Soot-Covered Bird Specimens Hightlight a Century of Dirty Air From Coal Burning

Horned larks from the Field Museum, with gray birds from the turn of the century and cleaner birds from more recent years when there was less soot in the atmosphere

Ornithologists at the Field Museum have long noticed that many bird specimens in the collections there are visibly darker than expected. Little songbirds known as horned larks, for example, normally have white bellies with yellow chins, but the remains of several of these birds at the museum are instead mostly dingy grey.

Shane DuBay, a graduate student at the University of Chicago, said that he, Carl Fuldner, and several other scientists even started to notice that the gloves they wear after handling the birds are stained, “like when you get ink on your fingertips reading a newspaper.”

“We decided to focus on the soot because it was so conspicuous, and visually striking,” Fuldner told Seeker, explaining that the material is not just basic dust and dirt, but instead is a dark oily substance mostly consisting of black carbon, which is produced when fossil fuels or Earth’s biomass are burnt.

Combining their shared interests in wildlife photography and conservation, he and DuBay photographed over 1,300 bird specimens from not only Chicago’s Field Museum, but also the Carnegie Museum of Natural History in Pittsburgh, and the University of Michigan Museum of Zoology in Ann Arbor. The birds, all collected within the Rust Belt, date from about 1880 to the present and represent five species. Aside from the horned lark, these included the field sparrow, grasshopper sparrow, Eastern towhee, and red-headed woodpecker.

“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 explained to Seeker.

Field sparrows from the Field Museum's collections, with gray birds from the turn of the century and cleaner birds from more recent years when there was less soot in the atmosphere

Reflectance was important to the researchers as they photographed the birds because they measured the light that was reflected off of the specimens. Black carbon, one of the major components of soot, is a light-absorbing compound, so the images precisely revealed how dirty — or not — the birds tended to be during particular times.

The scientists charted the information, then compared the observed trends to the history of coal burning in the areas where the birds were collected. The findings, reported Oct. 9 in the journal Proceedings of the National Academy of Sciences, reveal that the soot on the birds closely tracks the usage of coal in the US.

The bird images suggest that there was a slight improvement in air quality after 1910. Fuldner said this was “because the reform movements in American cities to combat the problem were starting to mature.”

He shared that the first municipal anti-smoke ordinances, which date to the early 1880s, were mostly symbolic pieces of legislation. They typically exempted domestic chimneys, for example, and were rarely enforced in practice.

“Starting in the 1910s, we start to see a shift in approach, which emphasized education — teaching more efficient burning techniques, for example, rather than simply prosecuting parties who violated a local ordinance,” he said. “Combine that with some incremental technology improvements, such as mechanical stokers for steam boilers, and you start to see modest improvements.”

The next observed soot reduction happened during the Great Depression (1929–1939), when the burning of coal plummeted across the US. Levels of black carbon on the birds then began to sharply rise during World War II (1939–1945), when wartime manufacturing drove up coal use.

The levels then dropped again after the war, when people in the Rust Belt began to heat their homes with natural gas piped in from the West. Before then, residential furnaces, such as coal-burning stoves, were estimated to produce approximately half of all atmospheric black carbon in the US.

As for how the birds often accumulated soot over the decades, the researchers suspect that it came from both particulate matter in the air and from direct contact with the material in the birds’ environments. The bird specimens included in the study came from numerous types of locations, including those near coal-burning factories like steel mills, residential urban areas, and rural sites.

Red-headed woodpeckers from the Field Museum's collections, with gray birds from periods when there was more coal-associated soot in the atmosphere

Birds like to wash themselves in water, which prior research has determined helps not only to keep them clean and primed for flight, but also promotes positive feelings of relaxation. It might at first then seem surprising that so many of the birds were found to be soiled, but the oily soot can be very difficult to clean off, even by humans wielding scrub brushes and soap.

“You see evidence of this in the early anti-smoke efforts starting in the late-19th century, many of which were led by women's reform groups,” Fuldner said. “Soot covered everything in the city — buildings and trees, but also textiles, clothes, and drapes and the like — and it was a nuisance not only because it was unsightly, but also because it was quite difficult to wash.”

DuBay added that health data on humans strongly suggests that the soot-filled environments increased both cases of respiratory illnesses and mortality in cities like Chicago and Pittsburgh.

“I think it's really hard to imagine how bad the problem was in these cities,” Fuldner said. “There are anecdotes from Chicago where, on some days, the smoke was so bad that people couldn't see across to the other side Michigan Avenue. That type of account may actually involve less hyperbole than we would be inclined to believe.”

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