In this article we will discuss about:- 1. Migratory and Resident Birds 2. Kinds of Migrations 3. Modes of Flight 4. Problems 5. Advantages and Disadvantages.
Contents:
- Migratory and Resident Birds
- Kinds of Migrations
- Modes of Flight in Migration
- Problems of Migration
- Advantages and Disadvantages of Avian Migration
1. Migratory and Resident Birds:
Individual organisms often move about in their environment in response to a varied group of stimuli. When such movements result in the temporary or permanent absence from one home range and the establishment of a residence in another, the resulting movement is said to be a migration (L., migrare = to travel). Migration is a periodic passing of animals from one place to another (Cahn). Bird’s migration is a two-way journey, i.e., a regular, periodic, to-and-fro movement of some birds between their summer and winter homes or from a breeding and nesting place to a feeding and resting place.
ADVERTISEMENTS:
Majority of birds have the inherent quality to move from one place to another to obtain the advantages of the favourable condition. Such birds are called migratory birds. Some birds such as bobwhite and ruffled sand grouse, however, do not migrate at all and they remain throughout the year in a country. Such birds are called resident birds.
Migratory birds cover thousands of miles in their periodic seasonal journeys. Migration is a remarkable device to obtain the advantage of the favourable conditions in more northerly regions only during the summer. Migration occurs during the particular period of the year and the birds usually follow the same route.
2. Kinds of Migrations:
The migratory birds migrate in a variety of ways and accordingly following kinds of avian migrations have been recognised by different ornithologists:
ADVERTISEMENTS:
1. Latitudinal Migration:
The latitudinal migration usually means the movement from north to south and vice versa. Most birds live in the landmasses of the northern temperate and subarctic zones where they get facilities for nesting and feeding during summer.
During winter, when the northern hemisphere becomes covered with ice and snow, these birds move towards South for shelter. Several North American and Eurasian birds cross the equator to spend winter in deeper and warmer parts of South America and Africa.
The American golden plover (Pluvialis dominica) passes the nine months of winter 8,000 miles south in the plains of Argentina, thus, enjoying two summers each year and knows not a hint of winter. Ruff breeds at Siberia and travels to Great Britain, Africa, plains of India and Sri Lanka, thus, travelling a distance of 6,000 miles.
ADVERTISEMENTS:
An opposite but lesser movement also occurs in the southern hemisphere when the seasons are changed. Some tropical birds migrate to breed during rainy season to the outer tropics and return to the central tropics in dry season. Marine birds may also make extensive migrations. The Arctic tern (Sterna) breeds in the north temperate zone, and migrates to the Antarctic along both sides of the Atlantic. Penguins make migrations by swimming.
2. Altitudinal Migration:
Altitudinal or vertical migrations from high mountains in the summer to low valleys in the winter occur in many Indian and foreign mountaineer birds. In India a number of species during summer migrate from planes to the slopes of Himalayas ascending thousands of feet above sea level and return to planes on the commencement of winter, e.g., common wood-cock.
Bush chat and Scolopax ruticola. Such altitudinal migration also occurs in the grebes and coots of Andes in Argentina, violet green swallows of Great Britain, and the willow ptarmigan of Siberia. The brown plumage of willow ptarmigan turns white in winter, and diet of insects shifted to buds and twigs of alders and willows.
3. Longitudinal Migration:
Longitudinal or east-west and vice-versa migration also occur in some birds. The Patagonian plover visits the Falkland Island, and south Patagonian in September and October for breeding.
4. Partial Migrations:
Many species, such as blue birds and many blue jays of Canada and northern United States and bam owls (Tyto alba) migrate southwards to mingle with the sedentary populations of the southern states, are only partial migrants. In such cases, all the birds of a group of migratory birds do not leave the native land, visible throughout the year. Actually these are partial migrants, because the birds visible in winter are not the same as seen in summer. Songthrush, redbreast, titmouse, finch, etc., are partial migrants.
5. Erratic Migration:
ADVERTISEMENTS:
The erratic, vagrant, irregular or wandering migration occurs in great blue heron, cuckoos, thrushes and warblers. In such birds, after breeding, the adults and the young may stray from their home to disperse in all directions over many or a few hundred miles in search of food and safety from enemies. Sometimes hurricanes take the sea birds as far as 2000 miles away from home seas and there they die either due to exhaustion or due to unknown shores.
6. Seasonal Migration:
Some ornithologists of temperate countries have classified migratory birds according to seasons. Thus, in Britain, swifts, swallows, nightingales and cuckoos are summer visitors, because they arrive in spring from the south, remain there to breed and leave for the south in autumn.
Some birds, such as fieldfare, snow bunting and redwing, are winter visitors, as they arrive in autumn, chiefly from north, stay throughout the winter, and fly northwards again in spring. While some birds such as snipes and sandpipers are the birds of passage, seen for short time twice a year on their way to colder or warmer countries in spring and autumn.
Besides these kinds of avian migrations, following three kinds of migrations can also be recognised in different birds:
A. Climatic migrations occur as a result of daily or seasonal changes in the climate of the environment. The well-known north-south migration of many ducks and geese is a good example of climatic migration.
B. Alimental migrations occur as a result of food or water shortages and may occur at any time in a year.
C. Gametic migrations result from a need to occupy some special region or environment for some part of the reproductive process. Most migratory birds perform gametic migration.
3. Modes of Flight in Migration:
During their migration, most migratory birds display following significant features:
1. Time of Migratory Flights:
The most migratory birds either fly during daytime or nighttime and accordingly following two types of migration can be recognised:
(a) Diurnal Migration:
Some birds fly mainly by day, such as crows, swallows, robins, blackbirds, hawks, blue-birds, jays, cranes, loons, pelicans, geese, ducks, swans, and other shore birds. They may stop to forage in suitable places, but swallows and swifts capture their insect food in the air as they travel. These diurnal migratory birds often travel in flocks, which may be well-organised (ducks, geese and swans) or loose (swallows).
(b) Nocturnal Migration:
A vast majority of birds are nocturnal migrants. These include mostly small-sized birds, such as sparrows, warblers, thrushes, etc. These birds prefer to fly at night, under the protective cover of darkness, to escape their enemies. By flying at night, they arrive at the daybreak, take rest, procure food during daytime and then start again at the approach of night.
2. Range of Migration:
The range of migration commonly varies from one or few miles to thousands of miles in different groups of migratory birds but it is almost constant for a particular species. For example, the Himalayan snow partridges descend a few hundred feet and cover a distance of about one or two miles, Chicades travel (descend) about 8,000 feet, while golden plovers, sandpipers, boblinks and swallows cover 6,000 to 9,000 miles from the Arctic to the grassy plains of Argentina or the Patagonian beaches in South America.
Arctic tern breeds along the coast of Labrador in summer and then migrates to its home destination to the edges of Antarctica in winter which is about 11,000 miles. It again returns in the summer to the coast of Labrador travelling about the same distance. White stork of Europe migrates to South Africa in winter taking a journey of about 8,000 miles. It is calculated that 5,000 million land birds migrate from Europe to Africa each autumn and half of them succeed in returning next spring.
3. Altitude of Flight:
Some birds fly quite close to the earth, while most routine migration occurs within 3,000 feet of the earth. Certain small land birds have been reported to fly in night at 5,000 to 14,000 feet altitudes. Further, some avian species even cross the Andes and the Himalayas at altitude of 2,000 feet or more.
4. Velocity during Migration:
The speed or velocity of flight of migratory birds varies from individual to individual and species to species. It is affected by the speed of air and its direction, and it has been reported that birds travel faster during migration than at any other time.
The flight speeds of certain migratory birds have been tabulated in the following table:
During migration, thus, cranes, carrians, crows and finches fly with the speed of 30 miles per hour, while cross-bill flies with the speed of 30 miles per hour. The maximum speed recorded so far by E.C. Stuart is about 170-200 miles per hour (e.g., Indian swifts). Birds cover hundreds of miles nonstop in a day or a night with an average of about 500 miles. Migratory birds usually fly five or six hours per day and also take rest in between for drink and food. Golden plovers fly nonstop from Hudson Bay and Alaska to South America covering a distance of 2,400 miles.
5. Routes of Migration:
The migratory birds usually follow definite lines of flight. The route followed by them may be the same while going and returning back or may be different.
The nocturnal small birds migrate with the general air flow. In spring it takes place from South to North along warm air currents, and in autumn from North to South with the cool wind of North. Change in their course occurs due to configuration of land, coastline, path of great rivers and intervening mountain ranges, etc.
Different migratory birds may follow the following routes during their migration:
(а) Sea Routes:
Marine birds follow sea routes. The land birds are known to cross as much as 400 miles of ocean in a stroke but if there are intermediate islands the distance covered may be more. Certain birds have been seen crossing the Atlantic Ocean between Azores and Portugal (900 miles) and the ocean between the continents of North America and Bermuda, etc.
(b) Coastal Routes:
The coastal routes afford migration for a large number of migrants. Certain important migratory coastal routes are- East Atlantic coastline. West Atlantic coastline, East Pacific coastline. West Pacific coastline. East Indies coastline and West Indies coastline.
(c) River-Valley Routes:
While migrating from planes to hills and from hills to planes, the migratory birds cross rivers and river-valleys falling in the way.
(d) Mountain Ranges:
Very rarely the birds cross mountain ranges.
The river valleys, mountain ranges and coastal routes provide good landmarks for the migrating birds, which enable the birds to recognise and remember the routes and entrances to the countries. Deviations in path sometimes occur due to configuration of land, coastline, course of great rivers or intervening mountain chains.
6. Segregation during Migration:
Some birds such as kingfishers, swifts, and night-hawks travel in separate companies, but certain other birds such as swallows, vultures, blue birds, turkeys, etc., usually travel in mixed companies of several species due to similarity in their size, method of search of food, etc. In some avian species, the male and female individuals travel separately. Males arrive first to build the nests. The young birds usually accompany their mothers.
7. Order of Migration:
During migration the birds follow a definite order which is strictly followed. Normally the adults migrate first and they are followed by youngs. It has been found that urge of migration occurs due to the maturity of gonads which instigate them to migrate towards their breeding, grounds. Adult birds return to the same general and even detailed places at both ends of the journeys.
Young birds mostly do not learn from their elders, indeed may leave before them, flying off in a direction that is presumably genetically determined. Hence, the adults with ripe gonads start the migration.
During the return flight the order becomes reversed—the young birds start first and follow the same path which their parents had followed while coming from that place. In adult precedence, there is always a definite sequence, the adult males take the lead, adult females next in order and the birds of the year follow them and in the end come the weak and wounded birds.
8. Regularity of Migration:
Several species of migratory birds show a striking regularity, year after year, in their timings of arrival and departure. In spite of long distances travelled or vagaries of weather, they are often punctual within a day or two in their time of arrival. Further, most migratory birds come back to the same breeding place year after year.
4. Problems of Migration:
The phenomenon of avian migration has remained an ornithological riddle and its various aspects are still little understood.
Certain scientific explanations have been forwarded from time to time to explain certain problematical issues concerning the avian migration and they are the following:
1. Problem of Way-Finding or Navigation:
It is still a riddle that what guides the young ones of a migratory bird to migrate and follow the same course which their parents had taken. Probably it is genetically determined. Various explanations have been given for what determines the direction and course of migration.
(i) Topographic Cues:
Topographic cues can give long range guidance in daylight. The migratory birds utilise various natural structures or topographical features, such as great rivers, river valleys, coastal lines, chains of oceanic islands, mountain ranges, etc., as the landmarks during their flight.
Landmarks may be learned. Ducks and geese travelling in family groups transmit the information from generation to generation. Most birds make little use of landmarks except for the last stages of homing. They navigate mainly by the sun and stars.
(ii) Experience:
A few naturalists have suggested that the birds learn by experience. Some older members, benefited by a tradition of following a path in past several years, become leaders to guide the younger generations. However, birds certainly do not learn their migratory route from elders, as some of them do not fly in flocks at all.
(iii) Some ornithologists such as Von Middendroff and H.L. Yeaglev advanced the idea that birds navigate through responses to the earth’s magnetic field and their inner ear reacts to the mechanical Coriolis effect produced by the rotation of the earth. But there is no evidence that birds have some magnetic sense, etc.
(iv) The east-west and north-south gradients of gravity and the magnetic intensity are supposed to have some function in avian migration.
(v) Mathews (1955) and Saner (1957) have emphasised the position of the sun (during day time) or stars (during night) helps the birds to navigate along definite route. Experimental evidences by creating artificial planetarium have shown that the shifting of the position of sun causes a change of the migration during night.
Kramer (1951) experiments on caged starlings show restlessness at times when migration is due and flutters in the appropriate direction in relation to the sun. Kramer showed that birds have a clock that does this. It has been experimentally proved that pigeons return home after release from a distant place even if they have never been there before, nor have had any previous training in returning from situations out of sight from the loft. Mathews (1955) and others have shown that upon release the birds fly off towards home provided that they can see the sun.
Navigation by starlight has been studied by following the direction of restlessness movements of birds in a planetarium. Warblers and others altered their directions when the stars were shifted. The stars change and presumably it is a pattern that is recognised.
(vi) Telluric Currents:
According to some naturalists, homing instinct and telluric currents enable the migratory birds to migrate along definite routes. It is probable that much migration in fact makes use of the wind. The economy of energy moving down wind has obvious selective value and this has made birds into very good meteorologists. Birds often fly at the height where the wind is most favourable.
Inspite of the evidence for navigation by the sun and stars much remain uncertain. It is not clear what determines the initial compass bearing and what is responsible for the reversal of migration between spring and autumn. Birds usually avoid flying in clouds.
There is some evidence that birds can use magnetic cues. Pigeons released under conditions of total overcast 50 km from home take up an initial bearing towards home. Robins (Erithacus) oriented their nocturnal activity in the migratory direction even without visual cues. Particles of magnetite have been found in the brain, and muscles of the head and trunk of pigeons and sparrows. Magnetic forces may influence the cells and/or receptors in the muscles.
2. Problem of Origin of Migration:
This has remained a problem for biologists that what started migration or how and why phenomenon of avian migration is originated. One theory which seeks to explain the origin of bird migration assumes that the original home of birds was in the north at a time when these regions were warmer than they are now.
With the advent of colder winters the birds were driven farther and farther south to find adequate supplies of food and escape the A B C freezing weather. Each summer, however, they returned to their homelands to breed.
Another theory assumes that birds originated in more equatorial regions at a time when all the landmasses of the world were located there. Gradually, according to this theory, continents broke apart and drifted (the solid continental crust presumably “floats” upon the deeper, liquid basalt layers of the earth) to their present locations.
Birds attempted to reach their original homelands to breed each summer, and these homelands gradually drifted farther and farther to the north. It seems quite possible that such continental drift has occurred, but most geologists believe that it happened long before birds evolved and, therefore, could not have had any influence upon their migratory behaviour.
The most convincing theory explains the migratory behaviour in evolutionary terms. According to it, in the past, tropical birds probably spread out in colder northern latitudes, where food was abundant, but were forced south when winter came. The event or series of events became an inborn custom through the long history of the race.
3. Problem of Stimulus or Immediate Causes for Migration:
Several theories have been put forward to explain the causes of migration in birds.
A few important theories are following:
(a) Environmental Stimuli:
External ecological stimuli such as growing scarcity of food, decrease of day-length, or sun glare, increased cold and stormy weather, increase of atmospheric pressure, etc., are found to excite birds to migrate to the better suited places. The heat glare and drought also provoke migration among birds.
Migrations are obviously correlated with the seasons and changes in day-length certainly play a central part in initiating migration. It was first shown by Rowan in 1925 that juncos (Junco) or crows submitted to artificially increased day-length in winter show precocious migratory movements if released. The longer days also caused growth of the gonads, which regress in autumn.
The increase of food availability in spring and its decrease in autumn may be the actual triggers to the migration change.
(b) Gonadial Stimuli:
It has been suggested that the ripening of sex organs in birds causes a physiological change which leads to an impulse for migration and the bird is evoked to leave its winter quarters and reach the summer breeding grounds. According to Rowan (1925), the spring migration is stimulated by the hormones secreted by gonads (i.e., testes and ovaries) of birds and reduction of sex hormones produces the autumn migration.
Later work has shown that this is only partly true and that other endocrine influences are also at work. Many nocturnal migrants make characteristic calls while in flight and these noises overhead may stimulate birds on the ground to join in the movement.
(c) Thyroid Hypothesis:
According to this hypothesis certain thyroid hormones produce certain necessary changes in the metabolism of migratory birds during the time of migrations and these changes compel the bird to migrate.
(d) Antipituitary Hormone:
The antipituitary hormone is found to regulate the migration and instinct to migrate.
(e) Metabolic Hypothesis:
Experimentally produced artificial prolonged day length in several migrant species can produce hyperphagia, increased body weight, deposition of fat and restlessness even in late autumn. Non-migrants showed no such effects. The temporal programme of migration is an inherited species-specific character.
The pattern of migration of many species must have been evolved quite recently. At the end of last glaciation 18,000 years ago there can have been hardly any breeding territories north of Mediterranean. The maximum of northern breeding was probably about 5,000 years ago and human clearance has probably reduced it by more than half However, swallows (Hirundo), house martins (Delichon) and swifts (Apus) have taken advantage of human habitation.
5. Advantages and Disadvantages of Avian Migration:
Advantages:
If advantages of avian migration have not outweighed its disadvantages, it could not be undertaken. Indeed, such behaviour would never have been established through evolution had it not strong survival value for the species. Many species do not migrate, since there is no need for them to do so; other means of survival have been exploited instead.
Advantages to be expected from a change of living range (or migration) include better climate, new or more plentiful food supply, increased space for breeding and nesting, longer daylight, working hours for gathering food to feed offspring, and the opportunity for gene exchange with individuals of other parts of the range.
Disadvantages:
Migration is expensive in terms of food and energy requirements and it is dangerous in terms of risk of predation and exposure to climatic factors. Every year, hundreds of millions of migratory birds never reach their destination. The following factors create great problem to the migratory birds- cold weathers, snowstorms, hurricanes, shortage of food, strong current of wind, fog, man-made lighthouses, mountains, big buildings, television towers, ceilometer beams at the airports, etc.