Bird migration

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A flock of barnacle geese during autumn migration BrantaLeucopsisMigration.jpg
A flock of barnacle geese during autumn migration
Examples of long distance bird migration routes. Migrationroutes.svg
Examples of long distance bird migration routes.

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.

Flyway aerial route taken by migrating birds

A flyway is a flight path used by large numbers of birds while migrating between their breeding grounds and their overwintering quarters. Flyways generally span continents and often pass over oceans. Although applying to any species of migrating bird, it is waterfowl and shore birds for which the concept was first conceived. The flyways can be thought of as wide arterial highways to which the migratory routes of different species are tributaries. An alternative definition is that a flyway is the entire range of a migratory bird, encompassing both its breeding and non-breeding grounds, and the resting and feeding locations it uses while migrating. There are four major north/south flyways in North America and six covering Eurasia, Africa and Australasia.

Breeding in the wild is the natural process of animal reproduction occurring in the natural habitat of a given species. This terminology is distinct from animal husbandry or breeding of species in captivity. Breeding locations are often chosen for very specific requirements of shelter and proximity to food; moreover, the breeding season is a particular time window that has evolved for each species to suit species anatomical, mating-ritual, or climatic and other ecological factors. Many species migrate considerable distances to reach the requisite breeding locations. Certain common characteristics apply to various taxa within the animal kingdom, which traits are often sorted among amphibians, reptiles, mammals, avafauna, arthropods and lower life forms.

Bird Warm-blooded, egg-laying vertebrates with wings, feathers and beaks

Birds, also known as Aves or avian dinosaurs, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They rank as the world's most numerically-successful class of tetrapods, with approximately ten thousand living species, more than half of these being passerines, sometimes known as perching birds. Birds have wings which are more or less developed depending on the species; the only known groups without wings are the extinct moa and elephant birds. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in flightless birds, including ratites, penguins, and diverse endemic island species of birds. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming.

Contents

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.

Stork family of birds

Storks are large, long-legged, long-necked wading birds with long, stout bills. They belong to the family called Ciconiidae, and make up the order Ciconiiformes. Ciconiiformes previously included a number of other families, such as herons and ibises, but those families have been moved to other orders.

European turtle dove species of bird

The European turtle dove is a member of the bird family Columbidae, the doves and pigeons.

Swallow family of birds

The swallows, martins and saw-wings, or Hirundinidae, are a family of passerine birds found around the world on all continents, including occasionally in Antarctica. Highly adapted to aerial feeding, they have a distinctive appearance. The term Swallow is used colloquially in Europe as a synonym for the barn swallow. There are around 90 species of Hirundinidae, divided into 19 genera, with the greatest diversity found in Africa, which is also thought to be where they evolved as hole-nesters. They also occur on a number of oceanic islands. A number of European and North American species are long-distance migrants; by contrast, the West and South African swallows are non-migratory.

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.

Arctic tern A bird in the family Laridae with a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe, Asia, and North America

The Arctic tern is a tern in the family Laridae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe, Asia, and North America. The species is strongly migratory, seeing two summers each year as it migrates along a convoluted route from its northern breeding grounds to the Antarctic coast for the southern summer and back again about six months later. Recent studies have shown average annual roundtrip lengths of about 70,900 km (44,100 mi) for birds nesting in Iceland and Greenland and about 90,000 km (56,000 mi) for birds nesting in the Netherlands. These are by far the longest migrations known in the animal kingdom. The Arctic tern flies as well as glides through the air. It nests once every one to three years ; once it has finished nesting it takes to the sky for another long southern migration.

Antarctic region around the Earths South Pole

The Antarctic is a polar region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other island territories located on the Antarctic Plate or south of the Antarctic Convergence. The Antarctic region includes the ice shelves, waters, and all the island territories in the Southern Ocean situated south of the Antarctic Convergence, a zone approximately 32 to 48 km wide varying in latitude seasonally. The region covers some 20 percent of the Southern Hemisphere, of which 5.5 percent is the surface area of the Antarctic continent itself. All of the land and ice shelves south of 60°S latitude are administered under the Antarctic Treaty System. Biogeographically, the Antarctic ecozone is one of eight ecozones of the Earth's land surface.

Procellariiformes order of birds

Procellariiformes is an order of seabirds that comprises four families: the albatrosses, petrels and shearwaters, and 2 families of storm petrels. Formerly called Tubinares and still called tubenoses in English, they are often referred to collectively as the petrels, a term that has been applied to all Procellariiformes, or more commonly all the families except the albatrosses. They are almost exclusively pelagic, and have a cosmopolitan distribution across the world's oceans, with the highest diversity being around New Zealand.

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.

Historical views

In the Pacific, traditional landfinding techniques used by Micronesians and Polynesians suggest that bird migration was observed and interpreted for more than 3000 years. In Samoan tradition, for examgple, 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. [1] 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. The Bible also notes migrations, as in the Book of Job, [2] where the inquiry is made: "Is it by your insight that the hawk hovers, spreads its wings southward?" The author of Jeremiah [3] 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."

Ancient Greece Civilization belonging to an early period of Greek history

Ancient Greece was a civilization belonging to a period of Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Roughly three centuries after the Late Bronze Age collapse of Mycenaean Greece, Greek urban poleis began to form in the 8th century BC, ushering in the Archaic period and colonization of the Mediterranean Basin. This was followed by the period of Classical Greece, an era that began with the Greco-Persian Wars, lasting from the 5th to 4th centuries BC. Due to the conquests by Alexander the Great of Macedon, Hellenistic civilization flourished from Central Asia to the western end of the Mediterranean Sea. The Hellenistic period came to an end with the conquests and annexations of the eastern Mediterranean world by the Roman Republic, which established the Roman province of Macedonia in Roman Greece, and later the province of Achaea during the Roman Empire.

Hesiod ancient Greek poet

Hesiod was a Greek poet generally thought by scholars to have been active between 750 and 650 BC, around the same time as Homer. He is generally regarded as the first written poet in the Western tradition to regard himself as an individual persona with an active role to play in his subject. Ancient authors credited Hesiod and Homer with establishing Greek religious customs. Modern scholars refer to him as a major source on Greek mythology, farming techniques, early economic thought, archaic Greek astronomy and ancient time-keeping.

Homer name ascribed by the ancient Greeks to the legendary author of the Iliad and the Odyssey

Homer is the legendary author of the Iliad and the Odyssey, two epic poems that are the central works of ancient Greek literature. The Iliad is set during the Trojan War, the ten-year siege of the city of Troy by a coalition of Greek kingdoms. It focuses on a quarrel between King Agamemnon and the warrior Achilles lasting a few weeks during the last year of the war. The Odyssey focuses on the ten-year journey home of Odysseus, king of Ithaca, after the fall of Troy. Many accounts of Homer's life circulated in classical antiquity, the most widespread being that he was a blind bard from Ionia, a region of central coastal Anatolia in present-day Turkey. Modern scholars consider these accounts legendary.

Aristotle noted that cranes traveled from the steppes of Scythia to marshes at the headwaters of the Nile. Pliny the Elder, in his Historia Naturalis , repeats Aristotle's observations. [4]

Scythia multinational region of Central Eurasia in the classical era

Scythia was a region of Central Eurasia in classical antiquity, occupied by the Eastern Iranian Scythians, encompassing Central Asia and parts of Eastern Europe east of the Vistula River, with the eastern edges of the region vaguely defined by the Greeks. The Ancient Greeks gave the name Scythia to all the lands north-east of Europe and the northern coast of the Black Sea.

Nile River in Africa and the longest river in the world

The Nile is a major north-flowing river in northeastern Africa, and is the longest river in Africa and the disputed longest river in the world. The Nile, which is about 6,650 km (4,130 mi) long, is an "international" river as its drainage basin covers eleven countries, namely, Tanzania, Uganda, Rwanda, Burundi, the Democratic Republic of the Congo, Kenya, Ethiopia, Eritrea, South Sudan, Republic of the Sudan and Egypt. In particular, the Nile is the primary water source of Egypt and Sudan.

Pliny the Elder Roman military commander and writer

Pliny the Elder was a Roman author, a naturalist and natural philosopher, a naval and army commander of the early Roman Empire, and a friend of emperor Vespasian.

Swallow migration versus hibernation

Minoan fresco of swallows in springtime at Akrotiri, c. 1500 BC Fresque du printemps, Akrotiri, Grece.jpg
Minoan fresco of swallows in springtime at Akrotiri, c. 1500 BC

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" [5] 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. [6] 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", [6] 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". [7]

Elliott Coues American ornithologist

Elliott Coues was an American army surgeon, historian, ornithologist and author.

Gilbert White British priest and naturalist

Gilbert White FRS was a "parson-naturalist", a pioneering English naturalist and ornithologist. He remained unmarried and a curate all his life. He is best known for his Natural History and Antiquities of Selborne.

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. [4] 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", [8] and states the situation in Britain as follows:

The Rostocker Pfeilstorch, found in 1822, demonstrated that birds migrated rather than hibernating or changing form in winter. Rostocker Pfeilstorch.jpg
The Rostocker Pfeilstorch , found in 1822, demonstrated that birds migrated rather than hibernating or changing form in winter.

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 [9]

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 [10]

Pfeilstörche

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. [11] [12] [13] This bird was referred to as a Pfeilstorch , German for "Arrow stork". Since then, around 25 Pfeilstörche have been documented.

General patterns

Flocks of birds assembling before migration southwards (probably common starling) 101111 Maison 007.jpg
Flocks of birds assembling before migration southwards (probably common starling)
Migrating waders in Roebuck Bay, Western Australia Waders in flight Roebuck Bay.jpg
Migrating waders in Roebuck Bay, Western Australia

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. [14] Non-migratory birds are said to be resident or sedentary. Approximately 1800 of the world's 10,000 bird species are long-distance migrants. [15] [16]

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. [17]

The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter. [18] 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. [19]

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. [20] [21] [22] The higher concentrations of migrating birds at stopover sites make them prone to parasites and pathogens, which require a heightened immune response. [17]

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. [23] 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. [24] 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. [25]

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 [26] and the European tits of genera Parus and Cyanistes only migrate their first year. [27]

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. [17] 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. [28] [29] 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. [17]

Northern pintail skeletons have been found high in the Himalayas Northern Pintail.jpg
Northern pintail skeletons have been found high in the Himalayas

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. [30] 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. [31] Seabirds fly low over water but gain altitude when crossing land, and the reverse pattern is seen in landbirds. [32] [33] 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). [34]

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. [17]

Nocturnal migratory behavior

While participating in nocturnal migration, many birds give 'Nocturnal Flight Calls', which are short, contact-type calls. [35] These calls likely serve to maintain the composition of a migrating flock, and can sometimes encode the gender of a migrating individual. [36] They also likely serve to avoid collision in the air. [35] Nocturnal migration can also be monitored using weather radar data, [37] which can be used to estimate the number of birds migrating on a given night, as well as the direction of the migration. [38] Future research in this field includes the automatic detection and identification of nocturnally calling migrant birds, [39] 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. [40]

Nocturnal migrants minimize depredation, avoid overheating, and can feed during the day. [4] 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. [41]

Long-distance migration

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 [42] and the American goldfinch from taiga to wintering grounds extending from the American South northwestward to Western Oregon. [43] 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. [44] 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. [45] [46]

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). [14] 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. [47]

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. [48]

In waders

Bar-tailed godwit BartailedGodwit24.jpg
Bar-tailed godwit

A similar situation occurs with waders (called shorebirds in North America). Many species, such as dunlin Calidris alpina [49] and western sandpiper Calidris mauri, [50] 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. [51]

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. [52] [53]

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. [54] Prior to migration, 55 percent of their bodyweight is stored as fat to fuel this uninterrupted journey.

In seabirds

The Arctic tern migrates the longest distance of any bird. Arctic terns.jpg
The Arctic tern migrates the longest distance of any bird.

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. [55] 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. [56]

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. [57]

Diurnal migration in raptors

Griffon vulture soaring Vulture 19o05.jpg
Griffon vulture soaring

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, [58] 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. [59] 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. [60]

The bottleneck of the Strait of Messina, point of transit of the migrations, seen from the Peloritani mountains, Sicily The natural terrace of Mount Dinnammare on the Strait of Messina, sicily .JPG
The bottleneck of the Strait of Messina, point of transit of the migrations, seen from the Peloritani mountains, Sicily

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. [61]

Ruby-throated hummingbird Rubythroathummer65.jpg
Ruby-throated hummingbird

Short-distance and altitudinal migration

Cedar waxwing Cedar Waxwing-27527-1.jpg
Cedar waxwing

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. [62]

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. [63]

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. [64]

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. [65]

Altitudinal migration is common on mountains worldwide, such as in the Himalayas and the Andes. [66]

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. [67]

Irruptions and dispersal

Mediaeval sketch by Matthew Paris in his Chronica Majora (1251) recording that year's major irruption of red crossbills into England Matthew Paris 1251 sketch of Crossbill eating fruit.jpg
Mediaeval sketch by Matthew Paris in his Chronica Majora (1251) recording that year's major irruption of red crossbills into England

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. [64] Red crossbills Loxia curvirostra too are irruptive, with widespread invasions across England noted in 1251, 1593, 1757, and 1791. [68]

Bird migration is primarily, but not entirely, a Northern Hemisphere phenomenon. [69] 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. [70]

Physiology and control

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. [71]

Timing

The primary physiological cue for migration is 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. [72] 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. [73]

In polygynous species with considerable sexual dimorphism, males tend to return earlier to the breeding sites than their females. This is termed protandry. [74] [75]

Orientation and navigation

The routes of satellite tagged bar-tailed godwits migrating north from New Zealand. This species has the longest known non-stop migration of any species, up to 10,200 km (6,300 mi). Bar-tailed Godwit migration.jpg
The routes of satellite tagged bar-tailed godwits migrating north from New Zealand. This species has the longest known non-stop migration of any species, up to 10,200 km (6,300 mi).

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 (magnetoreception), use visual landmarks as well as olfactory cues. [76]

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. [77]

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. [78] 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. [79] 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. [80] [81]

Vagrancy

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.

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. [82]

Drift migration of birds blown off course by the wind can result in "falls" of large numbers of migrants at coastal sites. [83]

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. [84]

Migration conditioning

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. [85] [86]

Adaptations

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. [87] [88] 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. [89]

Evolutionary and ecological factors

Migration in birds is highly labile and is believed to have developed independently in many avian lineages. [90] 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. [91] This explains the rapid appearance of migratory behavior after the most recent glacial maximum. [92]

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. [93]

Climate change

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 [94] as well as population variations. [95] [96] Many species have been expanding their range as a likely consequence of climate change. This is sometimes in the form of former vagrants becoming established or regular migrants. [97] Currently legislation and protection mechanisms have been slow to recognise the concept of range-shifts from climate refugee birds.

Ecological effects

The migration of birds also aids the movement of other species, including those of ectoparasites such as ticks and lice, [98] 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. [99] Some viruses that are maintained in birds without lethal effects, such as the West Nile Virus may however be spread by migrating birds. [100] Birds may also have a role in the dispersal of propagules of plants and plankton. [101] [102]

Some predators take advantage of the concentration of birds during migration. Greater noctule bats feed on nocturnal migrating passerines. [21] Some birds of prey specialize on migrating waders. [103]

Study techniques

Radars for monitoring bird migration. Kihnu, Estonia. Linnuradarid linnutee randureid seiramas.jpg
Radars for monitoring bird migration. Kihnu, Estonia.

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. [104]

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. [105] Other techniques include radar [106] and satellite tracking. [107] 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. [108]

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. [109] [110]

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. [111]

Emlen funnel EmlenFunnel.svg
Emlen funnel

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. [112] [113]

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. [114] Other approaches used in pigeon homing studies make use of the direction in which the bird vanishes on the horizon. [115]

Threats and conservation

Migration routes and countries with illegal hunting in Europe Illegal hunting on migratory birds in europe.png
Migration routes and countries with illegal hunting in Europe

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. [116] and the African-Eurasian Migratory Waterbird Agreement [117]

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. [118]

Other significant areas include stop-over sites between the wintering and breeding territories. [119] 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. [120]

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. [121] Structures such as power lines, wind farms and offshore oil-rigs have also been known to affect migratory birds. [122] Other migration hazards include pollution, storms, wildfires, and habitat destruction along migration routes, denying migrants food at stopover points. [123] 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. [124] [125]

See also

Related Research Articles

Bird of prey species of bird that primarily hunt and feed on vertebrates that are large relative to the hunter

Birds of prey, or raptors, include species of bird that primarily hunt and feed on vertebrates that are large relative to the hunter. Additionally, they have keen eyesight for detecting food at a distance or during flight, strong feet equipped with talons for grasping or killing prey, and powerful, curved beaks for tearing flesh. The term raptor is derived from the Latin word rapio, meaning to seize or take by force. In addition to hunting live prey, most also eat carrion, at least occasionally, and vultures and condors eat carrion as their main food source.

Cuckoo family of birds

The cuckoos are a family of birds, Cuculidae, the sole taxon in the order Cuculiformes. The cuckoo family includes the common or European cuckoo, roadrunners, koels, malkohas, couas, coucals and anis. The coucals and anis are sometimes separated as distinct families, the Centropodidae and Crotophagidae respectively. The cuckoo order Cuculiformes is one of three that make up the Otidimorphae, the other two being the turacos and the bustards.

European roller species of bird

The European roller is the only member of the roller family of birds to breed in Europe. Its overall range extends into the Middle East, Central Asia and Morocco.

Tundra swan species of bird

The tundra swan is a small Holarctic swan. The two taxa within it are usually regarded as conspecific, but are also sometimes split into two species: Bewick's swan of the Palaearctic and the whistling swan proper of the Nearctic. Birds from eastern Russia are sometimes separated as the subspecies C. c. jankowskii, but this is not widely accepted as distinct, with most authors including them in C. c. bewickii. Tundra swans are sometimes separated in the subgenus Olor together with the other Arctic swan species.

White stork species of bird

The white stork is a large bird in the stork family, Ciconiidae. Its plumage is mainly white, with black on its wings. Adults have long red legs and long pointed red beaks, and measure on average 100–115 cm (39–45 in) from beak tip to end of tail, with a 155–215 cm (61–85 in) wingspan. The two subspecies, which differ slightly in size, breed in Europe, northwestern Africa, southwestern Asia and southern Africa. The white stork is a long-distance migrant, wintering in Africa from tropical Sub-Saharan Africa to as far south as South Africa, or on the Indian subcontinent. When migrating between Europe and Africa, it avoids crossing the Mediterranean Sea and detours via the Levant in the east or the Strait of Gibraltar in the west, because the air thermals on which it depends for soaring do not form over water.

Black stork A large migratory bird in the family Ciconiidae

The black stork is a large bird in the stork family, Ciconiidae. It was first described by Carl Linnaeus in the 10th edition of his Systema Naturae. Measuring on average 95 to 100 cm from beak tip to end of tail with a 145-to-155 cm (57-to-61 in) wingspan, the adult black stork has mainly black plumage, with white underparts, long red legs and a long pointed red beak. A widespread, but uncommon, species, it breeds in scattered locations across Europe, and Asia to the Pacific Ocean. It is a long-distance migrant, with European populations wintering in tropical Sub-Saharan Africa, and Asian populations in the Indian subcontinent. When migrating between Europe and Africa, it avoids crossing the Mediterranean Sea and detours via the Levant in the east or the Strait of Gibraltar in the west. An isolated, non-migratory, population occurs in Southern Africa.

Reverse migration (birds) phenomenon in bird migration

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.

Red knot species of bird

The red knot is a medium-sized shorebird which breeds in tundra and the Arctic Cordillera in the far north of Canada, Europe, and Russia. It is a large member of the Calidris sandpipers, second only to the great knot. Six subspecies are recognised.

Garden warbler A small migratory passerine bird

The garden warbler is a common and widespread small bird that breeds in most of Europe and in western Asia. It is a plain, long-winged and long-tailed typical warbler with brown upperparts and dull white underparts; the sexes are similar and juveniles resemble the adults. Its two subspecies differ only slightly and interbreed where their ranges overlap. Due to its lack of distinguishing features, this species can be confused with a number of other unstreaked warblers. The garden warbler's rich melodic song is similar to that of the blackcap, its closest relative, which competes with it for territory when nesting in the same woodland.

Pectoral sandpiper species of bird

The pectoral sandpiper is a small, migratory wader that breeds in North America and Asia, wintering in South America and Oceania. It eats small invertebrates. Its nest, a hole scraped in the ground and with a thick lining, is deep enough to protect its four eggs from the cool breezes of its breeding grounds. The pectoral sandpiper is 21 cm (8.3 in) long, with a wingspan of 46 cm (18 in).

Blackpoll warbler species of bird

The blackpoll warbler is a New World warbler. Breeding males are mostly black and white. They have a prominent black cap, white cheeks and white wing bars. The blackpoll breeds in forests of northern North America, from Alaska throughout most of Canada, to the mountains of New York and New England. They are a common migrant through much of North America. Come fall, they fly South to the Greater Antilles and the Northeastern coasts of South America in a non-stop long-distance migration over open water, averaging 2500 km, one of the longest distance non-stop overwater flights ever recorded for a migratory songbird. Rare vagrants to western Europe, they are one of the more frequent transatlantic passerine wanderers.

Worm-eating warbler species of bird

The worm-eating warbler is a small New World warbler that breeds in the Eastern United States and migrates to southern Mexico and Central America for the winter.

Animal migration seasonal movement of animals

Animal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migration in ecology. It is found in all major animal groups, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. The trigger for the migration may be local climate, local availability of food, the season of the year or for mating reasons. To be counted as a true migration, and not just a local dispersal or irruption, the movement of the animals should be an annual or seasonal occurrence, such as Northern Hemisphere birds migrating south for the winter; wildebeest migrating annually for seasonal grazing; or a major habitat change as part of their life, such as young Atlantic salmon or Sea lamprey leaving the river of their birth when they have reached a few inches in size.

Lepidoptera migration

Many populations of Lepidoptera migrate, sometimes long distances, to and from areas which are only suitable for part of the year. Lepidopterans migrate on all continents except Antarctica, including from or within subtropical and tropical areas. By migrating, these species can avoid unfavorable circumstances, including weather, food shortage, or over-population. In some lepidopteran species, all individuals migrate; in others, only some migrate.

Bird migration perils

Migrating birds face many perils as they travel between breeding and wintering grounds each year.

Altitudinal migration

Altitudinal migration is a short-distance animal migration from lower altitudes to higher altitudes and back. It is commonly thought to happen in response to climate and food availability changes as well as increasingly due to anthropogenic influence. These migrations can occur both during reproductive and non-reproductive seasons. Altitudinal avian migration is common, and can also be found in other vertebrates, and can be seen in some invertebrates.

Bird fallout or migration fallout is the result of severe weather preventing migratory birds from reaching their destination. This can occur while birds are traveling south or returning to their breeding grounds. Due to the distance travelled, birds will not have enough energy to continue flight when encountering high winds. This exhaustion results in many birds resting in one area. This may be very stressful on the birds and on the surrounding ecology. Bird fallout is not particularly common, as it stems from the chance event of severe winds found in inclement weather. Due to the rare occurrence of a migratory fallout, as well as the abundance of birds resting in a single location, it is a sought-after event for birders.

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