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As average summer temperatures rise, North American birds are getting smaller, according to a study of 70,716 specimens published yesterday (December 4) in Ecology Letters.

David Willard, a collections manager emeritus at the Field Museum in Chicago and a coauthor of the study, has been collecting and measuring birds that died by crashing into Chicago buildings for more than four decades. He and a team of researchers analyzed specimens collected from 1978–2016 that represent 52 common bird species, mainly sparrows, warblers, and thrushes. They found that during the past few decades, body mass decreased by 2.6 percent, leg bones shortened by 2.4 percent, and wing length increased by 1.3 percent on average.

The scientists don’t know why birds are shrinking, but they say they believe that it could be because smaller animals keep cool more easily, or because warm temperatures can stunt the growth of young birds. The finding that wings are growing longer surprised the authors, who say it could make the birds more efficient fliers, according to The Washington Post.

Other studies on alpine goats and salamanders have also showed them to be getting smaller as the climate warms, reports the BBC. “Warming temperatures seem to be having a pretty consistent and almost universal effect on a large number of different species, regardless of other aspects of their biology,” study coauthor Benjamin Winger, an ecologist at the University of Michigan, tells the Post.

Simon Griffith, an ecologist at Macquarie University in Australia who was not involved with the study, tells the Post that it is “just amazing” that Willard measured such a large number of birds himself, adding that “it reduces the noise in the data-set.”

See “North America Has 3 Billion Fewer Birds Than it Did in 1970”

Emily Makowski is an intern at The Scientist. Email her at emakowski@the-scientist.com.