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  • In the lush rainforests of Australia,

    在蓊鬱的澳洲雨林裡

  • 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

In the lush rainforests of Australia,

在蓊鬱的澳洲雨林裡

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B2 中高級 中文 美國腔 TED-Ed 鳥類 翅膀 捕食 鳥兒 體型

【TED-Ed】為什麼有些鳥兒就是不會飛? (Why can't some birds fly? - Gillian Gibb)

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    Vivian Chen 發佈於 2018 年 11 月 17 日
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