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Study: Birds are shrinking as the climate warms

A few of the thousands of birds in the Field Museum’s collections that collided with windows in Chicago. Photo by Ben Marks/Field Museum

North American migratory birds have been getting smaller over the past four decades, and their wings have gotten a bit longer. Both changes appear to be responses to a warming climate.

Those are the main findings from a new analysis of a dataset of more than 70,000 North American migratory birds from 52 species that died when they collided with buildings in Chicago.

Since 1978, Field Museum personnel and volunteers have retrieved dead birds that collided with Chicago buildings during spring and fall migrations. For each specimen, multiple body measurements are made.

The research team analyzed this remarkably detailed dataset to look for trends in body size and shape. The biologists found that, from 1978 through 2016, body size decreased in all 52 species, with statistically significant declines in 49 species.

Over the same period, wing length increased significantly in 40 species. The findings were published today in the journal Ecology Letters.

“We had good reason to expect that increasing temperatures would lead to reductions in body size, based on previous studies,” says study 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.”

Dave Willard, the scientist who began collecting birds that crashed into Chicago buildings and measured all the specimens used in this study, in the Field Museum’s collections. Photo by Greg Mercer/Field Museum

The study was made possible because in 1978, Dave Willard, now collections manager emeritus at the Field Museum, began picking up dead birds that had hit windows at Chicago’s McCormick Place convention center.

Over the years, Willard and volunteers with the Chicago Bird Collision Monitors collected over 100,000 birds that had hit McCormick Place and other buildings in downtown Chicago — they now make up 20 percent of the museum’s bird collections. Willard began to notice subtle changes in the birds’ measurements over time, but it wasn’t clear that statistically significant change was happening. “It’s a matter of millimeters, tenths of millimeters — it’s not something you know is happening until the analysis,” says Willard. The Ecology Letters study’s senior author, Ben Winger, began working on statistical analyses of the birds’ sizes when he was still a University of Chicago graduate student working at the Field Museum; now, he’s an assistant professor of ecology and evolutionary biology at the University of Michigan and an assistant curator at the U-M Museum of Zoology.

The team analyzed the measurements of 70,716 bird specimens representing 52 species; they found that the sizes of all of these species declined between 1978 and 2016. The birds’ body masses, lower leg bone lengths, and overall body sizes all went down — the leg bone length decreased by 2.4% across species. Meanwhile, the birds’ wingspans increased by 1.3%. The researchers suspect that these changes in body size are related to climate change.

Consistent large-scale response to warming

The new study is the largest specimen-based analysis of body-size responses to recent warming, and it shows the most consistent large-scale responses for a diverse group of birds, Weeks said.

Several lines of evidence suggest a causal relationship between warming temperatures and the observed declines in avian body size, according to the researchers. The strongest evidence is that — embedded within the long-term trends of declining body size and increasing temperature — there are numerous short-term fluctuations in body size and temperature that appear to be synchronized.

“Periods of rapid warming are followed really closely by periods of decline in body size, and vice versa,” Weeks said. “Being able to show that kind of detail in a morphological study is unique to our paper, as far as I know, and it’s entirely due to the quality of the dataset that David Willard generated.”

“It’s really been a herculean effort on the part of Dave and others at the Field Museum, including co-author Mary Hennen, to get such valuable data from birds that might otherwise have been discarded after they died from building collisions,” Winger said.

Thanks to the University of Michigan and the Field Museum for providing this news.

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