If you want to figure out how long fridge cheese has been hanging around, check its expiration date. The same, it turns out, goes for birds’ nests. For the first time, researchers have used expiration dates— printed on plastic waste incorporated into coot nests—to figure out when these structures were built. The findings, reported in Ecology, could help scientists better understand the histories of individual nesting sites and the history of the species breeding there.

“I was surprised by the sheer amount of trash in the main nest they described,” says Daniel Baldassarre, an ornithologist at the State University of New York Oswego who wasn’t involved in the study. In one case, he notes, the team was able to date a nest all the way back to 1991. “I never would have guessed that a coot nest location would last that long.”

Auke-Florian Hiemstra hit on the approach while studying the nests of common—or Eurasian—coots (Fulica atra) in Amsterdam’s city center. The birds—which weigh nearly 1 kilogram, with black plumage and white beaks—incorporate plastic debris between the twigs of their nests. Hiemstra, an urban ecologist at the Naturalis Biodiversity Center, wondered whether the trash could reveal when the nests were built.

“I’m not sure about the idea that incorporating artificial material would be beneficial to the birds,” Baldassarre says. “It seems equally, if not more likely, that the artificial materials are simply overabundant and readily available in the environment.”

As a tool for dating bird nests, Hiemstra believes he and his colleagues have just scratched the surface. There could be even older plastic-filled nests out there, he notes, and this strategy could help urban ecologists date nesting sites.