Some 500 million birds migrate each year from Africa, some weighing as little as nine grams (three­tenths of an ounce). To complete a voyage that can be thousands of kilometers long, birds have to fatten themselves up to twice their normal weight. Some even shrink their internal organs (收缩内脏) to become more fuel efficient, so any additional distance may be dangerous.

　　The study finds that from 2071 to 2100, nine out of 17 species examined are going to face longer migrations, particularly birds that cross the Sahara Desert. Some birds traverse the Sahara and the Mediterranean Sea in one go, while others have a break in northern Africa before crossing. Many fly at night, when temperatures are cooler.

　　A few—such as the Blackcap—have started to adapt by spending winters in Britain, but such behavior remains exceptional, the study said. The study forecasts that the migration distance of the Orphean Warbler will jump from 2,700 kilometers (1,700 miles) to between 3,050 and 3,350 kilometers (1,900 and 2,100 miles), with even longer increases for the Subalpine and Barred Warblers.

　　[语篇解读]　文章主要介绍了气候变化对鸟类产生的影响，迫使候鸟迁徙的更远。

　　1．What does traveling extra distance mean for some migrating birds?

　　A．Better living conditions.

　　B. Better physical health.

　　C．Stronger ability to fly.

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