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The dirty feathers of birds in some museum collections are giving scientists clues as to the level of carbon in the air over the years.

Horned larks are songbirds with white bellies and yellow chins—at least, now they are. But 100 years ago, at the height of urban smoke pollution in the United States, soot in the atmosphere stained their pale feathers dark gray.

A new paper in the Proceedings of the National Academy of Sciences shows that the discoloration of these birds can be used to trace the amount of black carbon in the air over time and measure the effects of environmental policy on pollution.

“…when you look at pictures of Beijing and Delhi, you get a sense for what US cities like Chicago and Pittsburgh were once like.”

“The soot on these birds’ feathers allowed us to trace the amount of black carbon in the air over time,” says author Shane DuBay, a graduate student in the Committee on Evolutionary Biology at the University of Chicago and The Field Museum. “We found that the air at the turn of the century was even more polluted than scientists previously thought.”

DuBay and coauthor Carl Fuldner, a graduate student in the art history department, analyzed more than a thousand birds collected over the last 135 years to determine and quantify the effects of soot in the air over cities in the Rust Belt.

“If you look at Chicago today, the skies are blue. But when you look at pictures of Beijing and Delhi, you get a sense for what US cities like Chicago and Pittsburgh were once like,” DuBay says. “Using museum collections, we were able to reconstruct that history.”

Flying air filters

Ornithologists at The Field Museum have long known that bird specimens in the collection from the early 1900s were visibly darker than expected, and atmospheric soot was the suspected cause.

During the Great Depression, there’s a sharp drop in black carbon on the birds.

“When you touch these birds, you get traces of soot on your hands. We’d wear white gloves while handling them, and the gloves would come away stained, like when you get ink on your fingertips reading a newspaper,” DuBay says, because the soot in the air clung to the birds like dust to a feather duster. “These birds were acting as air filters moving through the environment.”

Birds were also ideal candidates for the study because they molted and grew new sets of feathers every year, meaning that the soot on them had been accumulating only for the past year when they were collected. And there was an apparent trend: old birds were dirtier, and new birds were cleaner.

To measure the changes in sootiness over the years, researchers turned to a new approach: photographing birds and measuring the light reflected off of them. Fuldner, a photo historian who focuses on images of the environment, worked with DuBay to develop a method for analyzing the birds using photography.

Researchers photographed more than 1,000 birds for the study from all five species that bred in the Manufacturing Belt. They all have lots of white feathers that show off soot. The images, depicting the contrast between the soiled gray birds and the clean white ones, are dramatic.

“The photographs give the project a visceral dimension—you make a connection to the images,” Fuldner says.

“The fact that the more recent birds are cleaner doesn’t mean we’re in the clear.”

The researchers plotted the amount of light bouncing off the feathers according to the year the birds were collected. To make sense of their findings, they then delved into the social history of urban air pollution.

“The changes in the birds reflect efforts, first at the city level but eventually growing into a national movement, to address the smoke problem,” Fuldner says. “We are actually able to go back and see how effective certain policy approaches were.”

“We were surprised by the precision we were able to achieve,” DuBay says. “The soot on the birds closely tracks the use of coal over time. During the Great Depression, there’s a sharp drop in black carbon on the birds because coal consumption dropped—once we saw that, it clicked.”

The amount of soot on the birds rebounded around World War II, when wartime manufacturing drove up coal use. Then it dropped off quickly after the war, around when people in the Rust Belt began heating their homes with natural gas piped in from the West rather than with coal.

Less visible pollution

“The fact that the more recent birds are cleaner doesn’t mean we’re in the clear,” DuBay says. “While the US 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. Plus, many people around the world still experience soot-choked air in their cities.”

Analysis of atmospheric black carbon might help scientists who study climate change. “We know black carbon is a powerful agent of climate change, and at the turn of the century, black carbon levels were worse than previously thought,” DuBay says.

“I hope that these results will help climate and atmospheric scientists better understand the effects of black carbon on climate.”

“As a historian, one of the questions I always ask is, ‘What is the point of this research to the way we live now?’ In this case the answer quickly became clear,” Fuldner says. “Filling in a blank space in the historical record of something as large as air pollution in American cities, and being able to share that with atmospheric scientists who study the effects of black carbon on the climate, is extraordinary.”

“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 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.”

Researchers used collections from The Field Museum in Chicago, the Carnegie Museum of Natural History in Pittsburgh, and the University of Michigan Museum of Zoology in Ann Arbor in the work.

Source: University of Chicago