Question

Read the passage and fill in the blanks with proper words or the proper forms of the given words.

Why don’t birds get lost on their long migratory (迁移的) flights? Scientists tried their best 1. (find) the answer to this question for many years. The reasons 2. (discover) lately.

Not long ago experiments showed that birds depend 3. the sun to guide them during daylight hours. But what about birds 4. fly mainly at night? Tests with man-made stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights.

One such bird, a warbler (莺), had spent its lifetime in a cage and had never flown under a(n) 5. (nature) sky. Yet it showed its ability by birth to use the stars for guidance. The bird’s cage 6. (put) under a man-made star-filled sky at migration time. The bird tried to fly in the same direction as the one that 7. (take) by other birds in the building. Any change in the position of the make-believe (虚构的) stars caused a change in the direction of its flight.

Scientists think that warblers, 8. they fly in daylight, use the sun for guidance. But stars are clearly their important means of navigation (导航). What do they do when the stars are hidden behind the clouds? Clearly, they find their way by such landmarks as mountains, coastlines and river courses. But when it’s too dark 9. (see) these, warblers circle 10. (help), unable to find out where they were.

Answer

1.to find

2.have been discovered

3.on

4.that / which

5.natural

6.was put

7.is taken

8.when

9.to see

10.helplessly

【解析】

1.考查固定搭配。try one’s best to do sth.竭尽全力做某事，故答案为to find。

2.考查现在完成时态的被动语态。根据lately可知句子用现在完成时态，再根据句子主语reasons 和discover之间是被动关系，且主语是复数形式，所以句子用现在完成时态的被动语态，故答案为have been discovered。

3.考查固定搭配。depend on依赖，取决于，故答案为on。

4.考查定语从句。句意：但是，那些主要在夜间飞行的鸟类呢？此处birds是先行词，指物，后面的定语从句缺少主语和引导词，故答案为that/which。

5.考查形容词。此处sky是名词，用形容词来修饰，故答案为natural。

6.考查一般过去时态的被动语态。句意：在迁徙的时候，鸟的笼子被放在一个人造的布满星星的天空下。此处The bird’s cage 和put之间是被动关系，结合句意可知此处用一般过去时态的被动语态，故答案为was put。

7.考查一般现在时态的被动语态。句意：这只鸟试图飞向同一方向，就像那座楼里其他鸟儿所做的那样。此处the one 是先行词，在后面的定语从句中做主语，且the one 和take之间是被动关系，根据句意可知定语从句中指的是“经常性、习惯性的动作”，所以这里用一般现在时态的被动语态，故答案为is taken。

8.考查时间状语从句。句意：科学家们认为，莺在白天飞行时，会利用太阳来引导。结合句意可知此处是when引导的时间状语从句，故答案为when。

9.考查固定结构。句意：但是，当天太黑了，看不见这些东西的时候，莺们只能无助地盘旋，无法找到它们的位置。too---to---，太---而不能，故答案为to see。

10.考查副词。句意：但是，当天太黑了，看不见这些东西的时候，莺们只能无助地盘旋，无法找到它们的位置。副词修饰动词，结合句意可知答案为helplessly。