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 US 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.
The study focused on 52 species, mostly songbirds that breed in cold regions of North America and spend their winters in locations south of Chicago. The researchers measured and weighed a parade of birds that crashed into building windows and went splat onto the ground.
Over the four decades, body size decreased in all 52 species: the average body mass fell by 2.6 percent, and the leg bone length dropped by 2.4 percent.