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Plastic trash in bird nests documents the Anthropocene epoch

What if expiration dates could tell us more than when something goes bad? Scientists have found that dates on plastic food and beverage packaging can serve as markers of the Anthropocene, a period in Earth's history marked by the widespread impact of human activities on nature.

The Eurasian coot, a round and black waterbird with a white beak, is a common sight in the Netherlands, along the canals of Amsterdam. In the wild, coots usually avoid reusing their nests, as their preferred building materials are typically fast-decaying plant matter.

The urban population, however, have started supplementing their nesting material with something much more long-lasting—plastic trash created by humans. Since plastic never truly disappears, every bit of old nesting material remains as the birds stack new layers of material, one breeding season after another.

In published in Ecology, a team of researchers from the Netherlands revealed that plastic waste in bird nests can serve as a time capsule, allowing biologists to determine when the nests were built by examining expiration dates on the plastic food packaging. In one case, the team traced nest materials back to 1991.

Scientists have previously used expiration dates to track seafloor litter and piece together extreme flood events from the Anthropocene. Building on this idea, the researchers of this study collected abandoned common coot nests from central Amsterdam on September 22, 2021, after the breeding season ended.

Each nest was then deconstructed and its contents were divided into piles of natural (twigs) and artificial (near-complete packaging) materials. Each artificial item was then carefully examined for manufacturing dates, expiration dates, or any other markings that could reveal its age. The recovered packaging ranged from items like milk and avocados to chocolate packets and fast food wrappers dating back to 1996.

The researchers discovered that two of the collected coot nests had very distinct layers of plastic, making them ideal for stratigraphy—study and interpretation of the layers. One of the nests, which the scientists named "The Rokin Nest," contained plastic waste that was over three decades old.

Nestled at the base of the nest was a candy bar wrapper promoting the 1994 FIFA World Cup while the upper more recent layers hosted discarded face masks from the COVID-19 pandemic. This process of layered accumulation of contemporary human waste is also known as technostratigraphy.

Based on the stratigraphy results and tracking of nesting activity via analysis of archived Google Street View images, the researchers arrived at the conclusion that the Rokin Nest must have been home to at least three generations of coots, as their lifespan is somewhere between 5 to 10 years.

Plastic waste has enabled coots to reuse their nests, giving them more time to forage for food and defend their territory, but this luxury comes at a cost. The researchers noted that old nesting material can be host to harmful parasites like red mites and too much plastic in the nest increases the risk of entanglement for the birds, sometimes resulting in death.