birds roost in the low branches and amble across the forest floor,

鳥兒們在低枝上棲息、在森林中漫步

enjoying the shade and tropical fruits.

享受著林蔭及熱帶水果

But the jungle isn't theirs alone.

但叢林裡不是只有牠們而已

A dingo is prowling in the shadows, and fruit won't satisfy his appetite.

一隻澳洲野犬在暗處潛行，而水果無法滿足牠的胃

The birds flee to safety all but the cassowary, who can't clear the ground on her puny wings.

鳥兒們飛走以逃離牠的魔掌，但鶴鴕卻因其翅膀短小而無法飛離

Instead, she attacks, sending the dingo running for cover with one swipe of her razor-sharp toe claws.

相反地，牠採取攻擊，用那鋒利的的爪子一掃，讓那隻野犬嚇得尋找掩護

The cassowary is one of approximately 60 living species of flightless birds.

地球上大約有 60 種不會飛翔的鳥類，鶴鴕就是其中一種

These earthbound avians live all over the world,

這些只能待在地面上的鳥類足跡遍布全世界

from the Australian outback to the African savanna to Antarctic shores.

從澳洲內陸到非洲大草原，再到南極沿岸

They include some species of duck and all species of penguin, secretive swamp dwellers

種類包含了一些特定品種的鴨子、所有企鵝、住在沼澤地區的居民

and speedy ostriches, giant emus, and tiny kiwis.

跑得飛快的鴕鳥、體型巨大的鴯鶓和嬌小的奇異鳥

Though the common ancestor of all modern birds could fly,

雖然所有現代鳥類的共同祖先都會飛

many different bird species have independently lost their flight.

但很多不同種類的鳥卻都各自失去了飛行能力

Flight can have incredible benefits,

飛行能力可以帶來許多意想不到的優勢

especially for escaping predators, hunting, and traveling long distances.

特別像是逃離捕食者、狩獵及遠距離的旅行

But it also has high costs:

但擁有飛行能力的同時也要付出很大的代價

it consumes huge amounts of energy and limits body size and weight.

飛行會耗上很多體力，也會限縮鳥類的體型及體重

A bird that doesn't fly conserves energy,

不會飛的鳥可以保存他們的體力

so it may be able to survive on a scarcer or less nutrient-rich food source than one that flies.

所以跟會飛的鳥比起來，牠們較有可能從食物或營養匱乏中存活下來

The Takahe of New Zealand, for example, lives almost entirely on the soft base of alpine grasses.

以紐西蘭的南秧雞為例，牠們幾乎都生活在食物不豐的高山草原

For birds that nest or feed on the ground,

對那些在地面築巢或覓食的鳥類來說

this predisposition to flightlessness can be even stronger.

這種天生就不會飛的天性可以讓牠們更具優勢

When a bird species doesn't face specific pressures to fly,

當鳥類不用面對不得不飛行的壓力

it can stop flying in as quickly as a few generations.

經過幾代後，牠們可能就不再飛行

Then, over thousands or millions of years,

接著在幾千幾百萬年後

the birds' bodies change to match this new behavior.

鳥類的身形就會有所變化，來適應「不需要飛」的這個行為

Their bones, once hollow to minimize weight, become dense.

原先為了輕量化而中空的骨頭會變得結實

Their sturdy feathers turn to fluff.

柔韌堅挺的羽毛也變得蓬鬆柔軟

Their wings shrink, and in some cases disappear entirely.

翅膀長度會縮短，甚至是完全消失

And the keel-like protrusion on their sternums,

在牠們胸骨上如龍骨般的突出 (龍骨突)

where the flight muscles attach, shrinks or disappears,

原先是為了連接拍動翅膀的肌肉，也會跟著萎縮或消失

except in penguins, who repurpose their flight muscles and keels for swimming.

但企鵝除外，牠們轉而將飛行用的肌肉和龍骨突用來游泳

Most often, flightlessness evolves after a bird species flies to an island

在大多數的情況下，鳥類是在飛到一座島嶼後才演化成沒有飛行能力的

where there are no predators.

因為該座島上沒有牠們的捕食者

As long as these predator-free circumstances last,

如果這種沒有天敵的情況繼續下去

the birds thrive, but they are vulnerable to changes in their environment.

這些鳥就會大量繁殖，但牠們也會很容易受到環境的影響

For instance, human settlers bring dogs, cats, and stowaway rodents to islands.

比方說，移民者如果帶著狗、貓以及隨船偷渡的囓齒動物來到島上

These animals often prey on flightless birds and can drive them to extinction.

牠們通常會以這些不會飛的鳥兒為食，並導致牠們滅絕

In New Zealand, stoats introduced by European settlers

在紐西蘭，歐洲移民者在引進白鼬時

have threatened many native species of flightless bird.

就威脅到了許多不會飛行的原生鳥種

Some have gone extinct while others are endangered.

有些鳥類因此滅絕，而有些則瀕臨絕種

So in spite of the energy-saving advantages of flightlessness,

所以即使不會飛的鳥類有節省能量的優勢

many flightless bird species have only a short run before going the way of the dodo.

許多不會飛的鳥類也只存活了一小段時間而已，就像渡渡鳥一樣

But a few flightless birds have survived on mainlands alongside predators aplenty.

但少數幾種不會飛的鳥，也在充滿許多捕食者的內陸中存活下來

Unlike most small flightless species that come and go quickly,

不同於大多數不會飛的小型鳥類可以快速地移動

these giants have been flightless for tens of millions of years.

這些巨鳥從好幾千萬年就失去飛行能力了

Their ancestors appeared around the same time as the first small mammals,

牠們祖先出現的時間與第一批小型哺乳類相差不遠

and they were probably able to survive

牠們之所以能生存下來

because they were evolving and growing at the same time as their mammalian predators.

是因為牠們在演化過程中長到跟牠們的哺乳類掠食者一樣大

Most of these birds, like emus and ostriches,

大部分的這種鳥類，像是鴯鶓和鴕鳥

ballooned in size, weighing hundreds of pounds more than wings can lift.

體型就像吹氣球一樣，體重比牠們翅膀所能負荷的重量重好幾百倍

Their legs grew thick, their feet sturdy,

牠們的腿變得結實、腳爪變得粗壯

and newly developed thigh muscles turned them into formidable runners.

後來演化出的大腿肌更讓牠們成為了讓人望而生畏的飛毛腿

Though they no longer use them to fly,

雖然這些鳥類不再用牠們的翅膀飛翔

many of these birds repurpose their wings for other means.

牠們還是能用翅膀來做其他的事

They can be spotted tucking their heads beneath them for warmth,

人類發現成鳥會將幼鳥的頭塞在翅膀下取暖

flashing them at prospective mates,

對心儀的對象擺動翅膀求偶

sheltering eggs with them,

保護牠們的蛋

or even using them to steer as they charge across the plains.

甚至在草原上飛奔時，用翅膀來控制牠們的方向

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

牠們也許不會飛，但他們仍「飛」常厲害