Bird migration is a fascinating phenomenon which has provoked more speculation than any other aspect of Avian biology. It is a periodic movement made necessary mainly for food requirement and for reproductive purposes. It is a striking regularity year by year in the arrival and departure of migrants.
Migration is a dynamic response in every sense of the term. All the birds do not migrate, they are known as residents and those birds who necessarily migrate with regularity, are known as migratory birds. Migration has been observed not only in the terrestrial birds, but, the aquatic birds also migrate. The desire of migration is a hereditary impulse. Our knowledge of migration
of birds is still far from complete. Various aspects of the bird migration has been discussed below.
Types of Migration
(1) Latitudinal Migration : That the birds fly from north to south or vice-versa, linking the equatorial regions of each hemisphere with its temperate and cold areas, is an old concept. However, it is correct that latitudinal migration takes place for change of climatic conditions. Latitudinal migration can be seen in several birds e.g. Golden plover (Pluvialis dominica) of America which spends nine months of winter, 8000 miles south in Argentina.
(2) Altitudinal Migration : It is also known as vertical migration. Birds reach the mountain regions in the summer season and return to the plains during winder season. High elevation birds because of excessive cold are rather forced to come to the ground. In the spring, they again migrate to higher altitudes e.g. many hill birds (e.g. Scolopax rusticola).
(3) Longitudinal Migration: This sort of migration takes place in the East-West direction. The starling of eastern Europe passes through Atlantic coast to avoid continental water.
(4) Vagrant Migration : This migration is irregular and wandering type. In the case of herons and terns the adults and the young ones disperse in all directions after breeding.
(5) a) Summer visitors: During the spring season, some birds arrive from the south and stay to breed, and then leave for the south again in the autumn. These birds are known as summer visitors e.g. cuckoo, swallows etc.
b) Winter visitors : In the autumn some birds arrive from the north, they stay through out the winter season and with the advent of spring they fly northward again e.g. Redwing (Turdus musicus) in Britain.
c) Birds of Passage: These birds are seen for a short time twice a year when on their way to colder or warmer countries, e.g. Sand piper (Tringa hypolecos). They winter southern hemisphere. European Cranes and American Sanderling are also the birds of passage.
d) Resident birds : These birds are more or less static through out the year but they may show partial migration also. The American Red-eyed Vireo is an example of summer resident. Some European species of swallows are also good examples. They always breed in a particular area.
Other Less Important Types of Migration
1. Partial Migration : All the birds of a group of migratory birds do not leave the home land. They stay at the breeding place. Such a migration is known as partial migration, e.g. American blue jay (Cynocitta cristata).
2. Local Migration : Local migration is seen among the resident birds only. The local migratory movements are prominent when the change of season is more pronounced e.g. the paradise birds along the base of the Himalayas.
3. Abnormal local Migration : Under stress of abnormal conditions, birds can be driven out from one habitat to another for their survival.
Different Aspects of Migration
1. Nocturnal Migration : There are several migratory birds who prefer to travel at night. For example, passerine birds normally travel at night and take rest by day. This habit provides them better protection against birds of prey. During the day time they are engaged in collecting their food.
2. Diurnal Migration : There are several birds which prefer to travel during day time. For example, falcons, herons, swallows and humming birds etc. like to travel during the day time. But some birds like ducks, loons and terns etc. can travel both during the day time or at the night irrespective of light or darkness.
3. Range of Migration including velocity and altitude: Flight of flocks of geese has been recorded at a height of 8500 feet above the North Sea during the storm but in the normal condition the height is only 5000 feet over a distance of eighty miles. The flight of small passerine birds is very much affected by the force and direction of wind. The altitude is not more than 300 feet on this case. Radar plotting has shown that some geese fly at an average speed of twenty miles per hour. A finch can fly about 370 miles in a day and a Shrike upto 125 miles as per available records. The arctic tern (Sterna paradisea) covers a distance of about 22000 miles in going out and coming to the native place during its migration. The golden plover migrates from Hudson Bay and Alaska to South America covering a distance of 2400 miles in 48 hours which is a non-stop flight. The sand piper (Actitis macularis) flies at a speed of 25 miles an hour and the Hawks at 30-40 miles per hour.
The intensity of light, rain, visibility and other weather conditions affect the velocity of flight.
4. Methods of Migration : Some birds migrate in groups but there are some birds which prefer to travel alone. The Golden oriole and many birds of prey travel alone during migration. Most of the passerine birds travel in dense groups without particular formation. However, some birds like cranes, ducks, geese and storks etc. fly in line or V-formation. There is a continuous flight in a single direction. These flight patterns have certain advantages e.g. visibility is increased and all participants are able to see each other and they get encouragement and idea of the flight direction. Several birds reach their destination in a single flight without break of journey but some of the birds take rest in the way at some selected places when the route is longer. Examples have been cited later.
5. Course of Migration and Migratory routes of some birds : Direction and distance are the two important features of migratory routes. The migratory birds try to avoid great barriers like oceans, seas, deserts and mountain chains, normally.
The European migrant bird Hirundo rustica migrates from Europe to Africa. It resides in the temperate parts of Europe from April to September. The home land is its breeding place. It migrates to Sudan, in West Africa and to many parts of Southern Africa even at the cape of Good Hope where it winters from October to onwards.
Many passerine birds of Europe, migrating to Africa avoid long passages over the Mediterranean and prefer shorter route from Spain to Morocco. They also prefer to the routes from Italy and Sicily to Tunisia and Asia Minor to Egypt. In some cases deserts are also crossed and oases are used as resting place. Sahara desert has been used as winter retreat. Anser indicus and Grus grus cross the Himalayas in both directions when they fly between winter quarters in India. Water birds can cover vast stretches of the oceans during their migrations. The Tahitian or Bristle-thighed curlew (Numenius tahitiensis) travels thousands of miles across the Pacific between various islands where it winters. They make their nests at Alaska.
The European Quail (Coturnix coturnix) winders on the shores of the Mediterranean and even in Egypt and tropical Africa. It takes very lengthy migration. The breeding area has been extended far to the north only to return every year to the ancestral feeding area.
Terns rest in northern Europe, Asia and Alaska and travel to the Antarctic where they spend the southern summer. They have to fly between 7500 and 95 (X) miles each way along the sea coasts or over the sea. The Arctic Tern (Sterna paradisea) travels with Great shearwater (Puffinus gravis).
The Tern nests in the northern summer from June to August and the Petrel resides in northern Europe without nesting during that period. Between September and November both migrate to the southern hemisphere. Now, during the southern summer (from December to March) the shearwater nests in southern Africa and the Tern remains the resident. So, this migration becomes interesting. Between April and May both species return to the north.
6. Regularity of Migratory travels: Migration takes every vear with regularity and accuracy. Several birds visit the same locality every year and thev return to their original place when the purpose of migration is served. They have very highly developed place-memory. They have very keen power orientation and a sense of direction which enable them to recognize the concerned places during migration. At the time of returning to their native places even the young birds accomplish the journey nicely. The young birds have no previous experience of either route or destination. Naturally, it seems to be the hereditary impulse. Sensibility to terrestrial magnetism and visual recognition of land mark help the birds in finding their way. In the spring, the adult males are first to arrive on the breeding grounds. At the time of returning in the autumn, the young birds lead. Birds can grasp some idea of the earth’s movement which might help in locating various places on the globe. They are very sensitive to intensity of light also. In the spring season the adult males arrive on the breeding ground first and then they are followed by the females.
Causes of Migration
Though the desire to migrate is a hereditary impulse, however, it is influenced by some external and internal causes. The external causes include —
1) The scarcity of food in the original locality where the birds reside permanently.
2) Shortening of day length
3) Increase of cold and sudden changes in the weather conditions continued for a longer time
The internal stimulus is the maturation of gonads and secretion of certain concerned hormones. It has been suggested by Rowen (1925) that the birds migrate when the gonads are in changing condition. The secretion of interstitial cells in males might be providing stimulus for migration. Increase or decrease in day light affects the maturation of gonads, and thus, migration is affected.
The pituitary hormones are also concerned with the migration of birds as they stimulate the activities of the gonads in both the male and female individuals. Fat deposition in the migratory birds has also been considered as the controlling factor by some workers. Increasing day length affects the activities of the hypothalamus, Pituitary and gonads, which are functionally linked. Thus, the physiological changes in the body induce readiness to migrate. The thyroid hormones are also said to be directly or indirectly concerned with the migratory activities of the birds.
Advantages of Migration
The following are the main points of advantages daring migration period.
(i) Birds avoid excessive cold and stormy weather for their safety.
(ii) Short day light is avoided which is not sufficient and convenient for food collection.
(iii) Adverse conditions are avoided so that, scarcity of food and water can not be faced.
(iv) Suitable uncongested nesting places are made available.
(v) Long hours of day light becomes available
(vi) Birds get an abundance of food supply
The phenomenon of migration is very much essential for the birds. If the suitable breeding area and feeding area are not available to birds at the proper time, several bird species will be subjected to natural elimination. Suitable climatic conditions for safety and comfortable breeding are necessary for them.