英語聽力文摘 自然界的直升機(jī):蜂鳥

蜂鳥是雨燕目蜂鳥科動(dòng)物約600種的統(tǒng)稱，是世界上已知最小的鳥類。蜂鳥身體很小，能夠通過快速拍打翅膀懸停在空中，每秒約15次到80次，它的快慢取決于蜂鳥的大小。蜂鳥因拍打翅膀的嗡嗡聲(humming)而得名。蜂鳥是唯一可以向后飛行的鳥。蜂鳥也可以在空中懸停以及向左和向右飛。

Nature’s Helicopters

自然界的直升機(jī)

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.

大多數(shù)的鳥是上下拍打翅膀飛行，但蜂鳥不一樣，它是以八字形移動(dòng)翅膀。

蜂鳥每次拍打翅膀都會旋轉(zhuǎn)180度，以便為前后的拍打創(chuàng)造一個(gè)推力。

如果每次翅膀旋轉(zhuǎn)的力度一樣，蜂鳥就會盤旋在空中。它們只要往想去的方向傾斜翅膀，就可以前進(jìn)，后退，或橫向移動(dòng)。

就像直升機(jī)一樣。是的。

要盤旋在空中，蜂鳥必須得很強(qiáng)壯。它們翅膀的肌肉重量占身體整個(gè)體重的三分之一。一只小蜂鳥盤旋在空中時(shí)，翅膀的震動(dòng)頻率約為每秒80次。

蜂鳥每天吃的東西是它們體重的二分之一，這樣才能維持體力。它們吃的東西都是高熱量食物，能產(chǎn)生大量可重復(fù)使用的能量，像種子，堅(jiān)果，水果和花蜜。