英语听力文摘 English Digest 394、自然界的直升机:蜂鸟（在线收听）

蜂鸟是雨燕目蜂鸟科动物约600种的统称，是世界上已知最小的鸟类。蜂鸟身体很小，能够通过快速拍打翅膀悬停在空中，每秒约15次到80次，它的快慢取决于蜂鸟的大小。蜂鸟因拍打翅膀的嗡嗡声（humming）而得名。蜂鸟是唯一可以向后飞行的鸟。蜂鸟也可以在空中悬停以及向左和向右飞。

Nature’s Helicopters
自然界的直升机

Most birds flap their wings up and down, but hummingbirds move their wings in a figure-eight pattern.

With each flap they twist their wings one-hundred-eighty degrees so that they create a downward thrust on both the forward and backward strokes.

If each loop of the wings is of the same strength, they hover. They can go forward, backwards, or sideways by tilting their wings in the direction they want to go.

Kind of like a helicopter.Exactly.

In order to hover, hummingbirds have to be really strong. Their wing muscles are thirty percent of their total weight. When a small hummingbird hovers it flaps its wings about eighty times per second.

Hummingbirds have to eat close to fifty percent of their weight in food everyday to keep up their strength. They eat things that are packed with calories and produce lots of reusable energy, like seeds, nuts, fruit, and nectar.

大多数的鸟是上下拍打翅膀飞行，但蜂鸟不一样，它是以八字形移动翅膀。

蜂鸟每次拍打翅膀都会旋转180度，以便为前后的拍打创造一个推力。

如果每次翅膀旋转的力度一样，蜂鸟就会盘旋在空中。它们只要往想去的方向倾斜翅膀，就可以前进，后退，或横向移动。
就像直升机一样。是的。

要盘旋在空中，蜂鸟必须得很强壮。它们翅膀的肌肉重量占身体整个体重的三分之一。一只小蜂鸟盘旋在空中时，翅膀的震动频率约为每秒80次。

蜂鸟每天吃的东西是它们体重的二分之一，这样才能维持体力。它们吃的东西都是高热量食物，能产生大量可重复使用的能量，像种子，坚果，水果和花蜜。