Why can't some birds fly? - Gillian Gibb

翻译人员: Rui Tang 校对人员: Congmei Han

In the lush rainforests of Australia, birds roost

在澳大利亚茂盛的雨林里，

in the low branches and amble across the forest floor,

鸟类们栖息在低矮的树杈上， 从森林的地面上漫步走过，

enjoying the shade and tropical fruits.

尽情享受树荫和热带水果。

But the jungle isn’t theirs alone.

但丛林里并不只有它们。

A dingo is prowling in the shadows,

一只澳洲野狗正在隐蔽处悄然潜行，

and fruit won’t satisfy his appetite.

水果显然无法满足它的胃口。

The birds flee to safety

鸟儿们还是顺利的逃走了，

all but the cassowary,

但是食火鸡，

who can’t clear the ground on her puny wings.

却无法用它孱弱的翅膀飞离地面。

Instead, she attacks, sending the dingo running for cover

然而，它选择攻击澳洲野狗，

with one swipe of her razor-sharp toe claws.

用它锋利的脚趾给了 它的对手猛烈一击以把它赶跑。

The cassowary is one of approximately 60

食火鸡就是现存的大约 60 种

living species of flightless birds.

无法飞行的鸟类之一。

These earthbound avians live all over the world,

这种只在陆地上活动的鸟类 在全世界都存在，

from the Australian outback to the

从澳洲的腹地

African savanna to Antarctic shores.

到非洲的热带草原、再到南极海岸， 都能见到它们。

They include some species of duck and all

这些鸟类包含了一些种群的鸭子

species of penguin, secretive swamp dwellers

和全种类的企鹅， 以及秘密栖息在沼泽的鸟类、

and speedy ostriches, giant emus, and tiny kiwis.

快速移动的鸵鸟、 巨大的鸸鹋、小小的鹬鸵。

Though the common ancestor of all modern birds could fly,

虽然，所有现代鸟类的共同祖先 都能够飞翔，

many different bird species have independently

但唯独一些个别种类的鸟

lost their flight.

却丧失了飞翔的能力。

Flight can have incredible benefits,

飞行有着许多好处，

especially for escaping predators, hunting,

尤其是能够从逃脱被捕食、捕猎，

and traveling long distances.

并且到达很远的地方。

But it also has high costs:

但是，这也有很高代价：

it consumes huge amounts of energy

飞行需要消耗巨大的能量，

and limits body size and weight.

并且在体积和体重上也会很受限制。

A bird that doesn’t fly conserves energy,

一只鸟如果不飞就能节省体能，

so it may be able to survive on a scarcer

这可能会帮助它在面对食物匮乏

or less nutrient-rich food source than one that flies.

或食物短缺的情况时，相比于 会飞的鸟，更容易生存下来。

The Takahe of New Zealand, for example,

例如，新西兰的南秧鸟，

lives almost entirely on the

一生几乎只生活在

soft base of alpine grasses.

高山柔软的草地上。

For birds that nest or feed on the ground,

对于那些在陆地上 筑巢和进食的鸟类来说，

this predisposition to flightlessness

这种不会飞的特质

can be even stronger.

会令身体更强壮。

When a bird species doesn’t face

当一种鸟的种群不需要面对

specific pressures to fly, it can stop flying

明确的需要飞行的压力时，它们

in as quickly as a few generations.

在经历过几代更迭后 很快就停止飞翔了。

Then, over thousands or millions of years,

接着，千百万年后，

the birds’ bodies change to match this new behavior.

这种鸟类的体型就会改变 以适应这种新的习性。

Their bones, once hollow to minimize weight,

它们的骨头， 一旦缩小到更小的重量，

become dense.

就会变得密度更高。

Their sturdy feathers turn to fluff.

它们坚固的羽毛变得蓬松。

Their wings shrink,

它们的翅膀变小，

and in some cases disappear entirely.

甚至在有些个例，完全消失了。

And the keel-like protrusion on their sternums,

同时，它们胸骨上如同船的龙骨一般 突起的地方，

where the flight muscles attach, shrinks or disappears,

即飞行所需要的肌肉连接处， 逐渐缩小、消失，

except in penguins, who repurpose their flight muscles

然而企鹅却是一种例外，它们 改变了用途，将飞行所依靠的肌肉

and keels for swimming.

和骨头用于游泳。

Most often, flightlessness evolves after

通常来说，向不会飞的方向进化是

a bird species flies to an island

当一种鸟的种群飞到一个

where there are no predators.

没有捕食者的岛上，才逐渐开始的。

As long as these predator-free circumstances last,

只要这种无捕食者的环境还存在，

the birds thrive, but they are vulnerable

这里的鸟类就会兴盛繁衍，但是它们

to changes in their environment.

在环境发生改变时也容易受重创。

For instance, human settlers bring dogs,

例如，人类移民把狗、

cats, and stowaway rodents to islands.

猫还有其他的啮齿动物带到岛上。

These animals often prey on flightless birds

这些动物就会捕食 这些没有飞行能力的鸟，

and can drive them to extinction.

并逐渐把它们赶向灭绝。

In New Zealand, stoats introduced by European settlers

在新西兰，欧洲移民带来了白鼬

have threatened many native species of flightless bird.

对许多无法飞行的鸟类造成了威胁。

Some have gone extinct while others are endangered.

一些种类已经灭绝， 而其他的也处于危险之中。

So in spite of the energy-saving advantages

所以，尽管不会飞的鸟类 有着体能储存的优点，

of flightlessness, many flightless bird species have

但许多不会飞的鸟类

only a short run before going the way of the dodo.

难逃像渡渡鸟一样被灭绝的厄运。

But a few flightless birds have survived on

然而仍有些不具有飞行能力的鸟类

mainlands alongside predators aplenty.

在充满捕食者的大陆上生存着。

Unlike most small flightless species that

与众多小型的、能够快速移动的 无飞行能力的鸟类不同，

come and go quickly, these giants have been

这些体积巨大的鸟类

flightless for tens of millions of years.

在过去的千百万年里都无法飞行。

Their ancestors appeared around the same time

它们的祖先与最初的小型哺乳动物

as the first small mammals, and they were probably

出现在同一时期，并且它们

able to survive because they were evolving—

能幸存下来，可能是因为它们

and growing—at the same time

与这些哺乳动物一起

as their mammalian predators.

进化并且变得巨大。

Most of these birds, like emus and ostriches,

这些鸟类中的大多数， 如鸸鹋和鸵鸟，

ballooned in size, weighing hundreds of

体型激增，多达数百磅重

pounds more than wings can lift.

以至于翅膀无法带动它们飞翔。

Their legs grew thick, their feet sturdy,

它们的腿变细，脚变得坚硬，

and newly developed thigh muscles turned

新生的大腿肌肉群

them into formidable runners.

使它们成为优秀的奔跑者。

Though they no longer use them to fly,

虽然它们不再用翅膀飞翔，

many of these birds repurpose

但是许多这样的鸟类

their wings for other means.

把翅膀用在了其他用途上。

They can be spotted tucking their heads beneath them

它们可以把头埋在翅膀下面

for warmth, flashing them at prospective mates,

以保暖，向它们喜欢的伴侣炫耀，

sheltering eggs with them, or even using them

保护自己的蛋，甚至

to steer as they charge across the plains.

用于在平原上冲刺时控制方向。

They may be flightless, but they’re still winging it.

它们可能无法飞翔，但它们仍然扇动着翅膀。