Bird migration

General approach to bird migration

• Bird migration is a two-way journey from breeding or resting place to feeding or nesting place and vice versa.
• It is a periodic, regular, to and fro movement.
• Birds are the greatest migrants, however, some species of birds do not migrate. The birds which remain in a country throughout the year are called residents. Eg: Bobwhite, Rufflet, sand goose etc.
• The birds from the northern hemisphere are more migratory than the southern hemisphere. Eg: Siberian crane, golden plover, cuckoos etc.
• Bird migration may be diurnal, nocturnal or both. Eg: hawk, robin of diurnal, sparrow, warbler of nocturnal and duck, gull of both.
• Distance travelled by migratory birds depends on the local condition and species of birds. Eg; golden plover is the non-stop fighter, Arctic terns are the king of migrants.

Causes of bird migration:

• Due to seasonal changes, scarcity of food, the shortening of daylight, increase in cold, etc.
• According to Bissonnette, decrease in pituitary activity in winter cause bird to migrate southward. In summer, the pituitary activity becomes active and gonads develop, so, the birds return northward for breeding.
• Migration is a part of the sexual cycle. Birds begin to migrate northward as their gonads begin to swell.

Factors affecting on bird migration

Physical factor

Light, temperature, a velocity of the wind, etc. affect bird migration.

Metabolic factor

When birds consume more amount of food, the fats deposited under subcutaneous layer affects the nervous system, then, the birds migrate towards the breeding place.

Biological or Hormonal factor

Due to the intensity of light, the pituitary gland stimulated which secretes the gonadotrophic hormone (FSH and LH). These hormones activate gonads which affect nervous system then the birds migrate towards breeding place.

Shortage of food

In the northern hemisphere, due to ice sheet during winter, shortage of food occurs. So, the birds migrate towards southern hemisphere.

Types of bird migration

Latitudinal migration: North to south and vice-versa. Eg; Siberian crane.

Longitudinal migration: East to West and vice-versa. Eg; Starling bird.

Vertical migration: Up and down in the mountainous region. Eg; Daphe.

Partial migration: Within a single species, some do migrate and some do not. Eg; birds of the temperate region.

Irregular migration: Migrate in all direction covering few or many miles. Eg; Herons.

Regular migration: Birds came out from their nest in the morning to search food and return back in evening hour daily. Eg; Pigeon, Sparrow.

Seasonal migration: Some birds are winter visitors like snow bunting whereas some birds are summer visitors like swift.

Advantages of bird migration

• Birds get more food, better climatic condition, and breeding place.
• Birds get better adaptability in new places.
• There will be the wide geographical distribution of birds.

Disadvantages of bird migration

• Predator will die due to the shortage of food when birds of that place migrate.
• During migration, birds need to face several risks in which enormous numbers of birds die.
• They can carry the germs like bird flu.