Why don’t birds get lost on their long migratory (迁居的) flights? Scientists have puzzled over this question for many years. Now they’re beginning to fill in the blanks.
Not long ago, experiments showed that birds rely on the sun to guide them during daylight hours. But what about birds that fly mainly by night? Tests with artificial stars have proved conclusively that certain night-flying birds are able to follow the stars in their long-distance flights.
One such bird, a warbler(鸣禽) , had spent its lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use the stars for guidance. The bird’s cage was placed under an artificial star-filled sky at migration time. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the direction of the make-be-live stars caused a change in the direction of his flight. Scientists think that warblers, when flying in daylight, use the sun for guidance. But the stars are apparently their main means of navigation (导航). What do they do when the stars are hidden by clouds? Apparently, they find their way by such landmarks as mountain ranges, coast lines, and river courses. But when it’s too dark to see these, the warblers circle helplessly, unable to get their direction.
The reasons why birds don’t get lost on migratory flights__________.

A．have been known to scientists for many years

B．are known by everyone

C．have only recently been discovered

D．will probably remain a mystery

Warblers migrate__________.

A．from North America to South America

B．using what is apparently an inborn navigational ability

C．only once during their lifetime

D．when they are freed from their cages

When the stars are hidden by clouds, warblers find their way by__________ .

A．an artificial star	B．some landmarks
C．their inborn ability	D．A and B

This article is a good example of the way scientists__________ .

A．discover workable answers to universal questions by studying particular cases(特别案例)

B．jump to conclusions

C．find a law and then investigate（调查）

D．are disappointed by the habits of animals

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