Each one has been meticulously measured and recorded. 

每一份樣本的大小都被精心測量並記錄下來。 

But when researchers started analyzing those measurements in 2019, they noticed something strange.  

但當研究者們在 2019 年開始分析這些數據時，他們發現了一些不對勁的地方。

"These birds are pretty different in their behaviors: where they nest, where they winter, where they migrate.

「這些鳥類的習性截然不同：牠們築巢、過冬、遷徙的地點都不一樣。

But we found that everything — all of the species — had declined in size over the past 40 years."

但我們發現全部（所有品種的鳥類）在過去的四十年間，體積都縮小了。」

One of the body parts declining in size was a lower leg bone called the tarsus. 

鳥類身上縮小的其中一個部分，是被稱為跗骨的下腳骨。

And when they lined up the tarsus measurements with summer temperature records, changes in temperature were followed one year later by a change in leg size.

在研究者們將跗骨的尺寸與每年夏季的氣溫紀錄對照後，發現一年的氣溫與之前相比改變後，鳥類腳骨的尺寸也會跟著變化。

"When temperatures started to cool down the next year, birds started to get bigger, when temperatures started to warm up, birds started to get smaller."

「若氣溫在下一年開始下降，鳥類的尺寸便會變得更大，而若氣溫開始上升，鳥類的尺寸則會開始變小。」

On average, warming temperatures were making birds shrink. 

平均來說，逐漸變暖的氣溫正在讓鳥類的尺寸縮小。

But it's not just birds.

但這並不是只發生在鳥類身上而已。

The same effect has been observed among salamanders in Appalachia, red deer in Norway, woodrats in New Mexico, fish in the Atlantic and Gulf of Mexico, and beetles around the world. 

我們可以在阿帕拉契亞山脈的蠑螈、挪威的紅鹿、新墨西哥州的林鼠、大西洋與墨西哥灣中的魚類，以及全球各地的甲蟲身上看到同樣的現象。

These changes might seem small so far, but they could eventually push many species closer to extinction.

這個現象在目前看來或許沒什麼大不了，但長久下來可能會在最後使許多物種更瀕臨絕種邊緣。

So why is this happening? 

所以，為什麼會這樣？ 

Researchers are still in the process of answering that question, but there are two broad hypotheses. 

研究者們仍在尋求這個問題的解答，但現在主要推論出了兩個廣泛的假說。

"The mechanisms really fall into two different categories. There's selection and developmental plasticity."

「我們認為整個機制的運作方式分為兩大類，也就是天擇與表型可塑性。」

Selection pressure consists of changes that pass from one generation to the next because they're advantageous to survival, while developmental plasticity describes changes that happen during an animal's life due to environmental conditions.

天擇的壓力包含了要將身上的變化傳遞到下一代，因為這些特性是有利於生存的。而表型可塑性描述的則是在動物的生命歷程中，面對環境狀況所進行對自己的變動。

For a long time, researchers believed that shrinking was caused only by selection pressure because of a principle in ecology called Bergmann's Rule.

在很長一段時間中，研究者們因為一個叫做伯格曼法則的規律，而相信這種體積的縮小只是天擇所造成的結果。

"There's a really well known rule in ecology that individuals tend to be smaller in the warmer parts of their ranges.

「在生物學中有一個為人所熟知的法則，就是動物個體在較為溫暖的地方體積會變得更小。

So there is this long standing hypothesis that as we warm the world, things will get smaller."

所以一直以來都有隨著地球暖化，動物會變得越來越小的假說。」

That's because of warm-blooded animals' ratio of body size to surface area, which makes it easier for large animals to conserve heat when it's cold, and easier for small animals to cool off when it's hot.

這是因為恆溫動物的體積大小與相對的身體表面積比例成反比的緣故。也就是說大型動物更容易在天氣寒冷時保存體溫，而小型動物更容易在天氣炎熱時散熱。

"You could think about an ice cube melting into glass.

「你可以想像一顆在玻璃杯中慢慢融化的冰塊。

If you have a bunch of small ice cubes, they melt faster than one big ice cube. It's the same idea. "

如果你有一堆小冰塊，它們會融化得更快。而如果你有一顆大冰塊，則會融化得比較慢。這是同樣的道理。」

But cold-blooded animals don't generate body heat, and they're shrinking too, which means that selection pressure for conserving body heat isn't the only thing at play.

而雖然冷血動物並不會在體內產生熱能，牠們的體積卻還是正在縮小。這意味著保存身體熱能的天擇壓力並不是動物體積縮小的唯一因素。

Instead, cold-blooded animals are particularly susceptible to plastic changes: changes that occur during the individuals' lifetime and aren't passed on to the next generation.

相對的，冷血動物更容易受到表型可塑性的影響：也就是單一動物在生命中經歷的變化，且該變化不會傳遞給下一代。

With frogs, for example, warmer temperatures increase metabolism and make the transitions between life cycle development phases speed up.

拿青蛙作為例子，更炎熱的氣溫會提升牠們的新陳代謝率，並加速了牠們的生命周期發展階段。

But because their rate of growth doesn't change, they're smaller by the time they arrive at adulthood. 

但由於牠們的生長速度並沒有改變，所以牠們便提早到達了成年期。

What researchers are still trying to figure out is how much these two factors are shaping warm and cold-blooded species, and whether they can make predictions about what those changes will look like.

研究者們仍在試著瞭解這兩個因素對恆溫與冷血物種的影響有多大，還有他們能如何預測未來將會有哪些改變。

Plus, this trend isn't uniform: warming temperatures are making some species bigger.

另外，這個趨勢並不是放諸四海皆準的：溫暖的氣溫其實會讓部分物種的體積變得更大。

In high latitudes, increasing temperatures and precipitation have given shrews, otters, and martens more time and resources to grow before winter.

在高海拔地區，氣溫與降水的增加讓鼩鼱、海獺與貂類更多的時間與資源來趕在冬季來臨前成長茁壯。

But we do know that warming results in smaller animals on average, because Earth has been through periods of warming that have shrunk animal bodies before. 

但我們確實能知道全球暖化平均而言會導致動物的體積縮小，因為地球過去也有經歷過數次的暖化，並在當時讓動物的體積縮小。

In a warming event roughly 56 million years ago, temperatures increased between 5 to 8 degrees Celsius over 10,000 years.

在大約五千六百萬年前的暖化期中，氣溫在一萬年間上升了五到八攝氏度。

And we can see a noticeable dip in animal size in the fossil record.

而我們可以在當時的化石中看到動物的體積顯著縮小。

But today, we're warming the planet at an unprecedented rate — about 10 times faster than the average warming following historic ice ages, giving animals little time to adapt. 

但今天我們正以前所未有的速度加劇全球暖化－這個速度比過去冰河期前的平均暖化速度還要快上十倍，讓動物幾乎沒有時間適應。

"That's the problem with human-driven climate change.

「這就是人類所造成氣候變遷的問題。

It's the rate of change that's just orders of magnitude faster than what the natural world has had to deal with in the past. 

改變的速度實在太快，讓自然界在過去得處理的問題多了好幾倍。

Size is really important to survival, and you can't just change that indefinitely without consequence.

體積大小對生存來說至關重大，而你不可能直接在毫無代價的情況下永遠改變體積。

For one thing, I don't think it's feasible that species are going to be able to continue to get smaller and maintain things like a migration from one hemisphere to another."

首先，我不認為物種能夠在持續變小的情況下，還能同時進行如從南半球遷徙到北半球之類的活動。」

And since smaller bodies can hold fewer eggs, they result in fewer offspring, and a lower population size in the long run. 

而且由於身體變小了，能產下的蛋量也會變少，讓生下的後代數量跟著減少，並在長期下來降低整個族群的數量。

For amphibians who need to keep their skin wet in order to breathe, shrinking can mean higher chances of drying out in a drought because their bodies absorb and hold smaller quantities of water.

對需要讓皮膚保持濕潤才能呼吸的兩生類來說，體積的縮小代表其更可能因為身體能吸收與保存的水分減少，而在乾旱中因失去水分而死亡。

But the more concerning consequences have to do with how this could destabilize relationships between species.  

但更令人憂心的後果，其實是這樣的現象很可能會打破物種之間的平衡關係。

Because shrinking plays out at different rates for different species.

這是因為體積縮小的速度在不同的物種之間並不相同。

Predators might have to eat more and more of shrinking prey, for example, throwing a finely-tuned ecosystem off balance.

舉例來窩，掠食者可能得吃掉更多體積已經縮小了的獵物，讓原本和諧的生態系統失衡。

It's that mismatch that's particularly dangerous. 

而這樣的失衡狀況是非常危險的。