Streamline English Destinations 23

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• published: 10 Oct 2012
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Streamline English Destinations 23 MIGRATION One of the greatest mysteries of nature is the instinct to migrate. Every year millions of creatures feel the need to move for one reason or another. Most of us have seen the arrival or departure of migrating flocks of birds. Migration, however, is not confined to birds, but can be seen in reptiles (for example turtles, frogs), insects (butterflies, locusts), fish (eels, salmon, tunny) and mammals (reindeer, seals, lemmings, whales, bats). Many of these creatures succeed in navigating over long distances. How exactly they manage to do this still remains a mysteries. There are several possibilities. They may navigate by using one or more of the following: 1 The sun. 2 The stars. 3 The Earth's magnetic field. (When a small bar magnet is attached to a pigeon, it is unable to navigate.) 4 A sense of smell. 5 Geographical features. (Birds flying from North Africa to France seem to follow coastlines and valleys.) 6 Changes in temperature. (Salmon can detect a change in water temperature as small as 0.03°C.) 7 Sound. (Whales and bats seem to use sonar.) Experiments suggests that these navigational abilities are partly instinctive. In one famous experiment a young seabird from the island of Stokholm, off the Welsh coast, was taken across the Atlantic by plane to Boston, 5100 km away. It was released, and was back in its nest twelve and a half days later. The Arctic Tern This seabird holds the record for long-distance migration. Arctic Tern breed in Northern Canada, Greenland, Northern Europe, Siberia and Alaska. In late August they set off on a 17,500 km journey which takes them south, past the West coasts of Europe and Africa to the tip of Southern Africa (14,000 km in 90 days). They then fly round to the Indian Ocean and down to Antarctica, where they spend the Antarctic summer. On the way back they sometimes make a complete circuit of Antarctica before returning to their breeding-grounds. The round trip is over 35,000 km in eight months (240 km a day when they are flying). The Arctic Tern sees more hours of daylight than any other creature, because it experiences two summers a year, one in the Arctic region, and one in the Antarctic. These regions have almost constant daylight in summer. One tern, which was ringed in Norway as a chick, died in exactly the same place, twenty-seven years later. Presumably, it had made the journey twenty-seven times. The European Freshwater Eel European Freshwater Eel, which look like snakes but are really fish, begin and end their lives in the Sargasso Sea, southeast of Bermuda. As eggs and larvae they drift for three years towards Europe, changing both shape and colour as they reach the fresh-water estuaries of European rivers. They spend the next nine to nineteen years in rivers, streams, lakes and ponds. As they approach old age they seem to have an unexplained compulsion to return to the Sargasso Sea to breed. Many eels which have found their way into ponds and lakes come out of the water and travel overland, gliding through damp grass. When they reach the sea, they make their way to the Sargasso, where they breed and die. No eels make the journey twice. The eel has an acute sense of smell, which is used for navigation in local waters, but inherited memory seems the only explanation for their migration to the Sargasso. The Lemming The Brown Lemming is a small mammal (10-18 cm long) found all over Northern Canada, Scandinavia and Northern Russia. Lemmings usually make short, annual migrations in spring, travelling by night and feeding and sleeping by day. Every three or four years, however, they make much longer migrations in large numbers. The lemming population seems to change over a three or four year cycle, from one lemming per four hectares to between 400 and 700 lemmings per four hectares. Migration seems to be a method of population control, and is most spectacular in the well known "mass suicides", where thousands of lemmings plunge over cliff tops into the sea, and swim till they die of exhaustion. These "mass suicides" only occur infrequently, and then only in Norway where mountains touch the sea. Nobody knows what makes them do it, but there are two theories. One is that migrating lemmings cross rivers and lakes and can't tell the difference between a river and the sea. The other, more interesting theory is that they are migrating towards ancient breedinggrounds which existed beneath the North Sea millions of years ago, when the sea-level was lower.