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Every year, David Willard, collections manager emeritus at the Field Museum, measures the lifeless bodies of birds that die from building collisions in Chicago. Since 1978, this has been his routine every spring and fall, when millions of migrating birds pass through the city to reach their seasonal homes. All told, he’s gathered some 70,000 avian individuals from 52 species.

In a published today in Ecology Letters, researchers from the University of Michigan and the Field Museum put to use Willard’s 40 years of data and found that North American migratory birds have been shrinking throughout the decades, likely a result of the warming climate. As their bodies have gotten smaller, most of the species have also developed longer wings. 

“People have hypothesized that as the planet warms, we may see a temporal pattern of decreases in body size,” says lead author Brian Weeks. “We found that there were remarkably consistent declines over all 52 species, despite them being pretty different.” 

The researchers’ initial interest in studying avian body sizes came from the concept behind Bergmann’s Rule, which says birds and mammals in cold regions tend to be larger than individuals of the same species in warm regions. The reasoning is that a bigger body is more adept at holding in heat, while a smaller body can release heat more easily.

By using Willard’s decades of morphological data and temperature records from NASA, the researchers identified a significant relationship between birds’ body sizes and mean summer temperature on their breeding grounds. As temperatures increased, all of the species in the study became smaller, with, on average, a 2.4 percent reduction in tarsus length, a leg bone that acts as a body-size indicator.

The researchers also compared short-term fluctuations to corroborate the connection, finding that changes in body size consistently followed changes in temperature from the previous year. “As you can imagine, just because body size has declined over 40 years and temperature has increased, that doesn’t really suggest that there’s a causal link because lots of things have changed in the past 40 years,” Weeks says. “By looking at short-term fluctuations, we’re much better qualified to establish that connection.”

The reason behind the increase in wing lengths, which grew by an average of 1.3 percent in 40 species, is less clear, but the researchers believe it may be a way the birds are compensating for the smaller body sizes to continue completing an efficient migration, says Willard, who is also a co-author of the study. 

These changes in morphology could be an additional aspect to consider in how birds will respond to climate change, says senior author Ben Winger. “This adds to the body of evidence about the complexity of climate change’s impact on animals,” he says. “A lot of studies have focused on whether or not birds and other animals will shift their geographic ranges in response to climate change, but the degree to which their body size changes due to warming temperatures could potentially interact with these range shifts.”

For example, ���Ϲ���’s Survival by Degrees: 389 Bird Species on the Brink science report considers the shifts in birds’ migration range and timing with different degrees of warming, but as Weeks points out, perhaps shifts in morphology may be another aspect that scientists need to start taking into account. 

As these questions become increasingly pertinent, long-term data like Willard’s measurements will also become more necessary. For Weeks, the success of this study demonstrates the important yet underappreciated role that museum collections have in the science community. 

“Answering questions of global change requires a different type of data than what scientists are used to working with, and we’re increasingly relying on natural history museums to be those data sources,” he says. “I don’t think many people think of them as research institutions, but they contribute a lot, especially in avian research.”

A birder since he was 7 years old, Willard says he “wasn’t even thinking about climate change back in the late '70s” when he first started collecting the dead birds. After hearing that McCormick Place, North America’s largest convention center, was notable for bird collisions, he wandered around it one day before work and found a few casualties. Soon after, he was going back every spring and fall to monitor the situation and add dead birds to his growing museum collection. Volunteers from the Chicago Bird Collision Monitors also contributed annually to the vast dataset by sending him casualties of building collisions from all around downtown Chicago.

“These days we find almost 500 a year at McCormick Place, which is way, way down from what it historically was,” Willard says. “Back then we had as many as 2,000 a year.”

The convention center has achieved that reduction by becoming a participant of the Chicago ���Ϲ��� Society's program and taking preventative actions like turning lights off at night, but Willard says they are still working on getting the numbers down. 

“That’s one of the beauties of this study,” Weeks says. “It’s based on specimen opportunistically collected that would otherwise be thrown away by city authorities, and the data has also been used to prevent future collisions and morphed into conservation activism in Chicago.”