Migration of Birds
Migration
- The term “migration” originates from the Latin word migrare, meaning movement from one spatial unit to another.
- Any place occupied by an organism at a given time qualifies as a spatial unit.
- Migration is considered an annual journey of an individual or group between a
- In birds, migration refers to a two-way journey—from a breeding/nesting place (home) to a feeding/resting site (new place), and back.
- These movements occur seasonally and often along the same route.
KINDS OF MIGRATION
Migration is broadly classified into three main categories:
- Return Migration: Movement back to a previously visited location.
- Exploratory Migration: Journey into unknown spaces; though the ability to return exists, it is not used.
- Removal Migration: Movement to a comparable spatial unit without a return to the original site.
Based on the plane of movement, migration is divided into:
- Horizontal Migration: Migration perpendicular to Earth’s gravitational force.
- (a) Latitudinal Migration:
- Movement from north to south or vice versa.
- Example: Cuckoo breeds in India and migrates to South-East Africa (~7250 km).
- Ruff breeds in Siberia and migrates to Great Britain, Africa, India, Sri Lanka (~9650 km).
- (b) Longitudinal Migration:
- Movement in east-west direction.
- Example: Pentagonian Plover migrates to Falkland Islands and South Patagonia for breeding.
- (a) Latitudinal Migration:
- Vertical Migration: Movement in a plane parallel to gravity.
- Altitudinal Migration:
- Includes both horizontal and vertical components.
- Birds move up and down hills and mountains.
- Small passerines fly at ~60 m, while some birds migrate at 4000 m altitude.
- In India, some birds ascend Himalayan slopes during summer and return to plains in winter.
TYPES BASED ON REASONS FOR MIGRATION
- Climatic Migration:
- Triggered by climate changes.
- Example: North-south migration of ducks and geese.
- Alimental Migration:
- Due to shortage of food and water.
- May occur any time of the year.
- Gamatic Migration:
- Driven by reproductive needs.
- Most birds migrate to reproduce successfully.
TYPES BASED ON SEASON
- Summer Visitors:
- Arrive in spring to breed and leave in autumn.
- Examples: Swifts, swallows, nightingales, cuckoos.
- Winter Visitors:
- Migrate southward in winter and return north in spring.
- Examples: Fieldfare, snow bunting, redwing.
- Birds of Passage:
- Seen briefly twice a year (spring & autumn).
- Examples: Sandpipers, snipe.
CAUSES OF MIGRATION
Environmental Stimuli:
- Decline in temperature and food availability may trigger migration.
- Reduced daylight during autumn (temperate zones) initiates southward migration.
- Longer days and warmth in spring (Southern Hemisphere) trigger return migration.
- Species-specific sensitivity to weather changes.
Physiological Stimuli:
- Endocrine glands like pituitary, thyroid, gonads, adrenal, pineal influence migration.
- Pituitary stimulates gonadal growth and metabolic activity.
- Thyroid hormones may initiate migratory fat deposition.
- Gonadal hormones aid fat accumulation in migratory birds.
ADVANTAGES OF MIGRATION
- Provides better feeding and breeding grounds.
- Offers longer daylight for food search in warmer areas.
- Allows escape from unfavourable conditions, ensuring survival and reproduction.