Scientists believe that birds in North America are getting smaller because of rising temperatures associated with climate change.

A study published Wednesday in the peer-reviewedEcology Letters found not only that birds’ bodies are shrinking, but also that their legs are getting shorter and wings are getting longer.

David Willard, study co-author and collections manager emeritus at the Field Museum in Chicago, measured 70,716 North American migratory birds of 52 different species from 1978 to 2016. 

For nearly 40 years, scientists and researchers woke up at about 3:30 a.m. to collect and measure birds that crashed into the museum’s windows during the spring or fall migration period for Willard, according to the Field Museum. 

“It started as a very casual study – someone mentioned that birds sometimes run into McCormick Place, and I was curious, so I went for a walk around the building one morning,” Willard said in the museum’s press release. “I found a couple dead birds and I brought them back to the museum.” 

Willard discovered that over time the birds’ mass decreased by about 2.6% and leg length decreased by about 2.4%. He also found that their wing length increased by about 1.3%.

Scientists also measured temperature, precipitation, vegetation and other environmental factors. They found that warmer temperatures were associated with a smaller body size.

Co-author Benjamin Winger, who also studies the evolution and ecology of birds at the University of Michigan, told The Washington Post that warmer temperatures have had an “almost universal effect” of shrinking other animals.

According to the Field Museum, smaller bodies hold onto less heat and larger bodies hold onto more. The phenomenon is called Bergmann’s rule, and it helps animals stay a comfortable temperature in different environments. 

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“We had good reason to expect that increasing temperatures would lead to reductions in body size, based on previous studies,” said the study’s lead author, Brian Weeks, an assistant professor at the University of Michigan School for Environment and Sustainability. “The thing that was shocking was how consistent it was. I was incredibly surprised that all of these species are responding in such similar ways.” 

The findings were consistent with a 2017 study that researched the breeding of wild zebra finches across a range of temperatures. Scientists found that a hotter climate during breeding periods produced smaller birds.

Authors of the most recent study, however, also noted that other complications of climate change, such as food limitation, could have something to do with body size reduction. 

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