Since the late 1970s, many North American bird species have decreased in size, scientists say. But during that period, bigger-brained birds, like the song sparrow, did not shrink as much as their smaller-brained counterparts, like warblers.
That’s according to researchers at Washington University, who drew on a data set of approximately 70,000 birds killed by window strikes in downtown Chicago between 1978 and 2016.
“We were really struck by how some species seem to be decreasing a lot more than others,” said study co-author, Justin Baldwin, a Ph.D. candidate with the Botero Lab at Washington University.
And the reason, researchers believe, is rising temperatures. Baldwin said further research could shed light on how exactly climate change has catalyzed the differences in size. Right now, he sees two possible explanations.
As temperatures rise, natural selection might favor smaller birds because they have more surface area relative to their volume. “That aids their capacity to dissipate heat,” he said. “If temperatures are dangerously high — especially during the summer months — then it might be that individuals that are smaller do better and survive these extreme temperature events better.”
The other possibility is that birds with bigger brains relative to their body size might be better at avoiding the effects of heat waves. In birds, unlike many other animals, brain size is linked to increased cognitive flexibility — suggesting bigger-brained birds have found solutions that elude their smaller-brained counterparts. “Maybe they're hiding out in cooler places; maybe they're moving around a little bit better,” suggested Baldwin.
Wash U says the study is the first to identify a direct link between cognition and animal response to climate change.
Overall, climate change is having a big impact on birds. Higher temperatures have forced birds to move to new habitats. Birds have also adapted by migrating and breeding at different times of the year.
“Many birds have been moving towards colder places as the climate gets warmer,” Baldwin said. “They've also been changing how they live — like the times at which they migrate [and] the times at which they breed.”
Baldwin said he discovered birds as a research subject while an undergraduate and is amazed by what he’s learned.
“To me, they're just really magical. … They have figured out thousands of different ways of living. They've conquered all the continents of the Earth. Their success story spans 60 million years of evolution,” he said. “And there's a lot that's still unknown. People are still describing new species to science from remote undersampled places in the world.”
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