LECTURE NOTES ON MIGRTION OF BIRDS

Migration:In a broad sense, 'migration', as defined by Cahn, "is a periodic passing of animals from one place to another, (L. migrare, to travel)". The bird migration is a two-way journey. It means a regular, periodic, to-and-fro movement of a population of some birds between their summer and winter homes, or from a breeding and nesting place to a feeding and resting place. Not all species of birds take part in the great pageant of migration. Bobwhite and the ruffled sand grouse, do not migrate at all. Birds which remain throughout the year in a country are known as residents.

I.Kinds of migration

Migration in birds takes place in a variety of manners, some of which are as follows :

1. Latitudinal migration: The most familiar migrations are latitudinal that is, north to south and vice versa. These are pronounced in the Northern Hemisphere, having larger land masses. Birds move during summer over the temperate and sub-arc tie regions of Northern Hemisphere, where there are facilities for feeding and nesting. The birds return to the south for shelter during winter, when north is covered with ice and snow. The American golden plover (Pluvialis dominica) passes the nine months of winter 8,000 miles south in the pampas of Argentina, thus enjoying two summers each year and knows not a hint of winter. Some birds of Siberia visit the plains of Himalayas in India.

2. Longitudinal migration: Some birds make migrations from east to west and vice versa. Thus, the starling moves from a breeding area in east Europe or Asia towards the Atlantic coast, to avoid the continental winter.

3. Altitudinal migration: Wherever large mountains are found in temperate regions, the birds migrate regularly up and down their slopes, as the weather changes. The birds pass the summer in the mountainous regions, but retum to the lowlands in winter. It is merely a dispersal or short journey from the bleaker slopes to the more protected valleys and has been called altitudinal or vertical migration. It ocurs in the grebes and coots of Andes in Argentina, violet green swallows of Great Britain, and the willow ptarmigan of Siberia

4. Partial migration:A part of bird population migrates and the major part  remains in the native land.Thus, barn owls (Tyto alba), blue-birds and many blue jays of Canada and Northern United States travel southwards to mingle with the sedentaiy populations of the southern states. Song thrush, redbreast, titmouse, finch, etc., seen throughout the year, actually represent partial migrants, as the birds seen in winter are not the same as seen in summer.

5.Irregular or vagrant migration: In some birds, such as herons, 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 sea birds are taken by  hurricanes to as far as 2,000 miles away from home seas to drop exhausted or to die on unfamiliar shores.

6.Seasonal migration: Some birds migrates at different seasons of the year for food or breeding, called seasonal migration, e.g., cuckoos, swifts, swallows etc. In Britain swifts, swallows, nightingales and cuckoos are summer visitors, for they arrive in spring from the south, remain to breed and leave for the south in autumn. Some like fieldfare, snow bunting and redwing are winter visitors, as they arrive inautumn, chiefly from the north, stay throughout the winter and fly northwards again in spring.

II.Modes of flight in migration

1. Nocturnal. and diurnal flight. Many large birds fly in flocks mainly by day, such as the crows, swallows, robins, blackbirds, hawks,

bluebirds, jays, cranes, loons, pelicans, geese and other shore birds.  These are callec diurnal migrants .

Nocturnal migrants  include mostly small passerine birds, such as warblers, thrushes, sparrows, etc. They prefer to fly at night, under the protective cover of darkness, to escape their enemies.

2. Segregation during migration. Certain birds, such as night hawks, swifts and kingfishers, travel in separate companies, while swallows, turkeys, blue birds, etc., travel in mixed companies of several species, due to similarity in their size, method of search of food, etc.

3. Range of migration: The distances travelled by migratory birds depend upon local conditions and the species concerned. The Himalayan snow partridges descend a few hundred feet only and cover hardly a mile or two, while the chicades come down nearly 8,000 feet. The arctic tern spends the summer and breeds in the  Arctic circle. Then it travels a distance of 11,000 miles to reach its destination to the edges of Antarctica in winter.. European white stork winters in South Africa after a journey of about 8,000 miles.

4. Altitude of flight. Some birds fly quite close to the earth, while most routine migration probably takes place within 3,000 feet of the earth. Radar has shown that some small land birds, migrating at night, fly at 5,000 to 14.000 feet. Some species even cross the Andes and the Himalayas at altitudes of 20,000 feet or more.

5. Speed and duration of flight. Average flight velocity of most small birds seldom exceeds 30 miles per hour. The greatest speed, recorded in India, of two species of swifts by E.C. Stuart, is 171-200 miles per hour.

6. 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. Another remarkable feature, besides punctuality, is that they sometimes come back to the same breeding place year after year.

7. Routes of migration. :Sea,river, mountain,sea shore ,some land marls etc are major  routes of migrations

III. Causes of Migration:The major causes of migrations are

a. Instinct and Gonadal changes: The  impulse to migrate in birds is by instict and the migration towards the breeding grounds is associated with gonadal changes.

b. Scarcity of food - Scarcity of food is believed to produce endocrinal changes which initiate bird migration.

c.Shortening of daylight: Shortening of daylight is believed to produce endocrinal changes which initiate bird migration.

d. 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.

e. Seasonal variation:The internal condition of the gonads which are affected by seasonal variation.

f. Light:Light plays an important role in the development of gonads, which has indirect role on migration.

IV.Navigation: The following factors determines the direction and course of migration.

(a) Visual landmarks. The sense of direction has been attributed to obvious topographical features or landmarks, such as great rivers, river valleys, coastal lines, chains of oceanic islands, mountain ranges, etc.

(b) Experience. A few naturalists have suggested that the birds learn by experience. Some older members, benefiting by a tradition following a path in past several years, become leaders to

guide the younger generations.

(c) Telluric currents. The air-currents, which would lead the birds straight to their destination.

(d) Homing instinct. Some have spoken of a homing instinct, enabling the birds to return to a goal

(f) Celestial bodies. The late Gustav Kramer, a German Ornithologist, claimed in 1949, that the birds which travel by day use the sun as compass for orientation . German, Franz Sauer, who experimented with night-flying old-world warblers, made the astounding discovery that the nocturnal, migrants navigate by the constellations of the stars

Advantages of migration:The advantages of migration are

1.Migration helps birds avoid harsh climatic extremes.

2,Migration from higher altitudes and latitudes (breeding areas in the north) during winter affords protection from cold and stormy weather.

3.The migrants get more food and better environmental conditions .

Disadvantages of Bird Migration:

i. Many youngs are not, able to reach the destination and die due to extreme factors on the way.
ii. Sudden changes in the climate such as storms and hurricanes, strong current of wind, fog are the causes for the death of a significant number of migrants.
iii. Sometimes man-made high towers and light houses cause the death of migratory birds.
iv. Man themselves are responsible by hunting  at these poor birds just for their own leisure and amusement.