Bird Maps and Compasses (罗盘)
Every year hundreds of species of birds take long trips. ___26___ In the fall, they fly to warmer climates where food is plentiful all winter. When spring comes, they fly back to raise their young where they were born. For a long time people wondered where the birds went and what routes they took.
Researchers now know that migrating birds are guided by their own sorts of maps and compasses. But it has taken many decades to uncover the secrets of these navigation (导航) tools. In the 1800s, scientists started putting bands around birds' legs. ___27___ When people found the banded birds, they contacted the person named on the band and told him where and when they had found the bird. In this simple way, scientists learned a lot about where birds traveled, where they stopped, and how fast they moved.
Today scientists still put bands on birds' legs. They also use new ways of tracking birds such as airplanes, computers, and satellites. ___28___
Birds often migrate in groups. You may have seen Canada geese flying high in the sky in a V-form. Young birds follow their parents. ___29___ They may follow a river and remember what it looks like. Also, birds use the position of the sun and stars as a compass to find their way.
___30___ Chemicals in their brains allow them to sense the magnetic field and travel in the right direction. But scientists are still researching how this happens. They think some birds may actually be able to see the earth's magnetic field.