animals migrate?

Why do animals migrate?
Many types of birds, insects, marine mammals, reptiles, fish and large mammals migrate each year. The principal reasons for this are to reach suitable places for breeding, and to obtain better conditions for food, water and temperature.

Some examples of migration for reproduction are: fish, such as salmon, who migrate to spawning grounds (and change from saltwater to fresh water); sea birds, sea turtles and seals, who come ashore to breed; and whales, who journey thousands of kilometres to their breeding grounds.

Many birds migrate from cold regions to warmer zones in winter. Large herbivorous mammals, like moose, have winter and summer habitats, and some antelopes in Africa, migrate so as not to be caught in droughts.

How do they find their way?
There is evidence to suggest that animals that migrate possess a sort of inbuilt "compass". In one experiment, some starlings were taken from their usual habitat in Holland, to Switzerland. When these birds migrated, they went in the same direction as in previous years, but ended up in Spain instead of France.

Further experiments have shown that animals may use the sun, the stars (nocturnal birds), polarized light (on cloudy days), geomagnetic fields (pigeons, turtles) and landmarks (whales, insects) to help them find their way.

Irruptions
An irruption is an irregular movement of an animal population out of an area, with no return, caused by a sudden, explosive increase in that animal's numbers. The best-known examples of this involve lemmings and locusts. It is a common myth that lemmings commit mass suicide by throwing themselves over cliffs. What happens, in fact, is that every few years their numbers increase to such an extent that they must leave their usual habitat in search of food. Their instincts are so compelling that they will cross rivers or lakes by swimming, or continue their journey even if the land before them ends in a cliff or the ocean.

Some facts and feats
Passenger pigeons were once the most common bird in the whole world, occupying the whole of the eastern United States from Canada to the Gulf of Mexico, breeding in their northern habitats and going south to more temperate climes in the winter.

They migrated in such vast numbers that the sun would be blocked out as they passed. Ornithologist John James Audobon wrote in his book The Birds of America: "Before sunset I reached Louisville, distant from Hardensburgh fifty-five miles, the pigeons were still passing in undiminished number, and continued to do so for three days in succession." Another ornithologist, Alexander Wilson, noted "It was then half past one (when the birds first appeared in the sky). About four in the afternoon, the living torrent above my head seemed as numerous and extensive as ever." He estimated that in less than 3 hours he had seen a little more than two billion birds.

Other extraordinary accounts talk of the birds' droppings covering the countryside like snow, their cooing drowning the noise of hunters' guns, and nesting grounds covering more than 300 square kilometres in which enormous trees would crash to the ground under the weight of the birds.

Astonishingly, the passenger pigeon is now extinct. A combination of habitat destruction and commercial killing meant that the last surviving specimen died in Cincinnati Zoo in 1914.

Another example of an animal that once migrated in vast numbers, but was almost hunted to extinction, is the American bison. At the beginning of the 19th century, an estimated 50-60 million bison would migrate to richer pastures each year. "When the European settlers arrived … a mass slaughter began. At the peak of the slaughter, some 2.5 million bison were killed annually from 1870 to 1875 and the legendary 'Buffalo Bill' Cody claimed to have killed 4862 of the animals in one year alone". Unlike the passenger pigeon, however, conservation efforts saved the bison from extinction.

Guinness Book of Animal Records, p. 8

Other animals amaze by the great distances they cover during migration. The arctic tern is credited with the longest migratory flight. "It breeds mainly around the shores of the Arctic Ocean and then flies to the other side of the world to spend the remainder of the year in the Antarctic - a total distance, if it were to travel in a straight line, of at least 16,000 kilometres. In a lifetime, this is equivalent to flying to the Moon and back".

Guinness Book of Animal Records, p. 127

The grey whale is believed to undertake the longest known migration of any mammal. "Hugging the North American coastline, it swims from its winter breeding grounds in Baja California, Mexico, to its summer feeding grounds in the rich waters of the Bering Sea, and back again, every year. This amounts to a total annual distance of (up to) 20,000 kilometres".

Guinness Book of Animal Records, p. 117

The European freshwater eel is another great migratory creature. They are born in the Sargasso Sea, off the coast of the USA and the Caribbean, and then drift for some three years across the Atlantic Ocean towards Europe, where they live in rivers of the North Atlantic, Baltic and Mediterranean Seas. When the eels reach maturity, and conditions are right, they make their way down the rivers and out to sea to begin the long swim back to the breeding grounds of the Sargasso Sea.

For the peace of mind of any couch potatoes reading this, not all animals migrate enormous distances. The blue or dusky grouse "spends the winter in mountain pine forests and descends just 300 metres to nest in deciduous woodland where there is an early crop of fresh leaves and seeds".

Guinness Book of Animal Records, p. 128