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  3. Why don’ t birds get lost on their long flights from one place to another? Scientists have puzzled over this question for a long

Why don’ t birds get lost on their long flights from one place to another? Scientists have puzzled over this question for a long

admin2013-06-22  51
问题     Why don’ t birds get lost on their long flights from one place to another? Scientists have puzzled over this question for a long time. Now they’ re starting to fill in the blanks.
    Not long ago, experiments showed that birds use the sun to guide them during daylight hours. How do birds fly at night? Tests with artificial stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights.
    A dove had spent its lifetime in a cage and had never flown under a natural sky. But it had an inborn ability to use the stars for guidance. The bird’s cage was placed under an artificial star-filled sky. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the artificial stars caused a change in the direction of his flight.
    But the stars are apparently their main means of navigation. When the stars are hidden by clouds, they apparently find their way by such landmarks as river courses, mountain ranges, coast lines. But when it’ s too dark to see these things, the doves circle helplessly, unable to find their way.
The experiment with the dove indicates that______.
选项 A、birds have to be taught to navigate
B、a bird that has been caged will not fly long distances
C、some birds cannot fly at night
D、some birds seem to follow the stars when they fly at night
答案D
解析 题干意为“鸽子的实验表明 。”选项A从第三段“But it had an inborn ability to use the stars for guidance．”可以排除。解题关键是从第二段的最后一句话“Tests with artificial stars have proved that certain night—flying birds are able to follow the stars in their long—distance flights．”可排除B、C，并推理出D项为正确答案。
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