Since 1978, researchers have scooped up and measured tens of thousands of birds that died after crashing into buildings in Chicago during spring and fall migrations. Their work has documented what might be called the incredible shrinking bird.
A study published on Wednesday involving 70,716 birds killed from 1978 through 2016 in such collisions in the third-largest U.S. city found that their average body sizes steadily declined over that time, though their wingspans increased.
The results suggest that a warming climate is driving down the size of certain bird species in North America and perhaps around the world, the researchers said. They cited a phenomenon called Bergmann's rule, in which individuals within a species tend to be smaller in warmer regions and larger in colder regions, as reason to believe that species may become smaller over time as temperatures rise.
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