Soot-covered, 100-year-old birds reveal black carbon level

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Horned larks are small songbirds with white bellies and yellow chins. 

But one hundred years ago, at the peak of urban smoke pollution in the US, their pale feathers were stained dark gray by the soot in the atmosphere. 

A new study has shown that the discolored feathers of these old bird specimens can be used to trace the amount of black carbon in the air over time, along with the effects policy has had on pollution.

Researchers found that the air in the early 1900s was even more polluted than thought, and old birds were dirtier, while new birds were cleaner. 

Ten Horned Larks at The Field Museum. The five specimens on the left were collected in Illinois, inside the US Rust Belt. The five specimens on the right were collected along the western coast of North America. All 10 specimens were collected between 1903 and 1922

Ten Horned Larks at The Field Museum. The five specimens on the left were collected in Illinois, inside the US Rust Belt. The five specimens on the right were collected along the western coast of North America. All 10 specimens were collected between 1903 and 1922

Ten Horned Larks at The Field Museum. The five specimens on the left were collected in Illinois, inside the US Rust Belt. The five specimens on the right were collected along the western coast of North America. All 10 specimens were collected between 1903 and 1922

HOW THEY DID IT  

A new study has shown that the discolored feathers of old bird specimens can be used to trace the amount of black carbon in the air over time and what effects policy has had on pollution.

Researchers based at The Field Museum of Natural History in Chicago and The University of Chicago analyzed over 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, a regions from the Great Lakes to the upper Midwestern States, including states such as Pennsylvania, West Virginia, Ohio, and Indiana.

To measure the changes in the sootiness over the years, the researchers used a new method: they photographed the birds and measured the light reflected off of them. 

The birds photographed were all from five species that breed in the Rust Belt and have many white feathers, easily showing soot.

They plotted the amount of light bouncing off the bird’s feathers according to the year the birds were collected. 

Then, to understand their findings, the researchers studied the social history of urban air pollution. 

They found that during the Great Depression, there was a sharp drop in black carbon on the birds because coal consumption dropped. 

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

The research, published in the journal Proceedings of the National Academy of Sciences, involved analyzing the feathers of bird specimens at The Field Museum of Natural History in Chicago.

‘The soot on these birds’ feathers allowed us to trace the amount of black carbon in the air over time, and we found that the air at the turn of the century was even more polluted than scientists previously thought,’ says Shane DuBay, a graduate student at The Field Museum and the University of Chicago and one of the authors of the study.

He and co-author Carl Fuldner, also a graduate student at UChicago, analyzed over 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, a regions from the Great Lakes to the upper Midwestern States, including states such as Pennsylvania, West Virginia, Ohio, and Indiana. 

‘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.’ 

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

‘When you touch these birds, you get traces of soot on your hands,’ says DuBay. 

‘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.

‘That’s because the soot in the air clung to the birds’ feathers like dust to a feather duster. 

‘These birds were acting as air filters moving through the environment.’ 

According to the researchers, birds were ideal candidates for the study because they molt and grow a new set of feathers every year – meaning that the soot on them had only been accumulating for the past year when they were collected.  

To measure the changes in the sootiness over the years, the researchers used a new method: they photographed the birds and measured the light reflected off of them. 

The birds photographed, numbering over a thousand, were all from five species that breed in the Rust Belt and have many white feathers, easily showing soot. 

The photos the researchers took show the striking contrast between the gray, soot-covered birds and the clean white ones.  

Carl Fuldner, a photo historian who focuses on images of the environment, worked with DuBay to develop a method for analyzing the photos. 

They plotted the amount of light bouncing off the bird’s feathers according to the year the birds were collected. 

Old and young specimens. (A) Grasshopper Sparrows from 1907 (Upper) and 1996 (Lower) (B) Horned Larks from 1904 (Upper) and 1966 (Lower). (C) Eastern Towhees from 1906 (Upper) and 2012 (Lower). (D) Red-headed Woodpeckers from 1901 (Upper) and 1982 (Lower)

Old and young specimens. (A) Grasshopper Sparrows from 1907 (Upper) and 1996 (Lower) (B) Horned Larks from 1904 (Upper) and 1966 (Lower). (C) Eastern Towhees from 1906
(Upper) and 2012 (Lower). (D) Red-headed Woodpeckers from 1901 (Upper) and 1982 (Lower)

Old and young specimens. (A) Grasshopper Sparrows from 1907 (Upper) and 1996 (Lower) (B) Horned Larks from 1904 (Upper) and 1966 (Lower). (C) Eastern Towhees from 1906 (Upper) and 2012 (Lower). (D) Red-headed Woodpeckers from 1901 (Upper) and 1982 (Lower)

Then, to understand their findings, the researchers studied 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,’ says Fuldner. 

‘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,’ says DuBay. 

‘The soot on the birds closely tracks the use of coal over time. 

Soot in the air clung to the birds’ feathers like dust to a feather duster – Shane DuBay, graduate student at UChicago

‘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.’

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

‘The fact that the more recent birds are cleaner doesn’t mean we’re in the clear.’ DuBay said. 

‘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.’ 

Horned larks are small songbirds with white bellies and yellow chins. But one hundred years ago, at the peak of urban smoke pollution in the US, their pale feathers were stained dark gray by the soot in the atmosphere. Pictured is a clean horned lark 

Horned larks are small songbirds with white bellies and yellow chins. But one hundred years ago, at the peak of urban smoke pollution in the US, their pale feathers were stained dark gray by the soot in the atmosphere. Pictured is a clean horned lark 

Horned larks are small songbirds with white bellies and yellow chins. But one hundred years ago, at the peak of urban smoke pollution in the US, their pale feathers were stained dark gray by the soot in the atmosphere. Pictured is a clean horned lark 

Analyzing atmospheric black carbon may 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,’ says DuBay. 

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

Both DuBay and Fuldner said that being able to apply their research beyond their respective fields of evolutionary biology and photographic history was unexpected and rewarding. 

Scanning Electron Microscope micrographs, taken at different magnifications, from Field Sparrows. A¿D are from a soiled 1906 specimen, E¿H are from a clean 1996 bird

Scanning Electron Microscope micrographs, taken at different magnifications, from Field Sparrows. A¿D are from a soiled 1906 specimen, E¿H are from a clean 1996 bird

Scanning Electron Microscope micrographs, taken at different magnifications, from Field Sparrows. A–D are from a soiled 1906 specimen, E–H are from a clean 1996 bird

‘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,’ says Fuldner.

‘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,’ says DuBay. 

‘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.’ 

DuBay said that in addition to the environmental implications of the project, their work also shows the importance of museum collections like those they used 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. 

‘I hope this study exposes collections as a valuable resource to address present day environmental concerns,’ says DuBay. 

‘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.’

Images of the dorsal side of specimens from.These images show that even soiling appears over the entire bird, indicating that the soiled birds acquired black carbon from the environment while alive. Upper birds are older, soiled specimens, lower birds are recent, clean specimens

Images of the dorsal side of specimens from.These images show that even soiling appears over the entire bird, indicating that the soiled birds acquired black carbon from the environment while alive. Upper birds are older, soiled specimens, lower birds are r
ecent, clean specimens

Images of the dorsal side of specimens from.These images show that even soiling appears over the entire bird, indicating that the soiled birds acquired black carbon from the environment while alive. Upper birds are older, soiled specimens, lower birds are recent, clean specimens