What a lark: bird collections reflect air pollution

Researchers who examined a collection of dead and dirty birds hope their discoveries could provide valuable tools for scientists studying climate change.

University of Chicago graduate students Shane DuBay and Carl Fuldner this week published a paper in the Proceedings of the National Academy of Sciences, which shows that the amount of soot found on the feathers of horned larks (Eremophila alpestris) held in museum collections can be used to trace the amount of black carbon in the air over time, and the effects of environmental policy on pollution.

DuBay and Fuldner analysed more than 1000 larks collected over the past 135 years to determine and quantify the effects of airborne soot in cities in the US Rust Belt, including Detroit, Pittsburgh and Chicago.

“If you look at Chicago today, the skies are blue,” says DuBay. “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. Using museum collections, we were able to reconstruct that history.”

Soot clung to each bird’s feathers like dust to a feather duster, effectively turning them into flying air filters. 

Horned larks were ideal candidates for the study because they moult and grow a new set of feathers annually, meaning that the soot on them had only been accumulating for, at most, a single year when they were collected. 

To measure the change in sootiness through the study period, Fuldner, a photo historian, worked with DuBay to develop a method using photography to analyse the birds. Choosing the larks and four other species from the former industrial heartland of the US – all of which have lots of white feathers easily discoloured by soot – they photographed the birds and measured the light reflected off them.

The pair plotted the light bouncing off the feathers and correlated the data against the year each specimen was collected. They then looked into the social history of urban air pollution.

“The changes in the birds 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.”

DuBay adds they were surprised by the precision they were able to achieve. “The soot on the birds closely tracks the use of coal over time,” he says. “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.” 

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The study found that black carbon levels matched coal consumption until the mid-20th century. After that, soot levels started to decline even though coal use continued to go up. The decline, the study finds, was the result of improved burning efficiencies rather than emissions control.

Analysis of atmospheric black carbon might assist scientists studying global warming. 

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

DuBay notes that in addition to the environmental implications of the project, their work also shows the importance of museum collections like those they used in their study. “I hope this study exposes collections as a valuable resource to address present day environmental concerns,” he says. “This paper shows the ways that natural history collections can be used, underlining the value in collections and in continuing to build collections, to help us improve our understanding of human impacts on the natural world.”