MIGRATION OF 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 result- ing movement is said to be a migration (L., migrare to travel).
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 other birds, however, do not migrate at all and they remain throughout the year in a country.
Such birds are called resident birds. In birds, the migration means two- way journeys-onward journey from the 'home' to the 'new' places and back journey from the 'new' places to the home'.
This move- ment occurs during the particular period of the year and the birds usually follow the same route. Kinds of Avian Migrations.
The migratory birds migrate in a variety of ways and accord. ingly following kinds of avian migrations have been recognized by different ornithologists:
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1. Latitudinal migration:
The latitudinal migration usually means the movement from north to south and vice versa. Most birds live in the land masses of the northern tempe. rate 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, India and Sri Lanka, thus travelling a distance of 6,000 miles.
An opposite but lesser movement also occurs in the southern hemisphere when the seasons are changed. some tropical birds migrate during rainy season to the outer tropies to breed and Greturn to the central tropics in dry season.
2. Altitudinal migration:
Altitudinal or vertical mig. rations 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-cook, Bush chat and Scolopax rusticola.
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 migration includes the migration of birds living in southern hemisphere, which in east-west direction.
The Patagonian plover visits the Falkland island, and south Patagonian in September and October for breeding.
4. Partial migrations :
Many avian species (e.g., blue birds and many blue jays of Canada and northern United States, etc.) are only partial migrants. In such cases all the birds of a group of migratory birds do not leave the native land and hence are always represented by certain individuals. But these individuals are not always the same.
5. Erratic migration:
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 direc- tions over many or a few hundred miles in search of food and safety from enemies.
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 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.
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Modes of Migration:
During their migration, most migratory birds display following significant features:
1. Time of migratory flights:
The migratory flights of most migratory birds may occur during daytime or night-time and accordingly following two types of migratory birds can be recognised:
(a) Diurnal migrants:
Some birds fly mainly by day, such crows, swallows, robins, blackbirds, hawks, blue-birds, jaya, cranes, loons, pelicans, geese, ducks, swans, etc. 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-organized (e.g., ducks, geese and swans) or loose (swallows).
(b) Nocturnal migrants:
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 day break, take rest, procure food during daytime and then start again at the approach of night.
2. Range of migration:
The range of migration com- monly varies from one or few miles to thousand of miles in different groups of migratory birds but it is almost constant for a particu- lar group. For example, the Himalayan snow partridges cover a distance of about one or two miles, Chicades travel 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 migrates about 11,000 miles from the coast of Labrador to Antarctic.
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 birds have been reported to fly 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 has been tabulated in the following table:
During migration, thus, cranes, carrians, crows and finches flight with the speed of 30 miles per hour, while cross bill flights with the speed of 30 miles per hour. The maximum speed recorded so far is about 170 miles per hour, (e.g., Indian swifts).
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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. Different migratory birds may follow the following routes during their migration:
(a) 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 Burmuda, etc.
(b) Coast routes:
The coast routes afford migration highways for a large number of migrants. Certain important avian migratory coastal routes are-East Atlantic coast line, West Atlantic coast line, East Pacific coast line, West Pacific coast line, East Indies coast line and West Indies coast line.
(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 recognize and remember the routes and entrances to the countries. Deviations in path sometime occur due to configur. ation 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 separ ately. 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 and this order 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. Hence, the adults with ripe gonads start the migration.
During the back 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.
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