“We’re tracking a piece of the environment that has literally been accumulated by the specimen,” said Carl Fuldner, an art historian who specializes in photography. Fuldner and Shane DuBay, an evolutionary biologist, are authors of the new study, published Monday in the Proceedings of the National Academy of Sciences.

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It was clear to museum curators in the industrial Midwest that some bird specimens in their collection drawers were dirtier than others. Although carbon soot does not easily rub off, the birds left oily black smudges when handled with white gloves, similar to the way newspaper ink stains fingers. But until this study, no one thought to analyze the soot. (Museum records indicate that a few curators attempted to scrub dead birds clean, to limited success. Cleaned birds were excluded from this research.)

“This approach is new and old in the same way,” DuBay said.

Tracking environmental pollutants via natural history museum collections has precedent. Researchers have measured the thinness of eagle eggshells, DuBay said, as a proxy for exposure to poisons such as DDT. But what made this study different was that evidence of pollution was not secondhand — the soot itself collected on feathers, as though the songbirds were feather dusters floating above smokestacks.

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DuBay and Fuldner studied five species: horned larks, red-headed woodpeckers, field sparrows, grasshopper sparrows and Eastern towhees. All were housed at the Field Museum in Chicago, the Carnegie Museum of Natural History in Pittsburgh and the University of Michigan's Museum of Zoology in Ann Arbor. Records back to 1880 indicated the time and place the birds were collected.

These birds molt each year, replacing their feathers with a fresh set. That molt essentially wiped out the soot that had accumulated in their plumage. The dirt on the birds at the time of their death was a snapshot of that year in industrial history, the researchers said.

“I’ve never heard of anything like this. I think it’s quite clever,” said Tami Bond, an environmental engineering professor at the University of Illinois who was not involved with this research.

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The researchers met at an exhibition about early wildlife photography that Fuldner curated. They hit it off and began to brainstorm ways to use museum collections. When it came to the blackened birds, they at first had to rule out whether, perhaps, some of the birds' pigmentation had darkened in response to industrialization. In an oft-cited case of evolutionary selection at work, naturalists observed that peppered moths in Britain adapted to have black wings rather than their typical gray, to blend in with soot-darkened trees of the early 1800s.

But birds were not mutants, just dirty. Microscope images revealed globs of carbon stuck to their feathers. Using a photographic technique, the graduate students measured how poorly the birds' feathers reflected light — the less relative light a bird reflected, the dirtier it was.

The authors constructed a timeline of airborne black carbon, although they emphasized that the birds revealed only relative trends in atmospheric soot, not direct concentrations of black carbon emissions.

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The birds were dirtiest from 1880 until 1929, when the Great Depression hit. Coal consumption plummeted, only to rise again during World War II. The mid-1940s birds darkened in response. As power plants became more efficient and natural gas supplanted coal in homes, the birds lightened. A period of legislation — 1955's Air Pollution Control Act, 1963's Clean Air Act and 1970's Clean Air Act extension — held the birds to their cleanest levels. The birds from the 1980s onward are the least sooty in recent decades.

Relative dirtiness followed trends Bond and her colleagues modeled in a 2007 paper that traced black carbon emissions from 1850 to 2000 using historical records. “One of the shocking findings that we found was what we recover is very similar to these predictive models,” DuBay said. With one exception: The birds were far dirtier from 1880 to the early 1900s than Bond's carbon emission study would have suggested.

“There is a lot we don’t know about emissions during that period,” Bond said. “You can use ice cores, but of course that only works if you have glaciers.” (Researchers have estimated carbon emissions by the soot trapped in Greenland ice.) She noted that the study was constrained to urban areas, “and it would take some work to understand how that would connect to regional or global average concentrations.”

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Despite the recently collected birds' low soot and the blue skies above cities such as Chicago today, the researchers said we're not in the clear yet. Although coal is declining in the United States, globally humans are burning more coal than ever. Cities in Asia have particle pollution that the researchers likened to old Rust Belt hubs. DuBay said he has seen birds in Beijing darkened by air pollution.

Also, while visible particulate matter has largely been scrubbed out in North American and European cities, that does not necessarily mean a healthy atmosphere. DuBay cited a recent article in the Guardian that reported most Londoners were exposed to tiny air pollutants — named PM2.5 for their diameters of 2.5 micrometers or less — at concentrations 1½ times the limit suggested by the World Health Organization.

In a century of museum work, no one could have anticipated the birds would be used like this, not the naturalists who collected the animals nor the curators who made sure moths didn't eat their skin. Both DuBay and Fuldner emphasized that natural history collections have untapped potential beyond these birds.

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Some museums keep pickled bird lungs that could reveal what the birds breathed in while alive. Drawers full of dead plants or lichens could, hypothetically, contain the heavy metals that the living organisms absorbed from the soil.

“The birds are incidental,” Fuldner said. “We’re studying atmospheric science and the birds were a pathway to that.”