他们能连飞数天，而且飞行时甚至能睡觉！
They can fly for many days without stopping. They can even sleep while they are flying!
所以，飞机必须保持一个最低速度以产生升力，而且飞行的高度越高，飞机维系升力的速度也越高。
A plane must maintain a minimum speed to generate lift, and the higher it travels, the faster it must go.
他们使用新型枪支，鸟类在飞行中很容易被击落，而且数量很大，以至于商业捕猎鸭鹅成了一种行业。
They use new types of guns, and birds could easily be shot down in flight, and in such great numbers that commercial hunting of ducks and geese became an industry.
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