Why can't some birds fly? - Gillian Gibb

譯者: Regina Chu 審譯者: Marssi Draw

In the lush rainforests of Australia, birds roost

鬱鬱蔥蔥的澳洲雨林內

小鳥棲息在低矮的樹枝上 在林地穿梭漫步

in the low branches and amble across the forest floor,

enjoying the shade and tropical fruits.

享受著樹蔭及熱帶水果

But the jungle isn’t theirs alone.

但是這片叢林並不專屬於牠們

A dingo is prowling in the shadows,

一隻澳洲野犬在陰影中潛行覓食

and fruit won’t satisfy his appetite.

而水果不能滿足牠的胃口

The birds flee to safety

眾鳥逃往安全地

all but the cassowary,

除了食火雞

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

牠小小的翅膀幫不了牠逃命

Instead, she attacks, sending the dingo running for cover

相反的，牠攻擊！

用鋒利的爪子一揮 就讓小野犬跑去找掩護

with one swipe of her razor-sharp toe claws.

The cassowary is one of approximately 60

食火雞是大約六十種

living species of flightless birds.

不會飛的鳥類之一

These earthbound avians live all over the world,

這些束縛於地上的禽類 生活在世界各地

from the Australian outback to the

從澳洲內陸

African savanna to Antarctic shores.

到非洲草原，到南極海岸都有

They include some species of duck and all

包括某些種類的鴨子

species of penguin, secretive swamp dwellers

以及所有種類的企鵝 躲躲藏藏的沼澤居民

and speedy ostriches, giant emus, and tiny kiwis.

和飛奔的鴕鳥、巨大的鴯鶓

及微小的奇異鳥都是

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

很多不同的鳥類都分別

lost their flight.

失去飛行能力

Flight can have incredible benefits,

飛行可以帶來很多不可思議的好處

especially for escaping predators, hunting,

特別是逃避掠食者、獵食

and traveling long distances.

且能長距離旅行

But it also has high costs:

但是飛行也有極高的代價：

it consumes huge amounts of energy

它需要消耗極大的能量

and limits body size and weight.

體型及體重也都受限

A bird that doesn’t fly conserves energy,

不會飛的鳥可以節省能量

so it may be able to survive on a scarcer

所以牠比會飛的鳥更能在食物稀少

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

或食物營養較差的狀態下生存

The Takahe of New Zealand, for example,

以紐西蘭的南秧雞為例

lives almost entirely on the

幾乎全部生活在

soft base of alpine grasses.

高山草原的柔軟基底

For birds that nest or feed on the ground,

對在地面築巢或覓食的鳥類而言

this predisposition to flightlessness

這種不會飛的傾向

can be even stronger.

更加強烈

When a bird species doesn’t face

如果某種鳥類不用面對

specific pressures to fly, it can stop flying

一定要飛的特定壓力

牠就會停止飛行

in as quickly as a few generations.

只要幾代就會失去這種能力

Then, over thousands or millions of years,

然後，在數千或數百萬年後

the birds’ bodies change to match this new behavior.

鳥類的身體就會改變 以配合這種新行為

Their bones, once hollow to minimize weight,

牠們原本要減輕重量而中空的骨頭

become dense.

會變密

Their sturdy feathers turn to fluff.

牠們強健的羽毛會變鬆軟

Their wings shrink,

牠們的翅膀萎縮

and in some cases disappear entirely.

某些例子甚至完全消失

And the keel-like protrusion on their sternums,

而胸骨上的龍骨狀突起

where the flight muscles attach, shrinks or disappears,

即飛行肌肉連接之處 也會萎縮或消失

except in penguins, who repurpose their flight muscles

除了企鵝

牠們的飛行肌肉及龍骨有另外的用途

and keels for swimming.

就是為了游泳

Most often, flightlessness evolves after

最常見的是，鳥類失去飛行能力

a bird species flies to an island

是因為飛到某個島

where there are no predators.

而島上沒有掠食者才演化出來的

As long as these predator-free circumstances last,

只要這種沒有掠食者的情況持續下去

the birds thrive, but they are vulnerable

鳥類就繁衍興旺

但牠們很容易受到環境變化的影響

to changes in their environment.

For instance, human settlers bring dogs,

舉例來說，人類移民帶著狗

cats, and stowaway rodents to islands.

貓及偷偷跟來的鼠類到島嶼上

These animals often prey on flightless birds

這些動物經常捕食不會飛的鳥

and can drive them to extinction.

也可能使牠們滅絕

In New Zealand, stoats introduced by European settlers

在紐西蘭，由歐洲移民帶來的白鼬

have threatened many native species of flightless bird.

已經威脅到許多不會飛的原生鳥種

Some have gone extinct while others are endangered.

有些已經絕種，有些則瀕臨絕種

So in spite of the energy-saving advantages

所以儘管不會飛有節省能量的好處

of flightlessness, many flightless bird species have

許多不會飛的鳥都撐不了太久 就像渡渡鳥一樣絕種了

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

But a few flightless birds have survived on

但是有少數幾種不會飛的鳥

mainlands alongside predators aplenty.

在掠食者眾的大陸上存活下來

Unlike most small flightless species that

不像大多數小型不會飛的鳥種

come and go quickly, these giants have been

來得快去得也快

這些大型鳥已經數千萬年不會飛了

flightless for tens of millions of years.

Their ancestors appeared around the same time

牠們的先代出現的時間

as the first small mammals, and they were probably

大約與第一批小型哺乳類相同

牠們之所以能存活下來 是因為牠們演化

able to survive because they were evolving—

and growing—at the same time

同時也長大！

就像掠食牠們的哺乳類一樣

as their mammalian predators.

Most of these birds, like emus and ostriches,

大多數這些鳥，如鴯鶓及鴕鳥

ballooned in size, weighing hundreds of

體型像吹氣球一樣長大

比牠們的翅膀能承受的 還重好幾百磅

pounds more than wings can lift.

Their legs grew thick, their feet sturdy,

牠們的腿變粗，腳變結實

and newly developed thigh muscles turned

而新發展出來的大腿肌肉

them into formidable runners.

讓牠們成為飛毛腿

Though they no longer use them to fly,

雖然牠們不再用翅膀飛行

many of these birds repurpose

這些鳥讓翅膀

their wings for other means.

產生新的用途

They can be spotted tucking their heads beneath them

牠們常被看到把頭塞進翅膀下取暖

for warmth, flashing them at prospective mates,

揮舞著翅膀求偶

sheltering eggs with them, or even using them

用翅膀保護蛋

甚至在草原上飛奔時用它們控制身體

to steer as they charge across the plains.

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

牠們可能不會飛 但還是會展翅