MYSTERIES OF MIGRATION G3sr06

 6. MYSTERIES OF MIGRATION

Winter weather comes with its extreme coldness and low supply of food. Since most animals can not survive in it, they either sleep through it or migrate.

The urge to migrate is very strong in some birds. Each year the tern can produce offspring only during a two-month period. During those two months the tern lives in the region near the North Pole. In the ten months between this time and the next, he flies all the way from the North Pole to the South Pole, and then back north again. That is a distance of about 22,000miles. To accomplish this, the tern must fly about 75miles every day for ten months!

What causes birds to migrate? How do they know when to migrate? How do they find their way on such long trips? And why do they come back from those warm comfortable places?

Studies in New York State show that it is the bird's biological time clock that causes a bird to migrate. A bird specialist kept some birds in a planetarium. He was surprised to find that they didn't pay any attention to the stars in the ceiling when migration time came. But then he gradually gave them periods of false daylight, making the days shorter and shorter, as they are in the fall. Within a few weeks the birds tried their best to fly south. He also found that the birds navigate by the North Star once they get the urge to migrate. In fact, when the North Star was removed from the false sky, the birds became completely confused and flew in all different directions.

Some birds find their way by the position of the sun in the sky. But most birds migrate at night. Apparently they migrate by the stars just as pilots of ships do.

This was shown scientifically by some studies in Germany recently. Native song birds were kept inside a planetarium. In the center of the floor of a planetarium a machine shoots lights on to the ceiling, making small points of light to look like stars. The operator can imitate the night sky over any part of the earth by changing the machine. Inside the building the birds all became excited on the day of their usual migration. They all tried to fly south to Africa to their winter home.

Then the experimenters changed the lights so that the sky on the ceiling was like that in Russia. At once the birds changed direction in the building to face what would have been the direction of Germany if they had been in Russia. When the false "sky" was slowly changed so that it was again like the one over Germany, the birds again turned to face Africa, trying to escape and fly a thousand miles.

If things are so good in the warm climates that birds return there every winter, why do they come back north every spring? The warm region is crowded with birds, and the short winter stay of the migrants puts space and food on short supply. The area also has many turtles, snakes, and other animals that steal from bird nests, so birds return north to lay their eggs.

Bird migration is the most familiar migration. But other animals migrate too. Every fall monarch butterflies all over Canada and the United States take an extremely long migration trip. They start out singly, then join others in crowds, moving in great numbers through the skies. Sometimes the butterflies fly in a single line, and other times they crowd together in thick black clouds, then stop in trees, making the covered branches move as if they were living.

When you hold a butterfly in your hand and see the thin wings, you wonder how the butterflies can make these long trips. But they fly thousands of miles, often ten to forty miles an hour. No one knows how they find their way to the same places in California, Florida, and Mexico—but they do. And. as did their parents and grandparents, they not only go to the same towns for the winter, but sometimes even land in the same trees. In the spring the butterflies leave the trees and return north again. There they lay their eggs on young plants that are just coming up above the ground. After they lay their eggs, they die.

Other creatures migrate too. Clouds of locusts sometimes cover the sky and eat everything visible. Eels leave the streams of Europe and America in the fall and travel as far as 5,000 miles to reach the Sargasso Sea in the Atlantic Ocean near Central America. Green turtles that live in Brazil set out every two or three years and travel across more than a thousand miles of open sea to lay their eggs on tiny islands in the South Atlantic Ocean. Salmon return to their home streams to lay their eggs, as do other fish.

Many other animals migrate, some short distances, some to the other side of the world.

Man, too, has been a great migrator. Many forces have kept man on the move—wars, religions, food, sickness. The most commonly known migrants were the groups who moved with the changes of seasons. These groups followed the animals north in the spring and south in the fall. For example, the American Plains Indian followed the buffalo for centuries.

Man started his life upon the earth moving from one place to another. Moving frequently, looking for food, hunting here, picking fruit there—this was the earliest human way. That urge to move is still in most of us, but it is also balanced with the opposite and newer urge to farm and build cities.