Why don’t birds get lost on their long flights from one place to another?
Not long ago, experiments showed that birds rely  【小题1  the sun to guide them during daylight hours. But what about birds 【小题2   fly by night? Tests with artificial（人造的）stars have proved that certain night flying birds are able to follow the stars in their long-distance flights.
A dove（鸽子） had spent its lifetime in a cage and had never flown under a  【小题3  (nature) sky. Yet  【小题4   showed an inborn ability to use the stars for guidance. The bird’s cage  【小题5  (place) under an artificial star-filled sky. The bird tried to fly in the same direction 【小题6(take) by his outdoor cousins. Any change in the position of the make-believe stars caused  【小题7change in the direction of his flight.
Scientists think that doves, when 【小题8  (fly) in daylight, use the sun for guidance. But the stars are apparently their principal means of navigation. What do they do 【小题9  the stars are hidden by clouds? 【小题10   (obvious), they find their way by such landmarks as mountain ranges, coastlines, and river courses. But when it’s too dark to see these, the doves circle helplessly, unable to get their position.