Bird Migration - a marvel of nature

Bird migration refers to the regular seasonal journeys undertaken by many species of birds. Migrations include movements of varied distances 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 or irruptions. Migration is marked by its annual seasonality. Many land birds migrate long distances. The most common pattern involves flying north to breed in the temperate or Arctic summer and returning to wintering grounds in warmer regions to the south. Between 1500 and 4000 species of birds migrate, the exact answer is not really knowy yet and the experts differ as to their answers.

The primary advantage of migration is energetic.The longer days of the northern summer provide greater opportunities for breeding birds to feed their young. The extended daylight hours allow diurnal birds to produce larger cluthces than related non-migratory species that remain in the tropics year-round.As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season. Many of the larger birds fly in flocks. Flying in flocks helps in reducing the energy needed. Many large birds fly in a V-formation and individual energy savings have been estimated in the range 12–20.eg.dalmatian pelicans.The V-shape can be easily seen in Bharatpur bird sanctuary in India.

All kinds of birds migrate, from large cranes, birds of prey, to tiny hummingbirds. Even flightless birds migrate! Emus move from breeding sites in the rainy season to more permanent water sources in the dry. Penguins migrate in the ocean. Auk babies migrate by swimming until they fledge and can fly! Even birds that spend their entire non-breeding time in flight, such as seagulls, also move around on the ocean to follow seasonal food abundances.The longest migration is undertaken by the Arctic Tern (Sterna paraisaea). It breeds in the Arctic North in the summer, then flies all the way to the other pole to spend winter on the Antarctic ice pack. The shortest distance between the two poles is 15,000km, but the birds usually travel a more circuitous route and can cover up to 20,000km; making a round trip of 30-40,000 km.

Birds exploit the winds to their favour so they can go the distance by burning minimal fuel.They may shift altitude to find the best wind "conveyor belt". Winds at high altitude may blow in the opposite direction from wind on the ground, and usually are blowing strongly. Larger birds rely on thermals (hot air) rising from the ground in the mornings to gain altitude by simply soaring. These birds usually migrate during the day. They may also follow strong updrafts along ridges.

Studies suggest birds orientate themselves to the compass points using the position of the sun during the day, and the stars at night. They can also sense magnetic north. In addition they use other clues such as visual layout of the land, smell (of the sea), sound (waves on shores, winds through mountain passes).

The most amazing aspect of bird migration is that the location, route and perhaps even the techniques are hard-wired into their brains. Many migrating birds abandon their young as soon as they fledge, and a short time later, the young make the migration on their own.

Sadly, in addition to surviving storms and bad weather, exhaustion and other natural obstacles, migrating birds are increasingly face human threats. Habitat destruction that affects staging posts handicap their ability to re-fuel. These include draining wetlands, cutting down forests. Pollution of the sea, water and air also affects them. Migrating birds are also distracted and killed by lit-up skyscrapers, lighthouses and other unnatural man-made formations that mislead them. Sadly, many migrating birds are also hunted, for food, and for sport or superstitious reasons .Human activities have threatened many migratory bird species Effects of hunting during migration 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 Afghistan Birds were last seen in their favourite wintering grounds in Bharatpur bird sanctuary in 2002.