Migration is the regular, seasonal, large-scale movement of bird populations between their breeding (summer) grounds and non-breeding (winter) grounds.
Drivers of Migration
Birds do not migrate simply to escape the cold, but rather to manage energy budgets and survival.
- Food Availability: The primary driver. Birds move from areas where resources are dwindling (e.g., winter in the Northern Hemisphere) to areas of seasonal abundance.
- Day Length (Photoperiodism): Increasing day length in spring triggers hormonal changes, stimulating the urge to migrate (Zugunruhe or migratory restlessness).
- Breeding Requirements: Some species require specific habitats or lower predator densities for nesting that are only available seasonally.
Navigation and Orientation Mechanisms
Birds possess a sophisticated multi-modal navigation system, often referred to as a “biological compass.”
- Magnetoreception: Birds can sense the Earth’s magnetic field. This is facilitated by cryptochromes (blue-light sensitive proteins in the eye) and iron-oxide (magnetite) clusters in the beak.
- Celestial Navigation: Many nocturnal migrants use the patterns of stars (the stellar compass), while diurnal migrants use the position of the sun (solar compass).
- Olfactory Cues: Some birds, like petrels and pigeons, use “smell maps” to locate specific nesting sites over long distances.
- Visual Landmarks: Following coastlines, mountain ranges, and river valleys to maintain direction.
Major Migratory Flyways
Flyways are the “highways in the sky” used by migratory birds. India occupies a strategic position in global avian migration.
- Central Asian Flyway (CAF): This is the most critical flyway for India. It covers a large continental area of Eurasia between the Arctic and Indian Oceans. India provides vital wintering grounds for waterbirds traveling from Siberia.
- East Asian-Australasian Flyway: Touches parts of North-East India.
Classification of Migration Patterns
| Type | Description | Example |
| Latitudinal | North to South movement (and vice versa). Most common type. | Siberian Crane |
| Altitudinal | Moving from high-altitude mountains to low-lying valleys in winter. | Himalayan Monal |
| Longitudinal | East to West movement (less common). | Starling |
| Partial Migration | Only some individuals of a population migrate while others stay. | Blue Jay |
| Vagrant/Accidental | Birds blown off course or appearing far outside their normal range. | Various rare sightings |
Physiological Preparations for Migration
To endure non-stop flights over thousands of kilometers, birds undergo extreme physiological changes:
- Hyperphagia: An instinctive drive to overeat before migration to build up fat stores, which serve as high-energy fuel.
- Atrophy of Organs: Some birds shrink their digestive organs during flight to reduce weight, while their heart and flight muscles increase in size.
- Unihemispheric Slow-Wave Sleep: Many migratory birds can sleep with one half of their brain at a time, allowing them to remain conscious and navigate during long-distance flights.
Important Migratory Species in India
- Amur Falcon: Known for one of the longest over-water migrations; they travel from East Asia to Southern Africa, using Nagaland (Pangti village) as a major stopover.
- Bar-headed Goose: Famous for flying over the Himalayas at extreme altitudes (nearly 29,000 feet) where oxygen is scarce.
- Siberian Crane: An endangered species that formerly wintered at Keoladeo National Park, Rajasthan (now considered locally extinct in India).
- Greater Flamingo: Migrates within the Indian subcontinent (e.g., to the Rann of Kutch for breeding).
International Treaties and Conservation
- Bonn Convention (CMS): The Convention on the Conservation of Migratory Species of Wild Animals. It provides a global platform for the conservation of terrestrial, aquatic, and avian migratory species.
- Ramsar Convention: Protects wetlands which serve as critical “stepping stones” or stopover sites for migratory waterfowl.
- Raptor MoU: An agreement under the CMS specifically for the conservation of migratory birds of prey in Africa and Eurasia.
Summary of Key Facts for Aspirants
- Arctic Tern: Holds the record for the longest migration, traveling from the Arctic to the Antarctic and back annually (approx. 70,000 km).
- V-Formation: Large birds fly in a ‘V’ shape to conserve energy; the lead bird breaks the air resistance, and the birds behind benefit from the “upwash” created by the wingtips of the bird in front.
- Stopover Sites: These are critical habitats where birds rest and refuel. Loss of even one major stopover site can lead to the collapse of an entire migratory population.