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  3. Birds are warm blooded animals. Though their feathers help to keep them warm

Birds are warm blooded animals. Though their feathers help to keep them warm

游客2023-10-17  33
问题     Birds are warm blooded animals. Though their feathers help to keep them warm, some birds such as ducks, 【C1】______(goose), and swans still can’t endure harsh winter temperatures. For these reasons, birds from cold climates fly to warmer climates 【C2】______the winter. This seasonal movement of birds is called migration. Birds migrate to warmer places, often hundreds of miles away, where they can have the best chance of 【C3】sur______.

    Birds migrate naturally. Certain clues from the environment cause hormone changes in the bird’s body. As the days get 【C4】______(short), for example, these hormones tell the bird’s body to store fat. This is because migrating takes an【C5】en______energy. Birds don’t have a lot of time to eat while migrating, so they rely on stores of fat.
    When birds migrate, they fly as a group. To minimize the energy needed to fly long distances, a group of birds 【C6】______(fly)together in a V-shape. The bird at the front of the "V" uses the most energy because the wind often blows 【C7】______him. Every so often, the birds change positions so that each bird has a turn at the front, and everyone gets a rest.
    【C8】Nav______is also an important part of the journey. Birds find where they are going by using visual clues, such as rivers, coastlines, and mountain ranges. In addition, they use the Sun and the star for guidance. 【C9】______(amaze), they also use the Earth’s invisible magnetic force for direction. This gives them a natural sense of north and south, like a kind of internal compass.
    Many studies indicate that migratory birds fly along the same course every year. Researchers decide to test this using "bird banding". They first capture a migratory bird and attach a tag to its foot. This tag has an ID number on it, 【C10】______is stored in a database. They then set the bird free and track its movements. Bird banding has shown that many birds follow the same route year after year. [br] 【C4】
选项
答案 shorter
解析 (前文提到鸟类在冬天来临之前进行迁移，由此可知此处指“随着白天逐渐变短，鸟类体内的荷尔蒙会告知鸟类的身体来储存脂肪”，故该空填人short的比较级shorter。)
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