Red-headed Woodpeckers from the specimen collection at The Field Museum, Chicago. Photo: Carl Fuldner and Shane DuBay

The layers of soot trapped in bird feathers has now provided a vital “air filter” time capsule of when industrial American cities had the dirtiest air to breathe.

The feathers from 1,097 birds collected in the U.S. Manufacturing Belt between 1880 and 2015 show that soot levels peaked at the cusp of the 20th century, and then were gradually reduced through reforms and legislation, writes a team from the University of Chicago in the latest Proceedings of the National Academy of Sciences.

“The precipitous drop in atmospheric black carbon at midcentury reflects policies promoting burning efficiency and fuel transitions rather than regulating emissions alone,” they write.

The birds were collected from Michigan, Wisconsin, Indiana, Illinois, Ohio, and Pennsylvania – a range spanning the industrial hubs of Chicago, Detroit, and Pittsburgh. The species included the field sparrow, the grasshopper sparrow, the eastern Towhee, the horned lark, and the red-headed woodpecker.

The birds were first assessed to make sure they had been collected during a non-molting period. The resulting specimens were analyzed with a scanning-electron microscope to determine that they were covered with soot.

They were, write the authors.

The light reflectance off those particles was then measured on the uniform white patch on the underside of the birds, using precision photography. (The data inputs were from median raw R, G/G2, and B channel sensor values, which were then compared and calculated using collection-specific regression equations.)

The findings lined up with some of the best atmospheric readings to date, specifically those from the ice cores of Greenland. But they also found the soot was present in much greater concentrations at the turn of the 20th century than had previously been believed. That could also indicate that environmental impacts were more extreme even before atmospheric readings were regularly taken (a practice which started post-World War II).

“Prevailing emission inventories underestimate black carbon levels in the United States through the first decades of the 20th century, suggesting that black carbon’s contribution to past climate forcing may also be underestimated,” they write.

Part of what changed by the mid-20th century were local regulations on pollution. For instance, Pittsburgh began subsidizing harder and low-volatile coal for domestic use in 1946. At the same time, the Steel City began to transition to different sources of energy; by 1950, some 66 percent of local households were heated with cleaner natural gas, they add.

The authors – graduate student Shane DuBay at the Committee on Evolutionary Biology, and Carl Fuldner, a graduate student in the school’s department of Art History – conclude that the birds provide a kind of air-filter time capsule.

“If you look at Chicago today, the skies are blue,” said DuBay, in a school statement. “But when you look at pictures of Beijing and Delhi, you get a sense for what U.S. cities like Chicago and Pittsburgh were once like. “When you touch these birds, you get traces of soot on your hands… These birds were acting as air filters moving through the environment.”

 “This research highlights the unexpected ways in which museum materials can yield insights about the physical and natural world and help address present-day environmental challenges,” they add in the study. “Our samples show that black carbon particulate covered the landscape along with its living inhabitants.”