in Chicago used a novel strategy to determine the ups and downs of U.S. airpollution levels over time: Measuring soot levels on birds kept in museums.
Soot on birds’ bellies tells a story of air pollution more than a century in the making. Researchers at the University of Chicago measured black carbon that clung to birds kept in Rust Belt museum collections and found a striking record of filthy air.
Over the course of 135 years, the dirtiness of the birds’ plumage rose and fell in line with coal legislation and followed social changes such as the switch in residential heating from coal to natural gas.
The soiled bird specimens also suggest that the air above Chicago, Pittsburgh and Detroit in the 1880s was even sootier than historians realized.
“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 in the Proceedings of the National Academy of Sciences.
Tracking environmental pollutants via collections at natural history museums has some precedent. Researchers have measured the thinness of eagle-egg shells, 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 birds were feather dusters floating above smokestacks.
DuBay and Fuldner studied five species: horned larks, redheaded 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 earlier 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.
The researchers, both graduate students, used microscope images to reveal globs of carbon stuck to the birds’ feathers. Using a photographic technique, they 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.
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 — visibly brightened the birds’ feathers. 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.”
Despite 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.
And while visible particulate matter has largely been scrubbed out in North American and European cities, DuBay cited a recent study that 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 preserved them. DuBay and Fuldner emphasized that natural-collections have untapped potential beyond these birds.
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.
“We’re studying atmospheric science,” Fuldner said, “and the birds were a pathway to that.”
Horned larks collected during the early 20th century. The birds at left were collected in Illinois, inside the U.S. manufacturing belt. The birds at right were from the Western coast, away from industry.