Climate change is shrinking our birds.
Birds in both North and South America are getting smaller as the planet warms, and the smallest-bodied species are changing the fastest, a study reported Monday.
At the same time, birds also are getting longer-winged, according to the study, which pinpoints human-caused climate change as the cause.
Monday’s study combines data from two papers that measured body-size and wing-length changes in more than 86,000 bird specimens collected over four decades in North and South America. One study examined migrating birds killed when they struck buildings in Chicago; the other looked at nonmigrating birds netted in the Amazon.
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Earlier studies have found similar results
This isn’t the first study to report such findings. According to a 2021 study in Science Advances, since the 1980s Amazonian birds’ bodies have shrunk and their wings have lengthened, the National Audubon Society said. “After accounting for other factors, the researchers found that the birds’ bodily changes were closely linked to the rising temperatures and shifts in precipitation caused by climate change.”
And according to UCLA, a study published last year in Nature Ecology and Evolution found that over the past three decades, the body mass of 105 bird species in the analysis declined by an average of 0.6% − but by as much as 3% in some species. Tree swallows, for example, got 2.8% smaller, American robins got 1.2% smaller, and downy woodpeckers got 2.2% smaller.
In the study published Monday, scientists found that despite being sampled from different environments using different methods, both datasets showed an overall decline in body size and concurrent increases in wing length.
“We found, in both (data sets), a consistent pattern wherein smaller species declined proportionally faster in body size and increased proportionally faster in wing length,” the study said.
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Why would climate change make birds smaller?
According to the Field Museum, smaller bodies hold on to less heat and larger bodies hold on to more. The phenomenon is called Bergmann’s rule, and it helps animals stay a comfortable temperature in different environments.
Meanwhile, the birds’ wingspans may have increased so the birds are still able to make their long migrations, even with smaller bodies to produce the energy needed for flight, the Field Museum said.
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What’s new in Monday’s study
The study published Monday is the first to report that smaller birds are changing the fastest.
But “why smaller-bodied species are changing faster is unknown,” the study said. It could be that smaller-bodied birds are adapting more quickly to evolutionary pressures. But the available data did not allow the research team to test whether the observed size shifts represent rapid evolutionary changes in response to natural selection.
“If natural selection plays a role in the patterns we observed, our results suggest that smaller bird species might be evolving faster because they experience stronger selection, are more responsive to selection, or both,” said study co-author Brian Weeks, an evolutionary ecologist at the University of Michigan.
“Either way, body size appears to be a primary mediator of birds’ responses to contemporary climate change.”
The study was published in the peer-reviewed journal Proceedings of the National Academy of Sciences.
Contributing: Adrianna Rodriquez, USA TODAY