bird migration
Bird migration: a blend of nature, habitat and geography
MUMBAI, Mar 24/2002: Birds use the sun as compass and stars as bearings to migrate across the globe to sustain themselves and further their flock, research shows.
The phenomenon of bird migration has, for long, been a subject of interest to ornithologists but diverse views exist about their migratory and navigational behaviour. Broadly, nature, habitat and geography are the factors that guide and facilitate birds’ migration.
Research at several places has proved beyond doubt that the sun, moon and stars play an important role in the migration of birds, sometimes over 10,000 miles across the skies.
Bird migration studies gained momentum in the last century, and their torchbearer in India is the Bombay Natural History Society (BNHS) — the premier body of ornithologists.
When clubbed together, the pattern of migration, path and reasons behind this phenomenon throw up some interesting facts.
More intriguing is the possibility of bird migration being linked with a natural phenomenon beyond our senses.
BNHS honorary secretary Dr J C Daniel says studies on the astronomical landmarks used by birds to chart their flight direction proved that they flied by day using the sun as compass. Similarly, night-flying birds used the stars as bearings. Society spokesperson Issac Kehimkar avers that bird migration studies are an important science and more facts would come out in the years ahead.
Dr Bharat Adur, a leading astronomer and senior scientist with the Nehru Planetarium here, says birds use two bright stars —"canopus (Agasthya) and sirius (Vyadha) — for navigation. "They also use the gravitational field and research is now focussed on this," he added.
The migratory distance varies from a few kilometres for some birds to 15,000 km in the case of the arctic tern. Birds flying from the north and north-east — Siberia to India — fly about 6,000 to 8,000 km.
The maximum distance covered in a single non-stop flight is about 3,000 km, which even man-made aircraft can’t achieve, says Dr S Balachandran, a senior BNHS scientist and an expert on bird-ringing.
Flight speeds range from 40 to 65 km per hour, resulting in single flights lasting 46 to 75 hours.
Comprehensive radar studies have shown that 95 per cent of migratory movements occur at less than 10,000 feet and the bulk of such movements occur under 3,000 feet.
However, birds can fly over 15,000 feet without any problems. Mountaineers in the Himalayas have reported geese flying at about 27,000 feet — the same level as airplanes, he informs.
Migration pattern is a blend of historic and present-day influences and it is an evolutionary process, Dr Daniel says.
"Migration is a deep-seated physiological phenomenon related to the cyclical changes of advantageous and adverse climatic conditions. It evolved during the long history of species itself.
"It is now believed that the life span of a bird species is about two million years. This is ample time for the paths of migration to be fixed in the genetic memory of the species," he avers.
Ornithologists have come out with the ‘northern ancestral home theory’, the ‘southern ancestral home theory’ and the ‘theory of continental drift’ to explain the migratory behaviour as the facts thrown up by experiments have failed to substantiate the evolutionary process of migration. Generally, winter is synonymous with migration. Not only birds, but also butterflies and insects visit India from far-flung places like Siberia, Australia and Central Asia.
These winged foreign guests chart their course through a combination of diverse factors which can be broadly categorised into two sets of parameters. They use geographical landmarks like rivers, coastlines and mountain ranges and also monitor the earth’s magnetic field with their developed visual systems and the tiny grains of magnetite — a mineral — in their heads.
Apart from the celestial bodies, the senses of smell and sight also help birds search for favourable climactic conditions.
Dr Daniel points out that the findings also indicate that hormones, secreted by glands like the pituitary gland, control migration impulses to a large extent. These secretions determine the reaction of the bird to external cues such as the shortening of the day in autumn and its lengthening in spring.
They also determine the amount of fat stored by the bird as fuel for the migration flight. Bird migration studies conducted by the BNHS showed birds were lighter when they touched down on their chosen land — their stored fat was exhausted during the flight, Dr Balachandran says.
Experts hope migratory routes and strategies will continue fascinating Indian ornithologists as Indian bird sanctuaries like the Keoladeo Ghana national park at Bharatpur in Rajasthan can provide a good ground for migration studies.
MUMBAI, Mar 24/2002: Birds use the sun as compass and stars as bearings to migrate across the globe to sustain themselves and further their flock, research shows.
The phenomenon of bird migration has, for long, been a subject of interest to ornithologists but diverse views exist about their migratory and navigational behaviour. Broadly, nature, habitat and geography are the factors that guide and facilitate birds’ migration.
Research at several places has proved beyond doubt that the sun, moon and stars play an important role in the migration of birds, sometimes over 10,000 miles across the skies.
Bird migration studies gained momentum in the last century, and their torchbearer in India is the Bombay Natural History Society (BNHS) — the premier body of ornithologists.
When clubbed together, the pattern of migration, path and reasons behind this phenomenon throw up some interesting facts.
More intriguing is the possibility of bird migration being linked with a natural phenomenon beyond our senses.
BNHS honorary secretary Dr J C Daniel says studies on the astronomical landmarks used by birds to chart their flight direction proved that they flied by day using the sun as compass. Similarly, night-flying birds used the stars as bearings. Society spokesperson Issac Kehimkar avers that bird migration studies are an important science and more facts would come out in the years ahead.
Dr Bharat Adur, a leading astronomer and senior scientist with the Nehru Planetarium here, says birds use two bright stars —"canopus (Agasthya) and sirius (Vyadha) — for navigation. "They also use the gravitational field and research is now focussed on this," he added.
The migratory distance varies from a few kilometres for some birds to 15,000 km in the case of the arctic tern. Birds flying from the north and north-east — Siberia to India — fly about 6,000 to 8,000 km.
The maximum distance covered in a single non-stop flight is about 3,000 km, which even man-made aircraft can’t achieve, says Dr S Balachandran, a senior BNHS scientist and an expert on bird-ringing.
Flight speeds range from 40 to 65 km per hour, resulting in single flights lasting 46 to 75 hours.
Comprehensive radar studies have shown that 95 per cent of migratory movements occur at less than 10,000 feet and the bulk of such movements occur under 3,000 feet.
However, birds can fly over 15,000 feet without any problems. Mountaineers in the Himalayas have reported geese flying at about 27,000 feet — the same level as airplanes, he informs.
Migration pattern is a blend of historic and present-day influences and it is an evolutionary process, Dr Daniel says.
"Migration is a deep-seated physiological phenomenon related to the cyclical changes of advantageous and adverse climatic conditions. It evolved during the long history of species itself.
"It is now believed that the life span of a bird species is about two million years. This is ample time for the paths of migration to be fixed in the genetic memory of the species," he avers.
Ornithologists have come out with the ‘northern ancestral home theory’, the ‘southern ancestral home theory’ and the ‘theory of continental drift’ to explain the migratory behaviour as the facts thrown up by experiments have failed to substantiate the evolutionary process of migration. Generally, winter is synonymous with migration. Not only birds, but also butterflies and insects visit India from far-flung places like Siberia, Australia and Central Asia.
These winged foreign guests chart their course through a combination of diverse factors which can be broadly categorised into two sets of parameters. They use geographical landmarks like rivers, coastlines and mountain ranges and also monitor the earth’s magnetic field with their developed visual systems and the tiny grains of magnetite — a mineral — in their heads.
Apart from the celestial bodies, the senses of smell and sight also help birds search for favourable climactic conditions.
Dr Daniel points out that the findings also indicate that hormones, secreted by glands like the pituitary gland, control migration impulses to a large extent. These secretions determine the reaction of the bird to external cues such as the shortening of the day in autumn and its lengthening in spring.
They also determine the amount of fat stored by the bird as fuel for the migration flight. Bird migration studies conducted by the BNHS showed birds were lighter when they touched down on their chosen land — their stored fat was exhausted during the flight, Dr Balachandran says.
Experts hope migratory routes and strategies will continue fascinating Indian ornithologists as Indian bird sanctuaries like the Keoladeo Ghana national park at Bharatpur in Rajasthan can provide a good ground for migration studies.
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