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In the lush rainforests of Australia, birds roost in the low branches and amble across the forest floor, enjoying the shade and tropical fruits.
在澳大利亚茂盛的雨林里，鸟类们栖息在低矮的树杈上，从森林的地面上漫步走过，尽情享受树荫和热带水果。
But the jungle isn't theirs alone. A dingo is prowling in the shadows, and fruit won't satisfy his appetite.
但丛林里并不只有它们。一只澳洲野狗正在隐蔽处悄然潜行，水果显然无法满足它的胃口。
The birds flee to safety all but the cassowary, who can't clear the ground on her puny wings.
鸟儿们还是顺利的逃走了，但是食火鸡，却无法用它孱弱的翅膀飞离地面。
Instead, she attacks, sending the dingo running for cover with one swipe of her razor-sharp toe claws.
然而，它选择攻击澳洲野狗，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 living species of flightless birds.
食火鸡就是现存的大约60种无法飞行的鸟类之一。
These earthbound avians live all over the world, from the Australian outback to the African savanna to Antarctic shores.
这种只在陆地上活动的鸟类在全世界都存在，从澳洲的腹地到非洲的热带草原、再到南极海岸，都能见到它们。
They include some species of duck and all species of penguin, secretive swamp dwellers and speedy ostriches, giant emus, and tiny kiwis.
这些鸟类包含了一些种群的鸭子和全种类的企鹅，以及秘密栖息在沼泽的鸟类、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, many different bird species have independently lost their flight.
虽然所有现代鸟类的共同祖先都能够飞翔，但唯独一些个别种类的鸟却丧失了飞翔的能力。
Flight can have incredible benefits, especially for escaping predators, hunting, and traveling long distances.
飞行有着许多好处，尤其是能够从被捕食、捕猎中逃脱，并且到达很远的地方。
But it also has high costs: it consumes huge amounts of energy and limits body size and weight.
但是，这也有很高代价：飞行需要消耗巨大的能量，并且在体积和体重上也会很受限制。
A bird that doesn't fly conserves energy, so it may be able to survive on a scarcer or less nutrient-rich food source than one that flies.
一只鸟如果不飞就能节省体能，这可能会帮助它在面对食物匮乏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, lives almost entirely on the soft base of alpine grasses.
例如，新西兰的南秧鸟，一生几乎只生活在高山柔软的草地上。
For birds that nest or feed on the ground, this predisposition to flightlessness can be even stronger.
对于那些在陆地上筑巢和进食的鸟类来说，这种不会飞的特质会令身体更强壮。
When a bird species doesn't face specific pressures to fly, it can stop flying in as quickly as a few generations.
当一种鸟的种群不需要面对明确的需要飞行的压力时，它们在经历过几代更迭后很快就停止飞翔了。
Then, over thousands or millions of years, the birds' bodies change to match this new behavior.
接着，千百万年后，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, become dense.
它们的骨头，一旦缩小到更小的重量，就会变得密度更高。
Their sturdy feathers turn to fluff. Their wings shrink, and in some cases disappear entirely.
它们坚固的羽毛变得蓬松。它们的翅膀变小，甚至在有些个例，完全消失了。
And the keel-like protrusion on their sternums, where the flight muscles attach, shrinks or disappears,
同时，它们胸骨上如同船的龙骨一般突起的地方，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 and keels for swimming.
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和骨头用于游泳。
Most often, flightlessness evolves after a bird species flies to an island where there are no predators.
通常来说，向不会飞的方向进化是当一种鸟的种群飞到一个没有捕食者的岛上，才逐渐开始的。
As long as these predator-free circumstances last, the birds thrive, but they are vulnerable to changes in their environment.
只要这种无捕食者的环境还存在，这里的鸟类就会兴盛繁衍，但是它们在环境发生改变时也容易受重创。
For instance, human settlers bring dogs, cats, and stowaway rodents to islands.
例如，人类移民把狗、猫还有其他的啮齿动物带到岛上。
These animals often prey on flightless birds and can drive them to extinction.
这些动物就会捕食这些没有飞行能力的鸟，并逐渐把它们赶向灭绝。
In New Zealand, stoats introduced by European settlers have threatened many native species of flightless bird.
在新西兰，欧洲移民带来了白鼬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 of flightlessness, many flightless bird species have only a short run before going the way of the dodo.
所以，尽管不会飞的鸟类有着体能储存的优点，但许多不会飞的鸟类难逃像渡渡鸟一样被灭绝的厄运。
But a few flightless birds have survived on mainlands alongside predators aplenty.
然而仍有些不具有飞行能力的鸟类在充满捕食者的大陆上生存着。
Unlike most small flightless species that come and go quickly, these giants have been flightless for tens of millions of years.
与众多小型的、能够快速移动的无飞行能力的鸟类不同，这些体积巨大的鸟类在过去的千百万年里都无法飞行。
Their ancestors appeared around the same time as the first small mammals,
它们的祖先与最初的小型哺乳动物出现在同一时期，
and they were probably able to survive because they were evolving -- and growing -- at the same time as their mammalian predators.
并且它们能幸存下来，可能是因为它们与这些哺乳动物一起进化并且变得巨大。
Most of these birds, like emus and ostriches, ballooned in size, weighing hundreds of pounds more than wings can lift.
这些鸟类中的大多数，如鸸鹋和鸵鸟，体型激增，多达数百磅重以至于翅膀无法带动它们飞翔。
Their legs grew thick, their feet sturdy, and newly developed thigh muscles turned them into formidable runners.
它们的腿变细，脚变得坚硬，新生的大腿肌肉群使它们成为优秀的奔跑者。
Though they no longer use them to fly, many of these birds repurpose their wings for other means.
虽然它们不再用翅膀飞翔，但是许多这样的鸟类把翅膀用在了其他用途上。
They can be spotted tucking their heads beneath them for warmth, flashing them at prospective mates,
它们可以把头埋在翅膀下面以保暖，向它们喜欢的伴侣炫耀，
sheltering eggs with them, or even using them to steer as they charge across the plains.
保护自己的蛋，甚至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。
