WASHINGTON (AP) — Scientists say more than 1,000 stuffed birds from Midwestern museums are helping them better understand a key global warming particle.

Researchers reported Monday that they found more soot on birds in museums in Chicago, Detroit and Pittsburgh than they expected. They also found more soot on the birds from the 1900s and 1910s than they did decades later, when people turned away from using coal to heat their homes.

Soot, also called back carbon, is important to climate change because it helps trap heat. But it has been difficult for scientists to study how levels of it have changed over time because it doesn’t last long in the atmosphere.

The study is in Monday’s Proceedings of the National Academy of Sciences.