Although bird migrations have been observed for thousands of years, it was not until the early nineteenth century that migration was accepted as an explanation for the winter disappearance of birds from northern climes (Lincoln 1979).
Bird migration has been studied using a variety of techniques, of which ringing is the oldest. Color marking, use of radar, satellite tracking, and stable Hydrogen and Strontium isotopes are some of the other techniques being used today to study the migration of birds (Font et al. 2007).
To identify migration intensity, one contemporary approach makes use of upward pointing microphones to record the contact calls of overflying flocks; these calls are then analyzed in a laboratory to measure time, frequency, and species (Farnsworth et al. 2004).
What is migration? Migration refers to directed, regular, or systematic movement of a group of objects, organisms, people, or animals. There are two type of migration such as immigration and emigration. Bird migration is one of the most fascinating views for bird watchers and it is refers to magnificent regular journeys to and from a given area undertaken by all or part of a bird population. The bird migration happens to temperate reagent birds but it also occurs in tropic birds. It is also known as nomadism, invasion, dispersal or irruption. Since migration takes a lot of time and energy, they must have really good explanations to do so. These also will increase the chance of survive and reproduce again so that they not become extinct.
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Migration is marked by its annual seasonality and bird movements include those made in response to changes in food availability, habitat or weather. Instead of difficulties to find foods and weather changes such as cold temperature, it’s been shown that at least in some birds there are also indicator such as environmental cue. The changes in day length cause pituitary gland (an endocrine gland that controls the release of hormonal stimuli) in the birds’ bodies to produce hormones that profound changes inside the birds, changes that prepare them for the flight south. As days grow shorter in the fall, fat accumulates under the skin and this will supplies energy needed for those coming days when the birds flying than they will eat during occasional rests. This shows the birds are already prepared with vary of probability.
Not all birds are long-distance migratory birds, some are short-distance migratory, some are residents and some are partial migratory. There are four types of residents such as permanent resident, summer resident, winter resident and transient. Permanent resident are non-migrating birds such as House Sparrows who remain in their home area all year round. Summer residents are migratory birds such as Purple Martins who arrive in our Northern in the spring, nest during summer and return south to wintering grounds in the fall. Winter residents are migratory birds who have come to south for the winter such as White-throated Sparrows, who are summer resident in much of Canada, are winter residents in much of the U.S.
Birds fly at varying altitudes during migration. 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 known as “leap-frog migration”. Within a population, there can also be different patterns of timing and migration based on the age groups and sex. These occur to some species such as Chaffinches. Only the female Chaffinches in Scandinavia migrate, while the males staying resident. This has given rise to the latter’s specific name of coelebs, a bachelor.
Besides that, there also an expedition to Mt. Everest found skeletons of Pintail and Black-tailed Godwit at 5000 m (16,400 ft) on the Khumbu Glacier. Bar-headed Geese have been seen flying over the highest peaks of the Himalayas above 8000 m (29000 ft) even when low passes of 3000 m (10000 ft) were nearby. 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 m (500 ft) to 600 m (2000 ft).
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Bird-hit aviation records from the United States show most collisions occur below 600 m (2000 ft) and almost none above 1800 m (6000 ft).
In contrast, some bird species migrate by swimming such as penguin. These routes can cover over 1000 km. Blue Grouse Dendragapus obscurus perform altitudinal migration mostly by walking. Emus in Australia have been observed to undertake long-distance movements on foot during droughts.
Migration often is concentrated along narrow belts that are well-established as traditional routes known as flyways. . There are four major flyways in North America: the Pacific, Central, Mississippi and Atlantic Flyway. The flyways are actually shaped by geographical, ecological, and even meteorological factors. It is typically follow mountain ranges or coastlines, and may have the 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 from the experiences. Some research state that the birds are guided by innate behaviors including hormonal signals that enable them to know when to depart and that orient them toward a specific location over long distances.
However, they also find their way to their destination in vary different ways such as they can navigate by the sun or the stars. An internal clock mechanism enables birds to use the sun as a point of orientation, determining the angle of the sun above the horizon. Nocturnal migrants may also use the stars to get their bearings. They have very good vision and watch for landmarks such as mountains and rivers. The birds like petrels and pigeons can also use their sense of smell but perhaps the most interesting navigation device are tiny grains of mineral called magnetite which is found just above their nostrils and which scientists think may act like compass, telling them which way to go. Many birds have been demonstrated to have a “compass sense;” i.e., they are able to fly in a particular constant direction, regardless of their release point.
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Despite the genetic and environmental factors that guide them along specific routes, migrating birds can still lose their way. In a phenomenon known as the “spring overshoot,” birds returning to their breeding areas overshoot their destination and end up further north than intended. “Drift migrations” of birds blown off course by the wind can result in “falls” of large numbers of migrants at coastal sites. The timing and response of migration are in large part genetically controlled. In contrast, the ability of migratory birds to navigate and orient themselves during migration is a much more complex phenomenon that may include both endogenous (internal) programs as well as learned behavior (Helm and Gwinner 2006).
Bird species have diverse modes of migration. The varied modes and patterns of bird migration may be as adaptations. In fact, migration itself has conferred an advantage to only certain bird species, while not evolving in other species that remain resident, or sedentary, year-round. Whether a particular species migrates depends on a number of factors. The climate of the breeding area is important, as few species can cope with the harsh winters of inland Canada or northern Eurasia. The nature of the staple food is also significant. Most specialist insect eaters that breed outside the tropics are long-distance migrants, and have little choice but to head south in winter.
The most common pattern involves flying north in the spring to breed in the temperate as such as Arctic summer and returning in the fall to wintering grounds in warmer regions to the south. The longer days of the northern summer provide greater opportunities for reproduce and raise their young. Long summer days in the north produce an abundance of insects and plants for birds and their young to eat. The extended daylight hours allow diurnal birds to produce larger clutches of eggs than those of related non-migratory species that remain in the tropics year round. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season. These advantages are offset the high stress, physical effort costs, and other risks of the migration such as predation (high and unpredictable).
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In many tropics regions have wet and dry season and there is little variation in the length of day throughout the year, and it is always warm enough for an adequate food supply. Apart from the seasonal movements of northern hemisphere wintering species, most species are in the broadest sense resident. However many species undergo movements of varying distances depending on the rainfall. Apart from the seasonal movements of Northern Hemisphere wintering species, most species are in the broadest sense resident. The Woodland Kingfisher migrates into the equatorial zone during the dry season. There are also notably cuckoos, which are pure long-distance migrants within the tropics. An example is the Lesser Cuckoo, which breeds in India and spends the non-breeding season in Africa.
The migratory routes of various large birds and many other birds of prey take advantage of ecological features like thermal columns to power their flight. Some large broad-winged birds rely on thermal columns of rising hot air to enable them to glide hover in the air. These include many birds of prey, such as vultures, eagles, and buzzards, as well as storks. Migratory species in these groups have great difficulty crossing large bodies of water, since thermals form over land only. The Mediterranean and other seas therefore present a major obstacle to soaring birds, which are forced to cross at the narrowest points. Massive numbers of large raptors and storks pass through areas such as Gibraltar, Falsterbo, and the Bosphorus at migration times.
Migrations may be diurnal (during the day) or nocturnal (at night).
By following established routes, some species risk predation during periods of peak migration. Many of the smaller insectivorous birds, including the warblers, hummingbirds, and flycatchers, are nocturnal migrants. By migrating at night, they minimize the risk of predation, and avoid the overheating that could result from the energy expended to fly such long distances. Those smaller species that migrate during the day tend to be those making movements that are relatively short and weather-driven, like the larks and finches, or that can feed on the wing, like swallows and swifts.
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To make the migration success and safe they have to adapt with many things such as the birds need to alter their metabolism in order 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 molted. The timing of this molt usually once a year but sometimes twice, it is varies with some species molting prior to moving to their winter grounds and others molting prior to returning to their breeding grounds. Apart from physiological adaptations, migration sometimes requires behavioral changes such as flying in flocks to reduce the energy used in migration or the risk of predation. Many large birds fly in a V-formation, which helps individuals save 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 per hour faster in flocks than when they were flying alone.
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. The concentration of birds during migration can put species at risk. Some spectacular migrants have already gone extinct, the most notable being the Passenger Pigeon (Ectopistes migratorius).
During migration the flocks were a mile (1.6 km) wide and 300 miles (500 km) long, taking several days to pass and containing up to a billion birds. Habitat destruction by land use changes is the biggest threat, and shallow wetlands that are stopover and wintering sites for migratory birds are particularly threatened by draining and reclamation for human use. 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 Water Bird Agreement.