A male bay-breasted warbler, one of the species of smaller-brained birds, researchers said were more affected by climate change.
A male bay-breasted warbler, one of the species of smaller-brained birds, researchers said were more affected by climate change.
Adobe Stock

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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.

climate change birds 1
Kate Golembiewski/Field Museum

Climate change could be making birds shrink in size, study finds

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.”

Since the 1970s, migratory birds that died after colliding with Chicago buildings have been collected and cataloged by the Field Museum. The data set is showing how birds are affected by climate change.
Since the 1970s, migratory birds that died after colliding with Chicago buildings have been collected and cataloged by the Field Museum. The data set is showing how birds are affected by climate change.
Bilgin S. Sasmaz/Anadolu Agency/Getty Images

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 wing of a Rufous-capped Antthrush (Formicarius colma).
The wing of a Rufous-capped Antthrush (Formicarius colma).
Vitek Jirinec

The climate crisis is messing with birds' body shapes

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.

A Blackburnian warbler, pictured here, was another of the species of smaller-brained birds that were more strongly affected by climate change.
A Blackburnian warbler, pictured here, was another of the species of smaller-brained birds that were more strongly affected by climate change.
Adobe Stock

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.

animals climate change shape shift scn
Ryan Barnaby

Animals are 'shape shifting' in response to climate change

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.”