Eastern Towhee Pipilo erythrophthalmus, top collected in Erie Township, Mich., 1936. Bottom collected in Ann Arbor, Mich., 2004. Image: Carl Fuldner and Shane DuBay

Horned Larks are cute songbirds with white bellies and yellow chins—at least, now they are. One hundred years ago, at the height of urban smoke pollution in the United States, their pale feathers were stained dark gray by soot in the atmosphere.

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

“The soot on these birds’ feathers allowed us to trace the amount of black carbon in the air over time,” said study 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 study co-author Carl Fuldner, a graduate student in the Department of Art History at UChicago, analyzed more than 1000 birds collected over the last 135 years to determine and quantify the effects of soot in the air over cities in the Rust Belt, including Detroit.

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

Over half of the bird specimens used in the study were provided by the University of Michigan Museum of Zoology. The others came from the Field Museum in Chicago and the Carnegie Museum of Natural History in Pittsburgh. "This study is a great example of the sometimes unexpected value of museum biodiversity collections: although museum curators had previously noted (anecdotally) that some older specimens were darker than more modern specimens, the extraordinary implications of this difference in coloration for environmental policy and regulations in the U.S was unknown until DuBay and Fuldner studied the specimens in detail,” said Benjamin Winger, assistant professor, Department of Ecology and Evolutionary Biology, U-M, and assistant curator of birds, UMMZ. 

“Museum curators are often asked to justify the value of keeping all that ‘stuff’ around, and this study is one more example of how specimens will continue to provide insights into our changing planet, sometimes in ways that were not imagined at the time of collection. The UMMZ houses over 200,000 specimens of birds from all over the world. Our international collections often garner attention for their role in describing biodiversity in remote corners of the earth; but as this study makes clear, our collections from areas close to Ann Arbor also have tremendous value, particularly when we can compare past to present."

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

The study, which analyzed specimens collected from 1880 – 2015, shows increases and decreases in black carbon deposition on bird specimens through time. This included a drop during the Great Depression, coinciding with a reduction in overall coal consumption during that time, a rebound during World War II, and a second and lasting drop postwar, when coal consumption declined as other fossil fuels gained traction.

By providing a more accurate, localized picture of historical trends in atmospheric black carbon, the results of the study yield a diverse set of implications that advance our understanding of human impacts on the physical and natural world, from assessing the impacts of black carbon on the environment to evaluating historical policies designed to clean up the air in some of the world’s smokiest cities, according to the PNAS paper.

The study also highlights the impact of environmental pollution on wildlife. How black carbon deposition on feathers has impacted signaling within and among species, for example, remains an open question.

University of Chicago press release

The story is receiving widespread media atttention including in The Washington Post, The New York Times and Science Magazine.