Often, this front narrows into one or more preferred routes termed flyways. These routes typically follow mountain ranges or coastlines, sometimes rivers, and may take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water. The specific routes may be genetically programmed or learned to varying degrees. The routes taken on forward and return migration are often different. A common pattern in North America is clockwise migration, where birds flying North tend to be further West, and flying South tend to shift Eastwards.
Many, if not most, birds migrate in flocks. For larger birds, flying in flocks reduces the energy cost. Geese in a V-formation may conserve 12–20% of the energy they would need to fly alone. Red knots Calidris canutus and dunlins Calidris alpina were found in radar studies to fly 5 km/h (3.1 mph) faster in flocks than when they were flying alone.
Birds fly at varying altitudes during migration. An expedition to Mt. 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. 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. Anecdotal reports of them flying much higher have yet to be corroborated with any direct evidence. Seabirds fly low over water but gain altitude when crossing land, and the reverse pattern is seen in landbirds. However most bird migration is in the range of 150 to 600 m (490 to 1,970 ft). 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 is not limited to birds that can fly. Most species of penguin (Spheniscidae) migrate by swimming. These routes can cover over 1,000 km (620 mi). Dusky grouse Dendragapus obscurus perform altitudinal migration mostly by walking. Emus Dromaius novaehollandiae in Australia have been observed to undertake long-distance movements on foot during droughts.
While participating in nocturnal migration, many birds give 'Nocturnal Flight Calls', which are short, contact-type calls. These calls likely serve to maintain the composition of a migrating flock, and can sometimes encode the gender of a migrating individual. They also likely serve to avoid collision in the air. 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. 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.
Nocturnal migrants land in the morning and may feed for a few days before resuming their migration. These birds are referred to as passage migrants in the regions where they occur for a short duration between the origin and destination.
Nocturnal migrants minimize depredation, avoid overheating, and can feed during the day. One cost of nocturnal migration is the loss of sleep. Migrants may be able to alter their quality of sleep to compensate for the loss.
The typical image of migration is of northern landbirds, such as swallows (Hirundinidae) and birds of prey, making long flights to the tropics. However, many Holarctic wildfowl and finch (Fringillidae) species winter in the North Temperate Zone, in regions with milder winters than their summer breeding grounds. 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. Some ducks, such as the garganey Anas querquedula, move completely or partially into the tropics. The European pied flycatcher Ficedula hypoleuca also follows this migratory trend, breeding in Asia and Europe and wintering in Africa.
Migration routes and wintering grounds are both genetically and traditionally determined depending on the social system of the species. In long-lived, social species such as white storks (Ciconia ciconia), flocks are often led by the oldest members and young storks learn the route on their first journey. In short-lived species that migrate alone, such as the Eurasian blackcap Sylvia atricapilla or the yellow-billed cuckoo Coccyzus americanus, first-year migrants follow a genetically determined route that is alterable with selective breeding.
Often, the migration route of a long-distance migratory bird doesn't follow a straight line between breeding and wintering grounds. Rather, it could follow a hooked or arched line, with detours around geographical barriers or towards suitable stopover habitat. For most land-birds, such barriers could consist of 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). 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.
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. 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.
A similar situation occurs with waders (called shorebirds in North America). 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.
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 Bay.
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. Prior to migration, 55 percent of their bodyweight is stored as fat to fuel this uninterrupted journey.
Seabird migration is similar in pattern to those of the waders and waterfowl. 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. 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. 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). Many tubenosed birds breed in the southern hemisphere and migrate north in the southern winter.
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. The tubenoses spread widely over large areas of open ocean, but congregate when food becomes available. Many are also among the longest-distance migrants; sooty shearwaters 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. Some Manx shearwaters Puffinus puffinus do this same journey in reverse. As they are long-lived birds, they may cover enormous distances during their lives; one record-breaking Manx shearwater is calculated to have flown 8 million km (5 million miles) during its over-50 year lifespan.
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, but also storks. 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. Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points. Massive numbers of large raptors and storks pass through areas such as the Strait of Messina, Gibraltar, Falsterbo, and the Bosphorus at migration times. More common species, such as the European honey buzzard Pernis apivorus, can be counted in hundreds of thousands in autumn. Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants. This is a notable factor in the Central American migratory bottleneck. Batumi bottleneck in the Caucasus is one of the heaviest migratory funnels on earth. 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. 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.
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.
Many long-distance migrants appear to be genetically programmed to respond to changing day length. Species that move short distances, however, may not need such a timing mechanism, instead moving in response to local weather conditions. 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. Other species such as merlin Falco columbarius and Eurasian skylark Alauda arvensis move further, to the coast or towards the south. 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.
Short-distance passerine migrants have two evolutionary origins. Those that have long-distance migrants in the same family, such as the common chiffchaff Phylloscopus collybita, are species of southern hemisphere origins that have progressively shortened their return migration to stay in the northern hemisphere.
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.
In the tropics there is little variation in the length of day throughout the year, and it is always warm enough for a food supply, but altitudinal migration occurs in some tropical birds. There is evidence that this enables the migrants to obtain more of their preferred foods such as fruits.
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. Several years may pass between visits to an area by a particular species.
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. 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. Red crossbills Loxia curvirostra too are irruptive, with widespread invasions across England noted in 1251, 1593, 1757, and 1791.
Bird migration is primarily, but not entirely, a Northern Hemisphere phenomenon. 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. In contrast, among (pelagic) seabirds, species of the Southern Hemisphere are more likely to migrate. This is because there is a large area of ocean in the Southern Hemisphere, and more islands suitable for seabirds to nest.
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 that may include both endogenous programs as well as learning.
The primary physiological cue for migration are the changes in the day length. These changes are also related to hormonal changes in the birds. In the period before migration, many birds display higher activity or Zugunruhe (German: migratory restlessness), first described by Johann Friedrich Naumann in 1795, 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 programs in controlling bird migrations. 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.
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 (magnetoception), use visual landmarks as well as olfactory cues.
Long distance migrants are believed to disperse as young birds and form attachments to potential breeding sites and to favourite wintering sites. Once the site attachment is made they show high site-fidelity, visiting the same wintering sites year after year.
The ability of birds to navigate during migrations cannot be fully explained by endogenous programming, even with the help of responses to environmental cues. The ability to successfully perform long-distance migrations can probably only be fully explained with an accounting for the cognitive ability of the birds to recognize habitats and form mental maps. Satellite tracking of day migrating raptors such as ospreys and honey buzzards has shown that older individuals are better at making corrections for wind drift. The birds navigate through an innate biological sense resulting from evolution. Migratory birds may use two electromagnetic tools to find their destinations: one that is entirely innate and another that relies on experience. A young bird on its first migration flies in the correct direction according to the Earth's magnetic field, but does not know how far the journey will be. It does this through a radical pair mechanism whereby chemical reactions in special photo pigments sensitive to short wavelengths are affected by the field. Although this only works during daylight hours, it does not use the position of the sun in any way. 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. 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. 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. 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.
Migrating birds can lose their way and appear outside their normal ranges. 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. Certain areas, because of their location, have become famous as watchpoints for such birds. Examples are the Point Pelee National Park in Canada, and Spurn in England.
A related phenomenon called "abmigration" involves birds from one region joining similar birds from a different breeding region in the common winter grounds and then migrating back along with the new population. This is especially common in some waterfowl, which shift from one flyway to another.
It has been possible to teach a migration route to a flock of birds, for example in re-introduction schemes. 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.
Birds need to alter their metabolism to meet the demands of migration. The storage of energy through the accumulation of fat and the control of sleep in nocturnal migrants require special physiological adaptations. In addition, the feathers of a bird suffer from wear-and-tear and require to be moulted. 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. 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.
Migration in birds is highly labile and is believed to have developed independently in many avian lineages. 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. This explains the rapid appearance of migratory behavior after the most recent glacial maximum.
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. However some species show circuitous migratory routes that reflect historical range expansions and are far from optimal in ecological terms. An example is the migration of continental populations of Swainson's thrush Catharus ustulatus, which fly far east across North America before turning south via Florida to reach northern South America; this route is believed to be the consequence of a range expansion that occurred about 10,000 years ago. Detours may also be caused by differential wind conditions, predation risk, or other factors.
Large scale climatic changes, as have been experienced in the past, are expected to have an effect on the timing of migration. Studies have shown a variety of effects including timing changes in migration, breeding as well as population variations.
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. 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. Some viruses that are maintained in birds without lethal effects, such as the West Nile Virus may however be spread by migrating birds. Birds may also have a role in the dispersal of propagules of plants and plankton.
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 routes have been studied by a variety of techniques including the oldest, marking. Swans have been marked with a nick on the beak since about 1560 in England. Scientific ringing was pioneered by Hans Christian Cornelius Mortensen in 1899. Other techniques include radar and satellite tracking. The rate of bird migration over the Alps (up to a height of 150 m) was found to be highly comparable between fixed-beam radar measurements and visual bird counts, highlighting the potential use of this technique as an objective way of quantifying bird migration.
Stable isotopes of hydrogen, oxygen, carbon, nitrogen, and sulphur can establish avian migratory connectivity between wintering sites and breeding grounds. Stable isotopic methods to establish migratory linkage rely on spatial isotopic differences in bird diet that are incorporated into inert tissues like feathers, or into growing tissues such as claws and muscle or blood.
An approach to identify migration intensity makes use of upward pointing microphones to record the nocturnal contact calls of flocks flying overhead. These are then analyzed in a laboratory to measure time, frequency and species.
An older technique developed by George Lowery and others to quantify migration involves observing the face of the full moon with a telescope and counting the silhouettes of flocks of birds as they fly at night.
Orientation behaviour studies have been traditionally carried out using variants of a setup known as the Emlen funnel, which consists of a circular cage with the top covered by glass or wire-screen so that either the sky is visible or the setup is placed in a planetarium or with other controls on environmental cues. The orientation behaviour of the bird inside the cage is studied quantitatively using the distribution of marks that the bird leaves on the walls of the cage. Other approaches used in pigeon homing studies make use of the direction in which the bird vanishes on the horizon.
Human activities have threatened many migratory bird species. The distances involved in bird migration mean that they often cross political boundaries of countries and conservation measures require international cooperation. Several international treaties have been signed to protect migratory species including the Migratory Bird Treaty Act of 1918 of the US. and the African-Eurasian Migratory Waterbird Agreement
The concentration of birds during migration can put species at risk. Some spectacular migrants have already gone extinct; during the passenger pigeon's (Ectopistes migratorius) migration the enormous flocks were a mile (1.6 km) wide, darkening the sky and 300 miles (480 km) long, taking several days to pass.
Other significant areas include stop-over sites between the wintering and breeding territories. A capture-recapture study of passerine migrants with high fidelity for breeding and wintering sites did not show similar strict association with stop-over sites.
Hunting along migration routes threatens some bird species. The populations of Siberian cranes (Leucogeranus leucogeranus) that wintered in India declined due to hunting along the route, particularly in Afghanistan and Central Asia. Birds were last seen in their favourite wintering grounds in Keoladeo National Park in 2002. Structures such as power lines, wind farms and offshore oil-rigs have also been known to affect migratory birds. Other migration hazards include pollution, storms, wildfires, and habitat destruction along migration routes, denying migrants food at stopover points. For example, in the East Asian–Australasian Flyway, up to 65% of key intertidal habitat at the Yellow Sea migration bottleneck has been destroyed since the 1950s.
The Atlantic Flyway is a major north-south flyway for migratory birds in North America. The route generally starts in Greenland, then follows the Atlantic coast of Canada, then south down the Atlantic Coast south to the tropical areas of South America and the Caribbean. Every year, migratory birds travel up and down this route following food sources, heading to breeding grounds, or travelling to overwintering sites.
This route is used by birds typically because no mountains block most of this path, though birds cross the northern, central and southern Appalachians to join the flyway. Good sources of water, food, and cover exist over its entire length. The warm climates found in the southern portion of the region are home to many northern birds in winter, while in summer the East Coast is home to many bird species from South America.Bear River Migratory Bird Refuge
Bear River Migratory Bird Refuge is a 74,000-acre (299 km2) National Wildlife Refuge in Utah, established in 1928. The refuge is part of a national system of fee ownership lands purchased from willing sellers, mostly private property owners.
The refuge encompasses the Bear River and its delta where it flows into the northern part of the Great Salt Lake in eastern Box Elder County. It includes a variety of habitats, such as open water, mudflats, wetlands, and uplands. The refuge hosts millions of migratory birds each year including species such as bald eagle and tundra swan. There are more than 41,000 acres (170 km2) of freshwater wetlands. The Refuge is approx. 80,000 acres of Federal and State lands that are managed by the U.S. Fish and Wildlife Service. The surrounding lands are occupied by multiple hunting clubs along the bird migration route.
The James V. Hansen Wildlife Education Center features interactive exhibits about the birds and wetlands of the Refuge and a 1/2-mile accessible walking trail a wetland habitat outside. Environmental education programs, symposiums and public events are offered.
There is a one-way 12 mile auto route through the Refuge, with an audio tour and map available for download through the official website.
Starting in 1983, rising floodwaters from the Great Salt Lake severely impacted the refuge. The flooding of the refuge is at the center of Terry Tempest Williams's noted nonfiction book, Refuge: An Unnatural History of Family and Place. In 2006, a new wildlife education center off Interstate 15 opened to attract visitors once more.Bolle di Magadino
The Bolle di Magadino is a 1500-ha Swiss natural reserve. It is located by the confluence of the Ticino with Lake Maggiore. It is a biotope where birds nest and stop during migrations.Central Flyway
The Central Flyway is a bird migration route that generally follows the Great Plains in the United States and Canada. The main endpoints of the flyway include central Canada and the region surrounding the Gulf of Mexico; the migration route tends to narrow considerably in the Platte River and Missouri River valleys of central and eastern Nebraska, which accounts for the high number of bird species found there. Some birds even use this flyway to migrate from the Arctic Ocean to Patagonia. Routes used by birds are typically established because no mountains or large hills block the flyway over its entire extent. Good sources of water, food, and cover exist over its entire length.
The other primary migration routes for North American birds includes the Atlantic, Mississippi and Pacific Flyways. The Central Flyway merges with the Mississippi Flyway between Missouri and the Gulf of Mexico.
The Central Flyway Council is composed of representatives from agencies responsible for migratory bird management in 10 states, two Canadian provinces and the Northwest Territories. Member states and provinces in the council are: Montana, Wyoming, Colorado, New Mexico, Texas, Oklahoma, Kansas, Nebraska, South Dakota, North Dakota, Alberta and Saskatchewan.China–Australia Migratory Bird Agreement
The China–Australia Migratory Bird Agreement (CAMBA) is a treaty between Australia and China to minimise harm to the major areas used by migratory birds which migrate between the two countries. CAMBA was first developed on October 20, 1986 and came into force on September 1, 1988.East Asian–Australasian Flyway
The East Asian–Australasian Flyway is one of the world's great flyways. At its northernmost it stretches eastwards from the Taimyr Peninsula in Russia to Alaska. Its southern end encompasses Australia and New Zealand. Between these extremes the Flyway covers much of eastern Asia, including China, Japan, Korea, South-East Asia and the western Pacific. It is especially important for the millions of migratory waders or shorebirds that breed in northern Asia and Alaska and spend the non-breeding season in South-East Asia and Australasia. In total, the flyway passes through 22 countries with approximately 55 migratory species travelling along it, equating to about 5 million birds.Flyway
A flyway is a flight path used in bird migration. Flyways generally span over continents and often oceans.GIUK gap
The GIUK gap is an area in the northern Atlantic Ocean that forms a naval choke point. Its name is an acronym for Greenland, Iceland, and the United Kingdom, the gap being the open ocean between these three landmasses. The term is typically used in relation to military topics.Migratory Bird Conservation Act
The Migratory Bird Conservation Act of 1929 of February 18, 1929, (also known as the "Norbeck-Andresen Act") created the United States Migratory Bird Conservation Commission (MBCC) to consider and approve any areas of land and/or water recommended by the Secretary of the Interior for purchase or rental by the U.S. Fish and Wildlife Service and to fix the price or prices at which such areas may be purchased or rented.
The Commission considered the establishment of new waterfowl refuges. While perhaps unimportant because of the Great Depression, the act was an important step for the conservation movement.Migratory Bird Treaty Act of 1918
The Migratory Bird Treaty Act of 1918 (MBTA), codified at 16 U.S.C. §§ 703–712 (although §709 is omitted), is a United States federal law, first enacted in 1916 to implement the convention for the protection of migratory birds between the United States and Great Britain (acting on behalf of Canada). The statute makes it unlawful without a waiver to pursue, hunt, take, capture, kill, or sell birds listed therein as migratory birds. The statute does not discriminate between live or dead birds and also grants full protection to any bird parts including feathers, eggs, and nests. Over 800 species are currently on the list.Some exceptions to the act, including the eagle feather law, are enacted in federal regulations (50 C.F.R. 22), which regulate the taking, possession, and transportation of bald eagles, golden eagles, and their "parts, nests, and eggs" for "scientific, educational, and depredation control purposes; for the religious purposes of American Indian tribes; and to protect other interests in a particular locality." Enrolled members of federally recognized tribes may apply for an eagle permit for use in "bona fide tribal religious ceremonies."The U.S. Fish and Wildlife Service issues permits for otherwise prohibited activities under the act. These include permits for taxidermy, falconry, propagation, scientific and educational use, and depredation, an example of the latter being the killing of geese near an airport, where they pose a danger to aircraft.
The Act was enacted in an era when many bird species were threatened by the commercial trade in birds and bird feathers. The Act was one of the first federal environmental laws (the Lacey Act had been enacted in 1900). The Act replaced the earlier Weeks-McLean Act (1913). Since 1918, similar conventions between the United States and four other nations have been made and incorporated into the MBTA: Mexico (1936), Japan (1972) and the Soviet Union (1976, now its successor state Russia). Some of the conventions stipulate protections not only for the birds themselves, but also for habitats and environs necessary for the birds' survival.
Constitutionally this law is of interest as it is a use of the federal treaty-making power to override the provisions of state law. The principle that the federal government may do this was upheld in the case Missouri v. Holland.Mississippi Flyway
The Mississippi Flyway is a bird migration route that generally follows the Mississippi River in the United States and the Mackenzie River in Canada. The main endpoints of the flyway include central Canada and the region surrounding the Gulf of Mexico. The migration route tends to narrow considerably in the lower Mississippi River valley in the states of Missouri, Arkansas, Mississippi, and Louisiana, which accounts for the high number of bird species found in those areas. Some birds use this flyway to migrate from the Arctic Ocean to Patagonia.
Typically birds use this route because no mountains or ridges of hills block the path over its entire extent. Good sources of water, food, and cover exist over its entire length. About 40% of all North American migrating waterfowl and shorebirds use this route.
The other primary migration routes for North American birds include the Atlantic, Central and Pacific Flyways. The Central Flyway merges with the Mississippi Flyway between Missouri and the Gulf of Mexico. In the northern portions of the Upper Mississippi River, the birds congregate in the Driftless Area, making use of the dams on the Mississippi.Pacific Flyway
The Pacific Flyway is a major north-south flyway for migratory birds in America, extending from Alaska to Patagonia. Every year, migratory birds travel some or all of this distance both in spring and in fall, following food sources, heading to breeding grounds, or travelling to overwintering sites.
Any given bird species travels roughly the same route every year, at almost the same time. Ornithologists and "bird lovers" can often predict to the day when a particular species will show up in their area.Reverse migration (birds)
Reverse migration also called reverse misorientation is a phenomenon in bird migration where a bird will fly in the opposite direction of what is species typical during the migration time.If a bird sets off in the opposite direction, shown by the orange arrow, it will end up in Western Europe instead of South East Asia. This is a mechanism that leads to birds such as Pallas's warbler turning up thousands of kilometres from where they should be. Keith Vinicombe suggested that birds from east of Lake Baikal in Siberia (circled) could not occur in western Europe because the migration routes were too north-south. Most of these lost young birds perish in unsuitable wintering grounds, but there is some evidence that a few survive, and either re-orient in successive winters, or even return to the same area.Rossitten Bird Observatory
The Rossitten Bird Observatory (Vogelwarte Rossitten in German) was the world's first ornithological observatory. It was sited at Rossitten, East Prussia (now Rybachy, Kaliningrad Oblast, Russia), on the Curonian Spit on the south-eastern coast of the Baltic Sea. It was established by German ornithologist Johannes Thienemann and operated until 1944. In 1945 East Prussia was divided between Poland, Russia and Lithuania, and most ethnic Germans expelled.Sempach Bird Observatory
The Swiss Ornithological Institute (Schweizerische Vogelwarte Sempach in German) is an ornithological research centre, which is based at the town of Sempach in the district of Sursee in the canton of Lucerne in Switzerland. Founded in 1924, it is a non-profit organization and the largest private field research institute in Switzerland. It carries out a bird ringing program as well as other ornithological research, conservation biology, and public education.Smithsonian Migratory Bird Center
The Smithsonian Migratory Bird Center is dedicated to fostering greater understanding, appreciation, and protection of bird migration.
This Smithsonian Institution research program was founded with Congressional support in 1991, and was incorporated in 1997 as part of the National Zoological Park, located in Washington, D.C.; it came under the Smithsonian Conservation Biology Institute, established in 2010.From an initial focus on the conservation biology of Neotropical songbirds, it now researches the role of disease in population declines in migratory birds, environmental challenges facing urban and suburban birds and their adaptation to changes in natural and anthropogenic habitats and climate, and the conservation biology of wetland birds. Their research group has long term research programs dealing with migratory birds in both their breeding and non-breeding areas, with studies of how specific breeding and non-breeding populations are connected by migration.In 1998 it developed the Bird Friendly coffee program that fosters management practices at coffee farms that are good for birds while remaining marketable. Coffee grown under the program is certified as shade grown and organic, with purchases supporting the conservation of migratory birds. The criteria for Bird Friendly certification was developed from basic research on migratory bird-habitat relationships by Smithsonian Migratory Bird Center scientists.
They sponsor advanced undergraduate and graduate students at collaborating institutions, as well as in-house post-doctoral fellowships. The center's education efforts include the creation of International Migratory Bird Day, a holiday which is celebrated on the second Saturday of May in the United States and Canada, and on the second Saturday of October in most of Latin America.
Neighborhood Nestwatch, its community-based science and educational outreach program, involves volunteers in monitoring the reproductive success and survival of birds in their communities.
Its Bridging the Americas/Unidos por las Aves program is an education program that partners elementary school classes in the Washington, D.C. area with classes in Latin America and the Caribbean. The goals of the program are
to teach students about the migratory birds that connect these two regions of the hemisphere and the need to protect their habitats, and
to stimulate an interest in learning about other countries and their cultures.Since 1993, over 17,000 students in grades 3 through 8 from 11 countries of the Americas have participated.Strait of Messina
The Strait of Messina (Italian: Stretto di Messina), is a narrow strait between the eastern tip of Sicily (Punta del Faro) and the western tip of Calabria (Punta Pezzo) in the south of Italy. It connects the Tyrrhenian Sea to the north with the Ionian Sea to the south, within the central Mediterranean. At its narrowest point, between Torre Faro and Villa San Giovanni, it is 3.1 km (1.9 mi) wide. At the town of Messina it is 5.1 km (3.2 mi) wide. The strait's maximum depth is about 250 m (820 ft).
The strait has strong tidal currents that create a unique marine ecosystem. A natural whirlpool in the northern portion of the strait has been linked to the Greek legend of Scylla and Charybdis. In some circumstances, the mirage of Fata Morgana can be observed when looking at Sicily from Calabria. With its bottleneck shape, it is also a compulsory point of transit of the migration of many bird species.
In 1957, a 220 kV overhead power line was built across the Strait of Messina. Its pylons are among the highest in the world. This power line has since been replaced by a submarine power cable, but the pylons remain and are protected as historical monuments (see Pylons of Messina).V formation
A V formation is the symmetric V-shaped flight formation of flights of geese, ducks, and other migratory birds. V formations also improve the fuel efficiency of aircraft and are used on military flight missions.Weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather.
During World War II, radar operators discovered that weather was causing echoes on their screen, masking potential enemy targets. Techniques were developed to filter them, but scientists began to study the phenomenon. Soon after the war, surplus radars were used to detect precipitation. Since then, weather radar has evolved on its own and is now used by national weather services, research departments in universities, and in television stations' weather departments. Raw images are routinely used and specialized software can take radar data to make short term forecasts of future positions and intensities of rain, snow, hail, and other weather phenomena. Radar output is even incorporated into numerical weather prediction models to improve analyses and forecasts.