Birds are warm blooded animals.though their feathers help to keep them warm ,some birds such as ducks,【A1】________(goose),and swans still can't endure harsh winter temperatures .for these reasons ,birdseed from cold climates fly to warmer climates 【A2】________the winter .this seasonal movement of birdseed is called migration .birds migrate to warmer places ,often hundreds of milestone away ,where they can have the best chance of 【A3】sur________.
Birds migrate naturally .certain clues form the environment cause hormone changes in the bird's body .as the days get 【A4】________(short), for example ,these hormones tell the bird's body to store fat .this is because migrating Takes an 【A5】en________energy .birds don't have a lot of time to eat while migrating ,so they rely on stores of fat .
When bird migrate, the fly as a group to minimize the energy needed to fly long distances ,a group of birds【A6】________(fly )together in a V-shape .the bird at the front of the "V"uses the most energy because the wind often blows 【A7】________him.every so often ,the birds change positions site that each bird has a turn at the front ,and everyone gets a rest .
【A8】Nav________is also an important part of the journey .birds find where they are going birds using visual clues ,such as Rivers coastlines,and mountain ranges.In addition ,they use the sun and the star for guidance .【A9】________(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 ,【A10】________is stored in a database .they then set the bird free and track its movement .bird banding has shown that many birds follow the same route year after year .

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