Birds are shrinking in size, and researchers believe it’s likely due to climate change

North American migratory birds have been getting smaller over the past four decades and the reason appears to be related to climate change, according to findings from a University of Michigan-led analysis of a large dataset.

The dataset included 70,716 North American migratory birds from 52 species that died when they collided with buildings in Chicago, a news article from the university said. 

Staff and volunteers at the Field Museum in Chicago have retrieved dead birds that collided with Chicago buildings during spring and fall migrations since 1978, the article said. Multiple body measurements are made for each specimen. 

“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,” the university’s article said. 


North American migratory birds have been getting smaller over the past four decades and the reason appears to be related to climate change, according to findings from a University of Michigan-led analysis of a large dataset. (Photo: Field Museum/Ben

Over the same period, the wing length increased significantly in 40 species, the dataset revealed. The wing length showed a mean increase of 1.3 percent. 

The findings were published on Wednesday 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. 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,” said study lead author Brian Weeks, an assistant professor at the U-M School for Environment and Sustainability.

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 article said. “The strong 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.”

The senior author is Benjamin Winger of the U-M Department of Ecology and Evolutionary Biology and the Museum of Zoology. Weeks worked on the project as a postdoctoral researcher in Winger’s lab. Co-authors include David E. Willard, the Field Museum ornithologist and collections manager emeritus.

“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.

The body size of the birds decreased significantly as temperatures warmed, the data analysis revealed. Temperatures at the birds’ summer breeding grounds north of Chicago increased around 1.8 degrees Fahrenheit over the course of the study. 

Studies of plant and animal response to climate change often focus on shifts in the geographical range of a species or the timing of events such as springtime flowering and migration. The consistency of the body-size declines reported in the new study suggests that such changes should be added to the list of challenges facing wildlife in a rapidly warming world, Weeks said, according to the article. 

“It’s clear that there’s a third component—changes in body size and shape—that’s probably going to interact with changes in range and changes in timing to determine how effectively a species can respond to climate change,” he said.

“Long-distance bird migration is one of the most impressive feats in the animal kingdom. The extreme energetic demands of flying thousands of miles have shaped the morphology of migrating birds—their form and structure—for efficient flight,” the article said. 

The authors of the Ecology Letters paper suggest that the body-size reductions are a response to climate warming and that increased wing length may help offset the body-mass losses, the article said. 

The researchers plan to do follow-up project that will again make use of the Field Museum dataset. They’ll look further into the mechanism behind the body size and shape changes and whether they are the result of a process called developmental plasticity, which is the ability of an individual to modify its development in response to changing environmental conditions, according to the university’s article. 

This story was reported from Los Angeles.