Last week, Chicago became the first city in the US to join the "Lights Out Plan". People turned off the lights in the city's tallest buildings, so that thousands of birds could get to their winter homes. Every year in autumn, some birds fly over Chicago on their way to the south, and come back the next spring.
Lights mean danger to birds. Birds find their way home by looking at the stars in the sky. Lights in tall buildings are so bright that birds may mistake them as stars. This means they cannot find the right way to go home.
This month, more than 300 kinds of birds are flying over Chicago to go back to the south to live through the cold winter. Birds fly to warmer places for a few reasons. Many birds cannot live in the cold, so they fly to warmer places. But there are other reasons, too. Birds need to look for food in the south. Another reason is that they want to have baby birds. Many birds like to have their baby birds in the same "house" they lived in last year, so they fly back there.
There are about 9, 000 kinds of birds in the world, and about 600 kinds of birds migrate（迁徙）. Some birds can fly a very long way. Sometimes they fly to other countries. We have fewer birds today because there are fewer places for birds.
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