Animal Migration: Journey to the Unknown
The word ‘migration’ usually refers to the movement of human, specially a whole group, from one place to another. The history of migration began as soon as human was created. The reason was easy – collecting food from the nature. Moreover, finding a safe place to protect themselves from natural disasters and wild animals encouraged to migrate. In-migration and out-migration are used for internal migration meaning inside the national territory while crossing the boundary is identified as international migration.
Animals also migrate from one place to another, like the humans, and this migration is more rapid than the humans. Some animals like wild beast and Sooty Shearwater traveled the world from their birth to death, thousands of miles every year. This traveling may cross the boundary or not, they dare about this, but is the only way to keep themselves alive in the world. Human migrates once, twice or more, but animals, like Pacific salmon, pass their life with migration. Some specifies of Pacific salmon born in freshwater streams, travel to ocean waters and then return to the stream where they were born to breed before dying.
Almost all animals migrate from one place to another for searching foods but some of them traveled long distances. Wildebeests, elephants, Reindeer, Zebra, and Caribou are some animals that traveled regularly. Fishes like various species of shark and whale, dolphins, and sea turtles traveled hundreds of miles every year. Birds, mostly known as migratory birds, are large among the migrates. About thousand types of birds travel the world every year; various types of goose like great crested grebe, widgeon, egret, large egret, heron, night heron, snipe etc. as well as fruit bat, nightingale, humming bird, cockatoo are mentionable.
Reasons of Migration
Some reasons lied behind the migration. Animals that live in habitats that are difficult to survive in year round, must evolve a way to cope with the difficult time of year. A common strategy is hibernation, used by many mammals and other species. Snakes, white bear and some animals in the winter season follow this technique. Others follow the rest option, migration, and move across long distances. They survive by leaving the area for part of the year, sometimes even part of their life, and move to habitats that are more hospitable.
Taking the advantages of food, shelter and water that vary with seasons, or life stage is the most common reason to migrate. The availability of food and water can change throughout the year. For instance, when winter suddenly halts the supply of insects, birds that insects must head for warmer climates where food is still bountiful. Similarly, when the cold settles in, small rodent and birds that are prey for predatory birds become scarce, so the North American red tailed hawk flies Mexico or the Gulf coast to find a more abundant food source. Sometimes, deep snow may make animals easier to catch by predators, or make the water ice. So finding a new place full of food is emergency case.
Plant eating mammals like buffalo and antelope; typically graze in herds, which can deplete the grass in an area. These animals, instead of waiting for grass, go a short distance to find new grass, circling back to the original area when grass is abundant again. But in winter, grass does not regrow, so the herds move back to the areas where they found good the previous season. During the climatic changes like drought, water holes draw both the predators and prey, making these areas both overcrowded and dangerous. In Africa, wildebeests, zebra, and other prey species therefore migrate to areas where water is more plentiful.
Bear the young in places relatively safe from predators and rich in resources, a more important reason of migration, also encourage the animals. For instance, some right whales leave their Antarctic feeding grounds where their primary food resource is plentiful. They travel to the relatively barren shores to bear their young.
Green turtle, another reproductive migrant, swim from their feeding grounds off the coast of Brazil during the time of laying eggs. After long swim, about 2000 km, they haul themselves onto sandy beaches, scrape out shallow nests, and deposits their eggs. Like the green turtles, freshwater eels trace ancient migratory patterns, swimming from each side of the Atlantic to the weedy Sargasso Sea and return after breeding.
Navigation of Migration
It is quiet a surprising matter about finding the same route while migration. The mystery of animal migration remains one of the most compelling in science. Much work has been done on orientation and navigation in migrating animals, although the subject is still not well understood. Studies of salmon indicate that they depend on the olfactory sense to locate and return to their stream of origin. Herbivorous mammals often follow well established trails and probably also use their sense of smell. Bats, whales, and seals use echolocation to navigate in the dark or underwater.
In shorter migration, animals do not need complicated navigation abilities. They can simply follow the food or the water, or head downhill to the valleys in winter and back up toward the ridges in summer. In longer migration, according to the scientists, animals use senses and sun, stars and geographic features. For instance, Starlings orient themselves using the sun, compensating for how the sun moves across the sky throughout the day. Mallard ducks can find north using the stars of the night sky. On the other hand, loggerhead turtles have the ability to sense the direction and strength of Earth’s magnetic field, which they use for navigating along the turtles’ regular migration route.
Animals can also use mental maps like human beings. They just become familiar with an area and navigate using mountain ranges, coastlines, rivers and even, like dolphins, the shape of the sea floor. Smell is also a powerful tool for navigation. Salmon uses smell to find the exact stream that they were born.
Migratory birds are believed to use the stars, sun and other geographic features as guides. Night migrating birds are sometimes disoriented in prolonged heavy fog. Day-flying birds navigate by the sun and also make some use of geographic features, particularly of shorelines. Most migratory birds travel within broad north-south air routes known as flyways. Some migratory birds winter only a few hundred miles from their breeding grounds, while other migrate between the cold or temperate zones of the two hemispheres. The longest journey is made by the arctic tern, a small bird that flies from the Arctic to the Antarctic and back again each year.
Hazards of Migration
Not every animals or birds can return to own land. Many died, some lost the track, some never return by nature. For example, sea turtles never saw their children. The animals face two types of hazards during the time of migration – natural and human made. Natural hazards include climatic changes, drought, food scarcity, predators and the individual physical demands of migration on the animal. Sometimes, the animal’s migratory behavior poses a considerable hazard as well. In southern Africa, for instance, springbok migrate in herds so dense that death from trampling, starvation, or drowning is not uncommon. Other animals caught in the springboks’ migration path suffer as well, often being swept along or trampled by the tide of rushing bodies.
Humans have also posed particular dangers to migrating animals. For instance, the caribou of arctic regions are hunted by Inuit who intercept hers along seasonal migration routes. Sport hunters acquaint themselves with migration routes as well.
Human made structures like skyscrapers and radio towers have caused the deaths of hundreds of migrating birds. Water pollution has caused the fishes not to migrate so some specific areas.
Every year, with the arrival of winter, migratory birds of various species throng some parts of Bangladesh. Jahangirnagar University, Hakaluki Hoar and Hyle Haor, Tangoar Beels, Nijhum Dwip, Chittagong Hill tracks, Mirpur Zoo, Mirpur Ceramic Lake, Netrokona Haor and many small as well as large lakes are some of the get together place of migratory birds. Most of the birds come from the mountainous northern parts of the subcontinent namely Himalayas and beyond. Some species come from different parts of Europe and as Far East as Siberia.
Thousands of hundred types of duck visit every year along with the native birds. Lesser Whistling Teal (Chhota Sarali), Greater Whistling Teal (Bara Sarali), Cotton Pigmy Goose (Balihansh), Pochard (Bhutihansh), Darters (Snake Bird), Pintail Duck, Gurgani etc are some of the birds.
However, the number of migratory birds in Bangladesh is decreasing every year due to the scarcity of wetlands and poaching. As the population is increasing and forests and wetlands are decreasing, we are loosing the beauty of nature. The poachers who trap and sell migratory birds also causes threaten to the ‘Guests’.
Animals are part of the world. Like humans, they breed, grow up, travel thousands of miles for searching food. They also need a peaceful living place. Only Human can save the animals from extinction. Birds and animals are the source of entertainment for the residents of the urban people. Sometimes these migratory animals threaten human society by spreading diseases like bird flu. However, this can be controlled by taking proper action. Steps should be taken to increase the arrival of migratory animals and to protect from extinction. Otherwise, we, including you, have no place to be entertained naturally!