A 1994 FIFA World Cup souvenir. A McDonald’s McChicken container from 1996. Decades-old plastic packaging. Look inside the nests of Eurasian coots, and you’ll find a surprising history of human plastic production. Much like tree rings hold climate records, bird nests preserve an unintentional archive — not of weather patterns, but of pollution.

A Plastic Timestamp in Birds’ Nests

Eurasian coots (Fulica atra) are small black waterbirds. They’re highly adaptable, thriving in both natural and urban environments. Their nesting habits reflect this adaptability. Auke-Florian Hiemstra, a biologist at the Naturalis Biodiversity Center, in Leiden, Netherlands, first noticed years ago that coots were incorporating plastic into their nests. In previous studies, he even documented them using artificial plants for their nests.

This time, he wanted to see whether this plastic could tell its own history of bird use.

Birds have long been known to incorporate human-made materials into their nests. But in Amsterdam, where waterbirds like coots nest among the canals, something remarkable is happening. Plastic is no longer just a supplement to their nest-building — it has become the primary construction material.

There’s something else that’s unusual about how birds use plastic. Coots, like many other birds, traditionally build new nests each breeding season. However, plastic has introduced a change in their behavior. Natural plant materials decay quickly, forcing birds to rebuild nests annually. But plastic is durable.

With nests made from artificial materials, coots have begun reusing structures from previous years — something rarely observed before. This adaptation may save them time and energy, allowing them to breed earlier and defend their territories more effectively. However, it also raises concerns: older nests could harbor parasites or attract predators.

Archives of the Anthropocene

Hiemstra and colleagues devised a careful plan to cause as little disturbance to wildlife as possible. They gathered nests in 2021, after the breeding season was over. They also carefully checked that there were no hibernating smooth newts, which are known to use waterfowl nests, and then hauled 15 nests to the lab for analysis.

The most striking discovery was a nest built on an abandoned foundation pile in Amsterdam’s Rokin canal. Over decades, it had accumulated 635 artificial items, including plastic dating back to the early 1990s. The researchers used date-stamped food wrappers and packaging to reconstruct a 30-year history of nest-building at the site.

‘The oldest layer is as old as me — all my life a bird was nesting here,’ Hiemstra says in a statement.

Another nest included layers of plastic with identifiable dates from multiple years, topped with face masks from the COVID-19 pandemic—providing an unmistakable timestamp for recent years.

These findings suggest that birds are inadvertently creating a stratigraphic record of the Anthropocene, the geological epoch characterized by significant human impact on Earth’s climate, ecosystems, and geology. Just as geologists use layers of rock to understand past epochs, scientists could now examine plastic-streaked bird nests to trace human impact over time.

Researchers suggest that these nests function like technofossils, similar to how ancient civilizations left behind pottery shards or metal tools. McDonald’s wrappers and face masks could become the defining fossils of the Anthropocene.

Resilience and warnings

Many scientists view the presence of plastic in bird nests as a troubling sign of environmental degradation. However, Hiemstra sees it as both a warning and a testament to the birds’ resilience. Urban birds, including swallows and crows, have been observed using plastic, cigarette butts, and even anti-bird spikes in their nests. Some of these materials offer advantages — plastic is strong, and chemicals in cigarette butts may reduce parasites — but they also introduce risks that birds likely don’t understand.

Coots seem to be an extreme case, providing the most concrete evidence of long-term plastic use. But this is a larger-scale issue and the odds are that many birds incorporate plastic in their nest-building.

This raises pressing questions, particularly about how the long-term use of these materials affects developing chicks and even mature individuals. For now, these urban bird nests stand as monuments to human influence — accidental archives of the Anthropocene.