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In this article we will discuss about the Migration of Birds:- 1. Definition of Bird Migration 2. Types of Bird Migration 3. Causes 4. Guiding Mechanisms 5. Disadvantages.
Contents:
- Definition of Bird Migration
- Types of Bird Migration
- Causes of Migration
- Guiding Mechanisms in Bird Navigation
- Disadvantages of Bird Migration
1. Definition of Bird Migration:
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The word “migration” has come from the Latin word migrara which means going from one place to another. Many birds have the inherent quality to move from one place to another to obtain the advantages of the favourable condition.
In birds, 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 movement occurs during the particular period of the year and the birds usually follow the same route. There is a sort of ‘internal biological clock’ which regulates the phenomenon.
Definition:
According to L. Thomson (1926), bird migration may be described as “changes of habitat periodically recurring and alternating in direction, which tend to secure optimum environmental conditions at all times”.
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Bird migration is a more or less regular, extensive movements between their breeding regions and their wintering regions.
2. Types of Bird Migration:
All birds do not migrate, but all species are subject to periodical movements of varying extent. The birds which live in northern part of the hemisphere have greatest migratory power.
Migration may be:
(i) Latitudinal,
(ii) Longitudinal,
(iii) Altitudinal or Vertical,
(iv) Partial,
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(v) Total,
(vi) Vagrant or Irregular,
(vii) Seasonal,
(viii) Diurnal and
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(ix) Nocturnal.
(i) 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 temperate and subarctic zones where they get facilities for nesting and feeding during summer. They move towards south during winter.
An opposite but lesser movement also occurs in the southern hemisphere when the seasons are changed. Cuckoo breeds in India and spends the summer at South-east Africa and thus covers a distance of about 7250 km.
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Some tropical birds migrate during rainy season to the outer tropics to breed and return to the central tropics in dry season. Many marine birds also make considerable migration. Puffinus (Great shearwater) breeds on small islands and migrates as far as Greenland in May and returns after few months.
It covers a distance of 1300 km. Penguins migrate by swimming and cover a considerable distance of few hundred miles. Sterna paradisaea (Arctic tern) breeds in the northern temperate region and migrates to the Antarctic zone along the Atlantic. It was observed that Sterna covers a distance of 22 500 km during migration!
(ii) Longitudinal migration:
The longitudinal migration occurs when the birds migrate from east to west and vice- versa. Starlings (Sturnus vulgaris), a resident of east Europe and west Asia migrate towards the Atlantic coast. California gulls, a resident and breed in Utah, migrate westward to winter in the Pacific coast.
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(iii) Altitudinal migration:
The altitudinal migration occurs in mountainous regions. Many birds inhabiting the mountain peaks migrate to low lands during winter. Golden plover (Pluvialis) starts from Arctic tundra and goes up to the plains of Argentina covering a distance of 11 250 km (Fig. 9.54).
Birds migrate either in flocks or in pairs. Swallows and storks migrate a distance of 9650 km from northern Europe to South Africa. Ruff breeds at Siberia and travels to Great Britain, Africa, India and Ceylon thus travelling a distance of 9650 kilometers.
(iv) Partial migration:
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All the members of a group of birds do not take part in migration. Only several members of a group take part in migration. Blue Jays of Canada and northern part of United States travel southwards to blend with the sedentary populations of the Southern States of U.S.A. Coots and spoon bills (Platalea) of our country may be example of partial migration.
(v) Total migration:
When all the members of a species take part in the migration, it is called total migration.
(vi) Vagrant or irregular migration:
When some of the birds disperse to a short or long distance for safety and food, it is called vagrant or irregular migration. Herons may be the example of vagrant or irregular migration. Other examples are black stork (Ciconia nigra), Glossy ibis (Plegadis falcinellus), spotted eagle (Aquila clanga), and bee eater (Merops apiaster).
(vii) Daily migration:
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Some birds make daily journey from their nests by the influence of environmental factors such as temperature, light, and humidity also. Examples are crows, herons and starlings.
(viii) Seasonal migration:
Some birds migrates at different seasons of the year for food or breeding, called seasonal migration, e.g., cuckoos, swifts, swallows etc. They migrate from the south to the north during summer. These birds are called summer visitors. Again there are some birds like snow bunting, red wing, shore lark, grey plover etc. which migrate from north to south during winter. They are called winter visitors.
Nocturnal and Diurnal Flight:
(i) Diurnal migration:
Many larger birds like crows, robins, swallows, hawks, jays, blue birds, pelicans, cranes, geese, etc. migrate during daytime for food.
These birds are called diurnal birds and generally migrate in flocks.
(ii) Nocturnal birds:
Some small-sized birds of passerine groups like sparrows, warblers, etc. migrate in darkness, called nocturnal birds. The darkness of the night gives them protection from their enemies.
3. Causes of Migration:
Most species of birds migrate more or less on schedule and follow the routes in a regular fashion. The actual causative factors determining the course and direction of migration are not clearly known.
The following factors may be related to the problems of migration:
i. Instinct and Gonadal changes:
It is widely accepted that the impulse to migrate in birds is possibly instinctive and the migration towards the breeding grounds is associated with gonadal changes.
ii. Scarcity of food and day length:
Other factors, viz., scarcity of food, shortening of daylight and increase of cold are believed to stimulate migration. Migration in birds depends upon two important factors— stimulus and guidance.
Scarcity of food and fall of daylight are believed to produce endocrinal changes which initiate bird migration.
iii. Photoperiodism:
The increase of day length (Photoperiodism) induces bird’s migration. The day length affects pituitary and pineal glands and also caused growth of gonads which secret sex hormones that are the stimulus for migration. In India, Siberian crane, geese, swan those come from central Asia, Himalayas, begin to return from March and onwards with the increase of day length.
iv. Seasonal variation:
The north-to-south migrations of birds take place under stimulus from the internal condition of the gonads which are affected by seasonal variation.
v. Light:
The experiments of Rowan with Juncos (summer visitor to Canada) have established that light plays an important role in the development of gonads, which has indirect role on migration. If the gonads undergo regression, the urge for migration is not felt. So the seasonal changes in illumination appear to be a crucial factor for determining migration.
Despite all these suggestions, it is not clear how birds — through successive generations — follow the same route and reach the same spot. The instinctive behaviours like migration, breeding, moulting are phasic occurrences in the annual cycle which are possibly controlled by the endocrine system. In all migratory birds, accumulation of fat takes place for extra fuel during prolonged flight in migration.
For more than a century the celestial navigations of birds have fascinated the ornithologists. Different explanations have been advanced to explain how birds navigate. It is difficult to generalize on the means of orientation and navigation in migration. The different groups of birds with different modes of existence have evolved different means of finding their way from one place to another (Pettingill, 1970).
The other reasons may be:
Fat deposition:
Migratory birds become greedy and fat is deposited in the subcutaneous region of the body. The fat deposition plays an important role in the migration of birds. Birds, those migrate a long distance, reserve enough fat which provides energy in their arduous journey and helps the birds to reach its destination, following a particular route. After fat deposition, restlessness (Zugunruhe) is seen among birds for migration.
Inherited instinct:
Birds that take part in migration or follow a more or less definite goal, evidently possess an inherited instinct. Both the direction and the goal must have been implanted in the bird’s genetic code when a population can adjust to a particular location or environment.
Experienced Lead the Flock:
The theory is sometimes advanced that old and experienced birds lead the way and thereby lead the whole route and show the whole route the younger generation. This theory may be applicable to some birds like swans, geese and cranes because they fly in flocks but not applicable in all species where old and youngs migrate at different times and mainly youngs start ahead of the adult.
Werner Ruppell of Germany, a leading experimenter on avian migration, found that Starlings of Berlin find their way back to their nestling places from about 2000 km away. A sea bird named Manx shearwater collected from the western coast of England after being flown by plane to Boston was found back in its nest in England within 12 days.
The shearwater had flown its own way about 4940 km across the unknown Atlantic Ocean! The golden plover of North America migrates from its winter home in the Hawaiian islands to its breeding place in northern Canada.
This bird lacks webbed feet and it is quite natural that it must fly for several weeks over thousands of kilometers of ocean to reach its destination. The birds have wonderful power of navigation and orientation to find their destination even under odd conditions.
There are many theories regarding the phenomenon of migration in birds.
Various theorists propose that birds are guided by a number of agencies:
a. Earth’s magnetic field—as the guiding factor:
Some ornithologists believed about the existence of a “magnetic sense” as the important factor in the power of “geographical orientation”. The theory was conceived as early as 1885 but conducted by Yeagley in 1947 and 1951. Yeagley suggested that birds are sensitive and guided by the earth’s magnetic field.
The Coriolis force arising from rotation of the earth plays the guiding role in migration of birds. The basic question of the theory may be asked — “can birds detect such minute differences in the earth’s magnetic field and can these forces affect bird’s behaviour?”
Attempts to demonstrate by experimental evidences have not supported Yeagley’s experiment. Experiments, in which the earth’s magnetic field was changed, had no effect on the direction which the birds undertook.
b. Sun—the guiding agent in diurnal migration:
The concept that birds are guided by the position of the sun was advanced by Gustav Kramer in Germany and G. V. T. Matthews in England. They have shown by intensive experimentations those homing pigeons and many wild birds use the sun as the compass and that they possess a ‘time sense’ or ‘internal clock’ which allows them to take account of motion of the sun across the sky.
Kramer (1949, 1957, 1961) performed experiments on Starlings (diurnal migrants) and showed that these birds use the sun for setting their migratory course. When the sky remains clear, the Starlings succeed in taking the right direction.
If the sky remains overcast they become bewildered and fail to orient themselves. Mechanical placement of a mirror which deflects rays of the sun result into considerable deviation of orientation to a predictable extent. The experiments of Kramer and others failed to explain the navigation and orientation of night migrants. This aspect was extensively worked out by E.G.F. Sauer (1958).
c. Stars—the guiding agent in nocturnal migration:
The warblers and many other birds orient themselves during navigation by the sun during daytime. But the warblers as well as many other birds navigate mainly at night. What sorts of system do these birds use to the pathways during navigation at night?
Sauer performed experiments on white throat warblers to give an insight to the problem. Sauer put the birds in a cage placed in a planetarium having an artificial replica of natural sky. When the light of the planetarium was poorly illuminated, i.e., when the stars were not visible, the warbelers failed to orient themselves.
When the illumination was better and the planetarium sky matched with the natural night sky, the birds followed up the proper direction. It has also been shown by Sauer that a warbler which has spent its life in a cage (i.e., never navigated in natural sky) has an inborn ability to follow the stars to navigate along the usual route the members of the species follow.
Sauer has suggested that the warblers possess hereditary mechanism to orient themselves by the stars during nocturnal migration. The warbler can adjust the direction perfectly at the latitude.
Suggestions have been advanced by many workers that the configuration of the coastline possibly helps in navigation, but Sauer has disproved the idea and advocated that the birds are exclusively guided by the stars during night.
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d. The ‘compass’ and the ‘internal clock’ in bird migration:
It is a known fact that millions of birds fly to their winter ‘home’ in every autumn. In doing so they cover often thousands of kilometers from their native ‘home’. In the following spring they again return to their breeding grounds. This is a regular biological phenomenon in avian life.
It has been established that the young birds caught during migration, when released afterwards, follow exactly the original route their undisturbed fellows followed. This phenomenon suggested the presence of a sort of ‘compass’ the birds use during navigation.
But Kramer’s experiment gave a clue to the problem. The position of the sun is vital in controlling the navigation pathways. During the day the position of the sun in the sky is changed from east to west via the south. Despite such changes birds tried to navigate in the same direction. This means they have the inherent ability to make appropriate allowance for the time of day.
How do the birds know the time of day? They have possibly a built-in timekeeping mechanism (internal clock) which is synchronized with the earth’s rotation. The ‘internal clock’ can be made to synchronize with external happenings.
Existence of biological clocks is a property of living organisms. It is not confined to animals, it is found in plants and even in simple cells too. It is a common experience that if we are in the habit of getting up every day at a particular time, we frequently wake up at the same time. Besides, many of our bodily functions have a rhythm of their own. These are possibly controlled by an ‘internal clock’ of which we are normally unaware.
Telemetry means methods of tracking of the movement of birds or other migratory animals by using radio. This is the most promising method that has been applied to trace the route of bird’s migration. The method consists of attaching a small radio transmitter, weighing about 2-3 gm. that sends periodic signals or “beeps”.
The miniature transmitter can be placed on birds and it does not interfere flight and the signals can be detected by means of a receiving set mounted on vehicles or aero planes that can detect the routes of migratory birds.
Though there are some limitations of telemetry but this technology gives encouraging results. More recently researchers are engaged largely to track the routes of the migratory birds with the aid of satellites and radar tracking instruments.
5. Disadvantages of Bird Migration:
i. Many youngs are not, able to reach the destination because they die during the course of the continuous and tiresome journey.
ii. Sudden changes in the climate such as storms and hurricanes, strong current of wind, fog are the causes for the death of a sizeable number of migrants.
iii. Sometimes man-made high tours and light houses cause the death of migratory birds.
iv. Man themselves are responsible for the death of the migrants. They shoot at these poor birds just for their own leisure and amusement.
