They quantified the sootiness by taking pictures of the chests of sparrows, woodpeckers, larks and towhees and measuring how well they reflected light. Black carbon isn’t very reflective — which is why it’s been a problem in the environment. It absorbs sunlight and generates heat, leading to alterations in cloud cover and precipitation or acceleration of the melting of snow and ice.

The team found that dirty feathers corresponded to historical reports and other atmospheric measurements — and they were more localized in time and space. They hope that scientists and policy makers can use the data to inform climate models and environmental policy. And the birds, once dead and void of context in museum drawers, now play a part in environmental history.

Mr. DuBay and Mr. Fuldner have more questions: Did soot affect the health of birds and other wildlife, as it has in humans? Did the dirt alter their reproductive strategies? And could birds in other industrialized nations of the past and present like England or China, also reveal hidden measures of air pollution?

“These collections are hidden treasures, and who knows what other questions we’ll be able to ask by getting into these museums and exploring,” Mr. DuBay said.