Most of the plastic that city vultures eat comes directly from food outlets

Since the 1950s, humanity has produced an estimated 8.3 billion tons of plastic, adding another 380 million tons to that amount each year. Only 9% of this is recycled. The inevitable conclusion is that plastic is everywhere, from the depths of the oceans to the summit of Everest – and is known to be inside the tissues of humans and other organisms.

The long-term effects of ingested plastic on people are not yet known. But in rodents, ingested microplastics can impair the function of the liver, intestines, reproductive organs, and excretory secretions.

Scavenging birds are at particular risk from ingesting plastic. New World vultures, for example, regularly forage in landfills, and have been noted for their leisurely selection of synthetic materials such as boat seats, rubber seals, and roofing.

Now, researchers from the US have shown that the amount of plastic eaten by black vultures and turkeys (Coragyps atratus and Cathartes aura) can be predicted from their location on suburban and suburban maps. This is not just a distinction between country birds versus city birds. The amount ingested depends on the local intensity of human trade within the urban landscape. These results have been published in Frontiers in ecology and evolution.

“Here we show that black vultures and turkey vultures in areas with more urban development and higher density of commercial food providers ingest more plastic,” said Hannah Partridge, a doctoral student in the Department of Geography and Earth Sciences at the University of North Carolina. In Charlotte, first author of the study.

“It is possible that they are eating some of this plastic on purpose and not just by accident, as is commonly thought.”

Eagle pellets record plastic consumption

In 2021 and 2022, Partridge et al. studied eight communal roosts shared between black eagles and turkeys across the Charlotte metropolitan area (human population 2.8 million and still growing). Perches usually host between 20 and 500 vultures. Under the perches, they collected a total of 1,087 grains of undigested material regurgitated by the vultures.

Of these pellets, 60% were found to contain plastic, making up 2.7% of the total mass on average. Other components included vegetation, dirt, rocks, animal remains, minerals, textiles, paper, wood, and glass. The authors were able to identify the types of plastics using Fourier transform infrared (FTIR) spectroscopy. The most common are silicone rubber (7.5% of the analyzed samples), high-density polyethylene (7.0%), polyethylene (6.4%), and biosilicate polyethylene (5.3%).

Next, the researchers looked for correlations between the amount of plastic within the pellets and four measures of human growth at increasing distances — from 400 meters to 20 kilometers when the eagle flies — from the perch. These were the density of commercial food providers (from convenience stores and food trucks to supermarkets and restaurants), the density of livestock and game producers, the amount of land developed, and the distance to the nearest landfill.

Food stores and restaurants

Exploratory statistical analyzes showed that the mass fraction of granules constituting plastic increased strongly with increasing land cover in urban areas and increasing the density of food providers within 20 km. From these results and direct observations, the authors concluded that black vultures particularly in the metropolitan Charlotte area may ingest plastic items directly in the garbage containers of food providers.

Partridge noted that “black vultures often roost overnight on a transmission tower next to a fast food restaurant and fly straight to the dumpster early in the morning”. “Turkey vultures do this more often; they prefer more rural areas and natural food sources.”

Curious birds

But are vultures eating all this plastic on purpose or by accident? The researchers hypothesized that vultures might commonly mistake plastic for nutritious bone fragments, which they usually get from carrion.

“Vultures are curious and are always looking for new sources of food, so they may swallow plastic thinking it is food,” Partridge said. “But they may sometimes ingest the plastic on purpose, to accumulate large amounts to help regurgitate indigestible parts of carrion such as hair.”

So what can we do to stop vultures and other vulnerable animals from eating plastic?

“Food providers such as restaurants and grocery stores can ensure that their rubbish is properly packed, that rubbish makes it to the bin, and that rubbish is closed and secured. We can also work to ban single-use plastics to protect vultures and other species from harm,” advised Dr. Sarah Gagner Assistant Professor in the same department.