单选题 Why don't birds get lost on their long flights from one place to another? 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 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. Yet it showed 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 principal means of navigation. When the stars are hidden by clouds, they apparently find their way by such landmarks as mountain ranges, coast lines, and river courses. But when it's too dark to see these, the doves circle helplessly, unable to find their way.

单选题 The reasons why birds don't get lost on long flights______.
A. have been known to scientists for years B. have only recently been discovered
C. are known by everyone D. will probably remain a mystery

单选题 During daylight hours, birds______.
A. fly aimlessly B. rely on landmarks
C. use the sun for guidance D. are more likely to get lost

单选题 By "his outdoor cousins" the author means______.
A. other experimenters B. the other doves of the same brood
C. doves under the natural sky D. other birds in general

单选题 The experiment with the dove indicated that______.
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

单选题 In total darkness, doves______.
A. use landmarks B. don't know which way to fly
C. fly back home D. wait for the stars to appear