Migratory songbirds in Illinois are no exception to Bergmann's rule, which posits specimens representing that same species tend to be smaller in warmer climates and bigger in colder climates.

New analysis of a robust bird-size data set suggests migrating songbirds are getting smaller as climate change continues to raise global temperatures.

Since 1978, researchers at Chicago's Field Museum have been collecting birds killed in collisions with the Second City's tallest skyscrapers. It isn't the most cheerful task, but the measurements have offered scientists novel insights into the physiological changes triggered by global warming.

"I would gladly exchange this data set for a world where no birds were dying by collisions with glass and the conclusions of this paper would have to have come by another route," David E. Willard, the Field Museum ornithologist responsible for the measurement program, told UPI. "However, I am glad that rather than being swept up and discarded, the birds killed were able to give us some insight into changes."

Over the last several decades, Williard weighed and measured more than 70,000 birds comprising 52 species.

"There is nothing particularly difficult about making the actual measurements -- there is a special ruler for measuring wings and we use a digital caliper for bill length and tarsus length and a digital scale for weight," Williard said. "The main challenge is finding time to measure the 5,000 to 6,000 birds that are coming to the museum each year."

Williard did find the time, and his work helped researchers at the University of Michigan suss out a link between rising temperatures and bird shrinkage.

The data -- detailed Wednesday in the journal Ecology Letters -- revealed statistically significant declines in the body size of 49 out of 52 species over the last several decades, corresponding with a steady rise in temperatures. The numbers also showed that short fluctuations in temperature matched accelerated changes in body size. Brief respites of cooling amidst the broader warming trends were reflected in small increases in bird body size.

The data also showed wing sizes of 40 of the 52 species increased over the lengthy study period.

"Wing length is an incredibly ecologically-important trait in birds," Brian Weeks, lead study author and an assistant professor of environmental science at University of Michigan, told UPI. "Our results suggest that increasing temperatures have driven declines in size, and that these declines are linked to increases in wing length, essentially adding new complexity to our expectations of biotic responses to climate change."

Researchers aren't yet sure why birds migrating back and forth past Chicago's skyline are shrinking. Animals in colder climates evolve larger bodies because it's more efficient to retain heat, but the reasoning for the flip side of Bergmann's rule is less obvious.

"There are two possible proximate mechanisms linking climate change and body size reduction," Weeks said.

"The first is developmental plasticity, in which individuals that mature in warmer temperatures tend to develop into smaller adults," Weeks explained. "The second is natural selection, in which smaller birds tend to do better -- in survival, reproduction, or both -- in warmer temperatures, leading to a shift in the average size of individuals in a population."

Getting smaller may have its advantages, but it's possible there are pitfalls, too -- like a reduction in metabolic efficiency.

"This reduction in efficiency could make migration more difficult, and may actually be what's driving the increase in wing length, which can make flight more efficient," Weeks said. "This is an important avenue of future work."

There are few bird-related data sets as lengthy and detailed as the one maintained by Williard, and Weeks and his colleagues plan to continue mining the measurements for fresh scientific insights.

Researchers are currently examining the same data set to better understand why the wing length of so many species increased over the last several decades. Scientists are also working to measure the bones of the birds in the collection to determine whether climate change is causing skeletal changes.

In the future, Williard hopes his research will help shrink the number of dead birds being admitted to the collection.

"There are a lot of questions that can be addressed with this data set," Williard said. "One practical one is to analyze the features of the buildings that are most hazardous for birds and advise the architectural community of ways to reduce the problem."

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