Sooty Bird Feathers Reveal a Century of Coal Emissions History

A story of pollution hides in the grime of museums’ birds specimens

Dirty Birds
Older, soot-covered horned larks on the left and cleaner specimens on the right Carl Fuldner and Shane DuBay, The University of Chicago and The Field Museum

As the Industrialization Revolution swept through the 18th and 19th centuries, so did the emergence of black carbon. Belched from factories, car tailpipes and more, these tiny bits of carbon come from the incomplete combustion of fossil fuels, including diesel and coal. And though they are small, these particles are a big problem for the health of both humans and the environment alike.

It's hard to say, however, how much has spewed into the skies over the years. But by better understanding the history of black carbon—a powerful driver of climate change—scientists hope to better understand how our climate could change in the future. So aMatt McGrath at the BBC reports, scientists turned to an unlikely marker to refine their records: bird feathers.

Over the years curators have noticed that some bird specimens were noticeably more dirty than others. Some even left black smudges when handled, reports Ben Guarino at The Washington Post. But until now, no one realized how useful this soot could be.

A pair of graduate students from University of Chicago spotted the potential of the black smudges. Because birds molt their feathers each year, the amount of soot on each bird would be a snapshot of the black carbon in the atmosphere for the year it was collected.

The duo—Shane DuBay, evolutionary biologist, and Carl Fuldner, an art historian—scoured museum collections at the University of Michigan in Ann Arbor, the Carnegie Museum of Natural History in Pittsburgh, and Field Museum in Chicago. In total, the pair photographed over 1,300 specimens collected over 135 years that span five species of birds, including horned larks, red-headed woodpeckers, field sparrows, grasshopper sparrows and Eastern towhees​.

“We chose those species because they all breed in the U.S. Manufacturing Belt, they are common enough in museum collections to provide a large sample size, and they have light, uniform breast and belly coloration, which maximizes the signal strength when tracking black carbon deposition on feathers as a function of reflectance,” DuBay tells Jen Viegas at Seeker.

The researchers then calculated the amount of light reflected off each bird and plotted that over time to get a relative measure of how soot increased and decreased in the atmosphere over the last century. They published their results in The Proceedings of the National Academy of Sciences. 

As Guarino reports, the dirtiness of the birds mirrored what we know about the rise and fall of fossil fuel use through the late 19th and 20th centuries. The dirtiest birds came from 1880 to 1929. But when the Great Depression struck, coal use plummeted in the United States and the birds' coloring lightened.

The soot kicked up again during World War II as the United States increased manufacturing for the war effort. But during the last half of the 20th century, Guarino reports, as new pollution control laws were passed—Air Pollution Control Act of 1955, Clean Air Act of 1963 and the Clean Air Act extension of 1970—the birds became gradually cleaner.

The most striking result is how dirty the oldest birds of the study were—far more than models have proposed for the late 1800's. “The big finding and implication of our study is that we are recovering relative concentrations of atmospheric black carbon that are higher than previously estimated from other methods,” DuBay tells McGrath. “It helps constrain and inform how we understand the relative role of black carbon in past climate and by understanding that we can more accurately model future climate scenarios.”

DuBay also notes that while the study shows the birds—and air—grow less sooty over the years, it doesn’t mean air quality problems are solved. Many microscopic particles that make it in the air do not discolor birds or buildings but can cause similar health problems as soot.

Even so, the gradually whitening birds suggests that there are solutions to air quality problems. “This study shows a tipping point when we moved away from burning dirty coal, and today, we're at a similar pivotal moment with fossil fuels,” DuBay says in the press release. “In the middle of the 20th century, we made an investment in infrastructure and regulated fuel sources—hopefully, we can take that lesson and make a similar transition now to more sustainable, renewable energy sources that are more efficient and less harmful to our environment.”

As McGrath reports, the researchers would like to continue the study and look at bird specimens from the United Kingdom, which has a much longer history of industrialization and a long tradition of natural history collection.

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