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