LIGHTS out! Believe it or not, the American city of Chicago (芝加哥) is going to make its lights weaker for birds.

  Last week, Chicago became the first city in the US to join the "Lights Out" programme. By turning off the lights in the city's tallest buildings, thousands of birds are more likely to reach their winter homes.

Every year in autumn, some types of birds migrate (迁移). They are flying over Chicago on their journey south and return the following spring.

Lights mean danger

Birds find their way by looking at the stars in the sky. Lights in tall buildings are too bright for the birds and they can mistake them for stars. This means they don't know which way to go, and can have sad results. Some birds die because they get too tired to fly, and some fly into the buildings. Warblers (鸣禽) and sparrows (麻雀)are especially confused by the lights.

This month, around 300 species of migratory birds are flying over Chicago to go back south to escape the cold winter.

Migratory birds travel every autumn and spring and follow a special route (路线).

Birds migrate for several reasons. Many birds cannot live in the cold, so they fly to warmer places in the south. But there are other reasons, too. Birds need to search for food. In winter, many plants die and insects hibernate (冬眠). When it get colder, birds know it's time to fly to warmer places with more food. Reproduction (繁殖) is another reason. Many birds like to reproduce in the same "house" they used last year, so they fly back there.

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There are around 9,000 kinds of birds in the world, and 600 types are migratory birds. Most migratory birds in China follow three routes. The first one is the western route, across the Qinghai-Tibet Plateau (青藏高原) to Sichuan. The second route is from the Taihang Mountain to Sichuan and other southern parts of China. Other birds fly along the coast (海岸) to eastern and southern China.

BIRDS can fly very long distances (距离) when migrating. Sometimes they fly to other continents (大洲). The longest a bird has flown is 22,000 miles (35,400km).