They may have yellow chins and white bellies but a century ago, horned larks were a much darker grey because of soot and dust in the air.

Scientists say this is because, in the 1900s, many US cities like Chicago had more industrial pollution that affected the plumage of urban birds.

Two researchers from the Field Museum and the University of Chicago studied more than a thousand bird specimens collected over the last 135 years to see if the amount of black carbon in the air changed in the last century.

“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,” said study author Shane DuBay.

Horned Lark specimens.
Horned larks in Chicago were darker 100 years ago (Carl Fuldner/Shane DuBay/The University of Chicago/The Field Museum)

DuBay and fellow researcher Carl Fuldner analysed the birds’ feathers to measure the effects of soot in the air over cities in the Rust Belt, a once-powerful industrial sector stretching from the Great Lakes to the upper Midwestern States of 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.”

To measure the changes in sootiness over the years, the researchers photographed over 1,000 birds and measured the light reflected off of them.

Horned Lark specimens
The researchers looked at more than 1,000 bird species (Carl Fuldner/Shane DuBay/The University of Chicago/The Field Museum)

The images showed a remarkable contrast between the older grey, soot-covered birds and the newer clean white ones.

The researchers also looked into the history of urban air pollution.

“We were surprised by the precision we were able to achieve,” DuBay said.

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

Field Sparrow specimens.
The researchers also compared field sparrow specimens (Carl Fuldner/Shane DuBay/The University of Chicago/The Field Museum)

The researchers found that the amount of soot on the birds rebounded during the Second World War, when coal usage increased, and dropped after the war, when residents began heating their homes with natural gas rather than with coal.

But does this all mean that the air is cleaner than it was 100 years ago? DuBay doesn’t think so.

“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,” he said.

The research is published in the journal Proceedings of the National Academy of Sciences.