September 24, 2008	Wednesday	Ramazan 23, 1429

In the fall you can look up in the sky and see beautiful formations of birds flying south.

Birds can’t pass a science test, but they know exactly when it is time to fly to warmer climates for the winter and when to go back home. This seasonal journey is called migration.

Scientists believe that birds detect changes in temperature and daylight. As the days get shorter and the amount of light decreases, birds produce certain hormones that seem to trigger the drive to migrate. A hormone is a chemical produced by the body.These chemicals cause the birds to store up fat in their bodies. The extra fat can be turned into energy that helps keep the birds going on their long flight.

Birds migrate because it is harder for them to find food (especially insects) when the ground begins to freeze. And ducks, geese and other birds that spend a lot of time in water can’t break through ice that forms in freezing temperatures.

Some birds start their southerly trek as early as July. Others wait until winter is nearly upon them.

The distances that birds migrate vary according to species. The blue grouse, for example, flies less than a mile. In an odd twist, it goes to a higher, colder spot in the North American mountains where it lives to avoid competition for food at lower levels.

Arctic terns fly from the Arctic, at the top of the planet, to their winter home in the Antarctic, at the bottom of the planet. And they go back again in the same year. That’s more than 20,000 miles a year. This migratory route is the longest of any animal.

Though most birds fly at heights of less than 500 feet during their everyday lives, many go much higher when they migrate. Some go up to 10,000 feet, or 1.9 miles. An airline pilot spotted bar-headed geese at 29,000 feet. That’s 5.5 miles!

Another amazing thing that birds can do well is navigate.

Some people can get off an escalator at a store and be confused about where they are. But birds can travel long distances and arrive at the same place each year.

Scientists don’t know how birds do this. They may have some kind of internal compass that detects Earth’s magnetic field and tells them which way is north.

You might think migration would be exhausting. Apparently not. The American golden-plover travels about 2,400 miles, from Nova Scotia to South America, in about 48 hours – without stopping! Birds show signs of being tired but not exhausted when they reach their destination.—Dawn/LT Times-Washington Post News Service