Bird Migration

An article by Mohan Pai

BIRD MIGRATION

Mystery of Nature

The Arctic tern flies a phenomenal round trip of 34,000 km per year from the Arctic to the Antarctic and back !

The longest known migratory journey is performed twice a year by the Arctic tern (Sterna paradisaea) which from the Arctic winter travels south, right across the world to the Antarctic summer and back again - a distance of over 17,000 km each way.

Arctic tern

What is bird migration ?

Bird migration refers to the regular seasonal journeys undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. These however are usually irregular or in only one direction and are termed variously as nomadism, invasions, dispersal or irruptions. Migration is marked by its annual seasonality. In contrast, birds that are non-migratory are said to be resident or sedentary.

Indian Migratory Birds

Indian subcontinent plays host to a number of migratory birds in summers as well as winters. It is estimated that over hundred species of migratory birds fly to India, either in search of feeding grounds or to escape the severe winter of their native habitat. This is because winds usually prevail at higher altitudes and at the same time, the cold temperature at these altitudes helps them in diffusing the body heat, which is generated by their flight muscles. The timing of the migration is usually a mixture of internal and external stimulus.
Migrating birds start on a journey when they feel that they have put on enough fat to provide them energy throughout the journey. Then, the tendency to aggregate into flocks is another determinant of the time of migration. Even after the flock, which has to fly together, has gathered, the birds keep on feeding till the weather conditions become favorable. Thus, apart from the internal clock of the birds and their flock, it is also the availability of food and the weather conditions that play a role in the determination of the time of migration.

Why birds migrate?

Food, water, protective cover, and a sheltered place to nest and breed are basic to a bird's survival. But the changing seasons can transform a comfortable environment into an unlivable one -- the food and water supply can dwindle or disappear, plant cover can vanish, and competition with other animals can increaseMost wild animals face the problem of occupying a habitat that is suitable for only a portion of the year. Fortunately, however, nature has provided methods for coping with the situation. One method, known as hibernation, involves entering a dormant state during the winter season. The other method, known as migration, involves escaping the area entirely. Because of the powers of flight, most birds adapt to seasonal changes in the environment by migrating. How they do it ? Some birds make the long journey in easy stages, stopping to rest on the way. Others fly great distances without pausing to rest and feed. Some fly by day, some both by day and by night, but most of them speed on their way through darkness after the sun has set. Birds usually travel in flocks. The V-shaped formation of cranes and geese attracts much attention as the bird's speed across the sky. Swallows, flycatchers, warblers, shore birds and water-birds being to gather in flocks- each with its own kind-and, after a great deal of excited fluttering, twittering and calling, they rise up into the air and away they go. Usually the male birds go first to their breeding grounds in bachelor parties and the female birds follow them in a few days!

Griffon Vulture

The movement of birds with the changing seasons was known from the earliest times, but people had strange ideas as to why the birds traveled, or where they went. To explain their absence from a place in a particular season, they said that the birds buried themselves in the mud and slept there throughout the winter! Later, detailed studies of migration started. Information was gained by directly observing the habits of birds, and also by ringing. Bird movements are also studied by creating artificial conditions and studying their effects on birds. Today, most of the information on migration has come from ringing young and adult birds. Ringing is done by capturing a bird and putting on to its leg a light band of metal or plastic. The band bears a number, date, identification mark, and the address to which the finder is requested to return the ring. The bird is then set free. The place where such a bird is shot captured or found dead gives clue to the direction and locality to which the bird has migrated.

Bird Sanctuaries in India

Among the most famous bird sanctuaries in India are, the Keoladeo Ghana National Park in Bharatpur, the Corbett National Park and the Sariska Wildlife Sanctuary, part of Project Tiger. These sanctuaries offer wide variety of bird species.

Keoladeo Ghana National Park is one of India's pioneer wildlife conservation centers. Considered to be the best sites for bird watching in the world, the sanctuary annually hosts thousands of visitors who come to view the spectacular wildlife Spread over an area of 30 square km of marshy swamp, kadam forests, woodland and shallow lakes, the sanctuary offers habitat to both nesting indigenous birds as well as migratory water birds. An amazing number of more than 330 species of birds have been spotted and identified in the sanctuary. The Siberian Crane, the finest and rarest of migratory birds, are the cynosure this sanctuary and are regular visitors.Siberian Crane is believed to have existed in this world for over one million years. However it is of great concern that only 125 pairs of these pure white, crimson-billed cranes estimated to survive worldwide. Profusion of marine vegetation, frogs, fish, insects and mollusks, as well fine setting for migratory birds go a long way to make Keoladeo Ghana National Park an ideal place for pelicans, storks, herons, egrets and kingfishers. Breeding females stay in peaceful co-existence and it is of no surprise that one tree can have nests of different birds. The sanctuary is know to have been the best breeding ground for more than a thousand species of birds. Migratory birds start arriving in the month of October. They include a variety of Geese, Ducks, Raptors, Geese, Warblers and Waders.

Extending over an area of 800 sq km, the Sariska Wildlife Sanctuary is located in the forest hills of the Aravalli ranges in the state of Rajasthan. It provides habitat to more than 200 species of birds including the Gray Hornbill, Crested Serpent Eagle, Black/Red Headed Bunting, Wryneck Woodpecker Babbler, White Breasted Kingfisher, Little Brown Dove, Small Minivet, Golden Oriole, Great Gray Shrike, Pale Harrier and Tailor Bird. An example of typical dry deciduous forest, the sanctuary remains lush and green during the monsoons and dry during the rest of the seasons.

Other place is the Pong Dam reservoir is 65 km Pathankot and 115 km from Dharamsala. Nestled in the sylvan surroundings of the Kangra valley, the sprawling Pong Dam wetland has emerged as a major habitat for migratory birds in the country as also an attraction for bird watchers. The most common bird species that have arrived and often visit this lake every year include ruddy-shell ducks (surkhab), bar-headed geese, mallards, coots, pochards and pintails besides rare red-necked grebe and gulls. These species come from as far as China, Siberia, Central Asia, Pakistan and Ladakh. According to a census, more than one lakh migratory birds visited the lake last year.

Apart from being home to the tiger, Corbett National Park is also noted for the bird watching. Considered to be one of the best bird watching sites in the world, the park is home to some 600 species of birds. This number exceeds the total number of bird species found in Europe and is about one fourth of the diversity found in India. A case in point is that out of the 69 species of raptors found in India, 49 can be seen in Corbett. Spreading out on an area of 520 sq km, the Corbett National Park is a hot destination for bird-watchers. Bird-watchers from across the world make a beeline to this park during winters when the bird diversity is at its zenith.

Painted Storks - Pic by Geeta Shankar

Threats and conservation

Human activities have threatened many migratory bird species. The distances involved in bird migration mean that they often cross political boundaries of countries and conservation measures require international cooperation. Several international treaties have been signed to protect migratory species including the Migratory Bird Treaty Act of 1918 of the US and the African-Eurasian Migratory Waterbird Agreement.The concentration of birds during migration can put species at risk. Some spectacular migrants have already gone extinct, the most notable being the Passenger Pigeon (Ectopistes migratorius). Other significant areas include stop-over sites between the wintering and breeding territories. A capture-recapture study of passerine migrants with high fidelity for breeding and wintering sites did not show similar strict association with stop-over sites.Hunting along the migratory route can also take a heavy toll. The populations of Siberian Cranes that wintered in India declined due to hunting along the route, particularly in Afghanistan and Central Asia. Birds were last seen in their favourite wintering grounds in Keoladeo National Park in 2002.Structures such as power lines, wind farms and offshore oil-rigs have also been known to affect migratory birds. Habitat destruction by land use changes is however the biggest threat and shallow wetlands which are stopover and wintering sites for migratory birds are particularly threatened by draining and reclamation for human use.

Refereces: Salim Ali ‘The book of Indian Birds’, Wikipedia, Indian Wildlife.htm, HSBC’s Environment Forum

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