阅读理解。
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 (人
造的) 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 principal 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.
1. The reasons why birds don't get lost on migratory flights _____. [ ]
A. have been known to scientists for many years
B. have only recently been discovered
C. are known by everyone
D. will probably remain a mystery
2. Warblers migrate _____. [ ]
A. from North America to South America
B. only once during their lifetime
C. using what is apparently an inborn navigational ability
D. when they are freed from their cages
3. When the stars are hidden by clouds, warblers find their way by _____. [ ]
A. an artificial star
B. their inborn ability
C. some landmarks
D. wind
4. This article is a good example of the way scientists _____. [ ]
A. jump to conclusions
B. discover workable (可行的) answers to general questions by studying particular cases
C. find a law and then investigate (调查)
D. are disappointed by the habits of animals
