Specimens from 1904 and 1966 look almost like different species of songbird. Courtesy Carl Fuldner and Shane DuBay, The University of Chicago and The Field Museum

How do you work out changes in air pollution over a century? You can’t get air samples from the past; buildings get cleaned up; historical data may be inaccurate. Scientists at the Field Museum and the University of Chicago have developed a novel approach: looking at the soot on the feathers of birds in museum collections. Where modern-day songbirds have dazzling white bellies, many specimens from the turn of the 20th century are blackened with soot. When birds fly through sooty, smoky air, black carbon dust collects in their wings and feathers. This, in time, stains them a deep gray.

“We went into natural history collections and saw that birds from 100 years ago that were soiled, they were covered in soot,” researcher Shane DuBay told the BBC. “We saw that birds from the present were cleaner and we knew that at some point through time the birds cleaned up—when we did our first pass of analysis using reflectance we were like, ‘wow, we have some incredible precision.’”

In their analysis of over 1,000 birds, DuBay and his study co-author Carl Fuldner were able to measure and quantify just how sooty the air of Rust Belt cities has been over the last 135 years. Black carbon levels, the birds revealed, hit their apex in the first ten years of the 20th century.

Measuring the soot levels turned out to be astoundingly accurate. On the one hand, birds molt and grow a new set of feathers every twelve months or so. That makes it easier to date soot levels to a particular year. On top of that, the degree of the color’s darkness is a clear indicator of how much soot there was in that year. To measure that precisely, DuBay and Fuldner photographed the birds and measured how much light reflected off them.

Soot levels on the birds closely matched what researchers already knew, or assumed, about coal use over time. During the Great Depression, for instance, coal consumption dropped. Birds, correspondingly, got a bit snowier. Throughout World War II, wartime manufacturing meant coal use soared, and the little songbirds once again became grayer and grayer. Then, in the 1950s, natural gas replaced coal to heat Rust Belt homes. Birds started to clean up once again.

Air pollution today might not dirty birds in the same way, but that doesn’t mean that all is well. “While the U.S. releases far less black carbon into the atmosphere than we used to, we continue to pump less-conspicuous pollutants into our atmosphere—those pollutants just aren’t as visible as soot,” DuBay said in a press release.