Migration

Back in the centuries when most people traveled very little and did not know much about the world we live in, they got some strange ideas about birds. They would see swallows gathering in large flocks in the fall, and overnight they would be gone. The next spring they would come back again. Where had they been? It was a well-known fact that frogs hid in the mud at the bottom of ponds in the fall, coming out when the weather got warm in the spring. So it was decided that the swallows must do the same thing! This was believed for hundreds of years.

Around 1700 someone published a paper suggesting that the birds that disappeared in the fall flew to the moon! It was thought that it took them about two months to make a one-way trip. It was probably not known at that time that there is no air out in space. But anyone should have been able to figure out that there would be no food or water or perching places.

Now we not only know that millions of birds move north in the spring and south in the fall, but we can pinpoint exactly where they go.

Around the beginning of the century scientists began putting light aluminum bands on the legs of waterfowl. Each band had a number and an address. If a bird was killed the band was to be sent to the address given. Or a bird might be captured, then released after the number and address had been copied. Other kinds of birds, too, are banded in this way. This has helped us to learn some astonishing things about birds.

Why do birds migrate? The explanation found in most books is that a long time ago a great glacier slid from the north pole toward the south pole and all living things were driven ahead of it. Then when the glacier started to melt, every spring the birds went as far north as they could, going back south in the fall. After the glacier was gone the birds kept going back and forth from force of habit. Even if that were true, it would not explain why some birds spend the winter in Hawaii and other South Sea islands. Nor would it explain how the old birds can leave their northern homes at one time and the young ones leave at an earlier or later date, and they all arrive at the same destination.

Nor would it explain why some birds fly half-way around the world twice a year. Some arctic terns live in the far north during the short summer when the sun shines all the time. When the sun starts to dip below the horizon at midnight, the birds head south. In the middle of winter in the north the sun is shining all the time at the south pole, so the arctic tern enjoys two summers in the year. He has to fly 22,000 miles a year to do it, but he has an easier trip than some birds. He can rest on the water and fish on the way. And the days are always much longer than the nights while he is traveling.

Many birds migrate much shorter distances. They may live in Canada or the northern U. S. and move to the southern states for the winter. Or they may live in the southern U. S. in the summer and move to northern South America for the winter.

How do birds that go south for the winter find their way back to the same apple tree to nest the next summer? Evolution doesn't have the answer!