(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."

美国有线电视新闻网）每年春天和秋天，成千上万的鸟类死于撞到芝加哥的摩天大楼，这些摩天大楼位于加拿大和拉丁美洲之间的主要迁徙路线上。

但这些鸟儿不会白白死去。自 1970 年代以来，其中许多已被从街上收集起来并由该市的菲尔德博物馆编目。这组独特而详细的数据是一项科学意外收获，表明北美候鸟数量似乎正在因气候变化而减少。

对这些数据的一项新研究强调了这一趋势中的一个重要细微差别：就体型而言，大脑较大的鸟类并没有像大脑较小的鸟类一样数量缩减的那么严重。

据圣路易斯华盛顿大学的研究人员称，这项研究首次确定了认知与动物对人为气候变化的反应之间的潜在联系。

“随着气温变暖，鸟类体型正在缩小，”华盛顿大学博士生贾斯汀·鲍德温在新闻发布会上说，他同时也是本周发表在《生态学快报》杂志上的这项研究的作者。 “但大脑较大的物种的衰退程度低于大脑较小的物种。”

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.

根据这项研究，大脑的相对尺寸通常被认为是鸟类行为灵活性的指标。鲍德温说，这个想法如果应用于其他一些动物时是有争议的，但它适用于鸟类。

“大脑的相对尺寸与学习能力的提高、记忆力的加强、寿命的延长和更加稳定的人口动态都息息相关，” 鲍德温说。

“在这种情况下，举例来说，大脑（相对体型）较大的鸟类可能可以通过找到更低温的微生态环境来减少其在升温环境下的暴露，”他说。

研究人员对1978 年至 2016 年间， 70,000 只撞到芝加哥的建筑物后死亡的鸟类的信息做了分析。 他们将 52 个中的 49 个种类的的脑容量测量和寿命数据添加到了数据库中。

研究发现，像北美歌雀和其他新大陆麻雀这些大脑相较于体型较大的鸟类，它们的的体型相较于据观测大脑较小的鸟类仅缩小了将近三分之一。林莺（林莺科）往往大脑更小，（体型）缩小的更严重。

“做出这份惊人研究的作者们分享了他们的原始数据……让我们可以有更多的研究和发现，”鲍德温通过电子邮件说。

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."

目前鸟类体型缩小的确切原因尚不可知。较大的体型帮助动物在寒冷地方保暖，而较小的体型只能保留较少的热量。

研究人员还发现，鸟类的翼展可能已经增加，可以在迁徙时相当长距离的飞行中，来补偿较小体型产生的较少的能量。

同样的，其他研究也发现一些动物正在长出更大的喙、腿和耳朵，使它们能够随地球变暖更好地调节体温。虽然大部分形态变化发生在鸟类身上，但蝙蝠和鼩鼱也受到了影响。气候变化甚至改变了人体。

然而，该研究的合著者、华盛顿大学生物学助理教授卡洛斯·博特罗在报告中表示，体型变小会让鸟类付出潜在的代价，成为捕食者的猎物，或和其他鸟类争夺资源变得更加困难，这样的风险会增加。

他说，正是在这个背景下，拥有更大的大脑，可能会获得更多的选择，而这些是小脑物种无法得到的。

“这些发现让我首先想到的是，我们已经能看到，气候变化已经对通过行为应对环境变化的能力较弱的物种产生了不成比例的影响，”博特罗说。

“这并不意味着气候变化不会影响那些聪明的鸟类......或者聪明的鸟类会没有问题。我们的研究结果表明，气候变化可以对不太聪明的鸟类产生更大的影响。”

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