(CNN)Thousands of birds die each spring and fall when they collide with Chicago's skyscrapers, which lie on a major migration path between Canada and Latin America.

But the birds don't die in vain. Since the 1970s, many of them have been collected from the street and cataloged by the city's Field Museum. This unique and detailed set of data has been a scientific windfall, revealing that North American migratory birds appear to be shrinking in response to climate change.

A new study of this data has highlighted an important nuance in this trend: Birds that have bigger brains, relative to their body size, are not shrinking as much as their smaller-brained counterparts.

The study is the first to identify a potential link between cognition and animal response to human-made climate change, according to the researchers from Washington University in St. Louis.

"As temperatures warm, body sizes are decreasing," said Justin Baldwin, a doctoral student at Washington University and author of the study that published this week in the journal Ecology Letters, in a news release. "But larger-brained species are declining less strongly than small-brained species."

美國有線電視新聞網(wǎng)）每年春天和秋天，成千上萬的鳥類死于撞到芝加哥的摩天大樓，這些摩天大樓位于加拿大和拉丁美洲之間的主要遷徙路線上。

但這些鳥兒不會(huì)白白死去。自 1970 年代以來，其中許多已被從街上收集起來并由該市的菲爾德博物館編目。這組獨(dú)特而詳細(xì)的數(shù)據(jù)是一項(xiàng)科學(xué)意外收獲，表明北美候鳥數(shù)量似乎正在因氣候變化而減少。

對這些數(shù)據(jù)的一項(xiàng)新研究強(qiáng)調(diào)了這一趨勢中的一個(gè)重要細(xì)微差別：就體型而言，大腦較大的鳥類并沒有像大腦較小的鳥類一樣數(shù)量縮減的那么嚴(yán)重。

據(jù)圣路易斯華盛頓大學(xué)的研究人員稱，這項(xiàng)研究首次確定了認(rèn)知與動(dòng)物對人為氣候變化的反應(yīng)之間的潛在聯(lián)系。

“隨著氣溫變暖，鳥類體型正在縮小，”華盛頓大學(xué)博士生賈斯汀·鮑德溫在新聞發(fā)布會(huì)上說，他同時(shí)也是本周發(fā)表在《生態(tài)學(xué)快報(bào)》雜志上的這項(xiàng)研究的作者。 “但大腦較大的物種的衰退程度低于大腦較小的物種。”

Relative brain size is often considered an indicator of behavioral flexibility in birds, according to the research. The idea is controversial when it's applied to some other animals, Baldwin said, but it works for birds.

"Relative brain size correlates with increased learning ability, increased memory, longer lifespans and more stable population dynamics," Baldwin said.

"In this case, a bigger-brained species of bird might be able to reduce its exposure to warming temperatures by seeking out microhabitats with cooler temperatures, for example," he said.

The researchers analyzed information from 70,000 birds that died when they collided with buildings in Chicago between 1978 and 2016. They added brain volume measurements and life-span data for 49 of the 52 species in the database.

Birds that had big brains, relative to their bodies such as the song sparrow and other New World sparrows, had body-size reductions that were only about one-third of those observed for birds with smaller brains, the study found. Wood warblers (Parulidae) tended to have smaller brains and tended to shrink more.

"The authors from that amazing study shared their raw data ... which allowed us to add to it and discover more," Baldwin said via email.

根據(jù)這項(xiàng)研究，大腦的相對尺寸通常被認(rèn)為是鳥類行為靈活性的指標(biāo)。鮑德溫說，這個(gè)想法如果應(yīng)用于其他一些動(dòng)物時(shí)是有爭議的，但它適用于鳥類。

“大腦的相對尺寸與學(xué)習(xí)能力的提高、記憶力的加強(qiáng)、壽命的延長和更加穩(wěn)定的人口動(dòng)態(tài)都息息相關(guān)，” 鮑德溫說。

“在這種情況下，舉例來說，大腦（相對體型）較大的鳥類可能可以通過找到更低溫的微生態(tài)環(huán)境來減少其在升溫環(huán)境下的暴露，”他說。

研究人員對1978 年至 2016 年間， 70,000 只撞到芝加哥的建筑物后死亡的鳥類的信息做了分析。 他們將 52 個(gè)中的 49 個(gè)種類的的腦容量測量和壽命數(shù)據(jù)添加到了數(shù)據(jù)庫中。

研究發(fā)現(xiàn)，像北美歌雀和其他新大陸麻雀這些大腦相較于體型較大的鳥類，它們的的體型相較于據(jù)觀測大腦較小的鳥類僅縮小了將近三分之一。林鶯（林鶯科）往往大腦更小，（體型）縮小的更嚴(yán)重。

“做出這份驚人研究的作者們分享了他們的原始數(shù)據(jù)……讓我們可以有更多的研究和發(fā)現(xiàn)，”鮑德溫通過電子郵件說。

Shape-shifters

It's not known exactly why birds are shrinking in size. Larger body size helps animals in cold places stay warm, while a smaller body retains less heat.

Bird wingspans may have increased to compensate for smaller bodies that produce less energy for the incredibly long distances traveled during migration, researchers have also found.

Similarly, other research has found that some animals are developing larger beaks, legs and ears that allow them to better regulate body temperature as the planet gets hotter. While most of the morphological changes have been in birds, bats and shrews have also been affected. Climate change has even altered human bodies.

However, downsizing comes at a potential cost for a bird, with an increased risk of falling prey to predators or making it harder to compete for resources with other bird species, said study coauthor Carlos Botero, assistant professor of biology at Washington University, in the statement.

It's in this context that having a larger brain may offer alternatives that are not available to small-brained species, he said.

"One of the first things that jumps out to me from these findings is that we can already see that climate change is having a disproportionate effect on species that have less capacity to deal with environmental change through their behavior," Botero said.

"This doesn't mean that climate change is not affecting brainy birds ... or that brainy birds are going to do just fine. What our findings suggest is that climate change can have a much stronger effect on the less-brainy birds."

目前鳥類體型縮小的確切原因尚不可知。較大的體型幫助動(dòng)物在寒冷地方保暖，而較小的體型只能保留較少的熱量。

研究人員還發(fā)現(xiàn)，鳥類的翼展可能已經(jīng)增加，可以在遷徙時(shí)相當(dāng)長距離的飛行中，來補(bǔ)償較小體型產(chǎn)生的較少的能量。

同樣的，其他研究也發(fā)現(xiàn)一些動(dòng)物正在長出更大的喙、腿和耳朵，使它們能夠隨地球變暖更好地調(diào)節(jié)體溫。雖然大部分形態(tài)變化發(fā)生在鳥類身上，但蝙蝠和鼩鼱也受到了影響。氣候變化甚至改變了人體。

然而，該研究的合著者、華盛頓大學(xué)生物學(xué)助理教授卡洛斯·博特羅在報(bào)告中表示，體型變小會(huì)讓鳥類付出潛在的代價(jià)，成為捕食者的獵物，或和其他鳥類爭奪資源變得更加困難，這樣的風(fēng)險(xiǎn)會(huì)增加。

他說，正是在這個(gè)背景下，擁有更大的大腦，可能會(huì)獲得更多的選擇，而這些是小腦物種無法得到的。

“這些發(fā)現(xiàn)讓我首先想到的是，我們已經(jīng)能看到，氣候變化已經(jīng)對通過行為應(yīng)對環(huán)境變化的能力較弱的物種產(chǎn)生了不成比例的影響，”博特羅說。

“這并不意味著氣候變化不會(huì)影響那些聰明的鳥類......或者聰明的鳥類會(huì)沒有問題。我們的研究結(jié)果表明，氣候變化可以對不太聰明的鳥類產(chǎn)生更大的影響?！?/p>

From <https://edition.cnn.com/2022/02/11/americas/birds-brains-climate-change-scn/index.html>