Mullion Island is a tiny, uninhabited island off the south coast of Cornwall in the United Kingdom, and no one is allowed to visit the island without a permit. So when thousands of multi-colored rubber bands kept showing up all over the island near the nests of gulls and other seabirds, researchers were stumped.

However, scientists may now have an answer: the birds think the bands are food, and bring them to the island to feed their young.

Members of the West Cornwall Ringing Group, a group of birders that monitors the breeding and movements of birds in the area, first noticed the bands in 2013, according to a National Trust press release. Over the years since, more and more rubber bands have accrued on the island. This year, the group decided to try and solve the mystery.

“We first noticed the bands on a monitoring visit during the breeding season and were puzzled why there were so many and how they’d got there,” says Mark Grantham, a member of West Cornwall Ringing Group. “To save disturbing the nesting birds, we made a special trip over in the autumn to clear the litter. Within just an hour we’d collected thousands of bands and handfuls of fishing waste.”

Among that waste were poo pellets with the rubber bands and fishing line embedded in them—telltale signs that the birds were mistaking the objects for food and likely trying to feed them to their nestlings. The investigators also found a gull that had died after getting caught on a large fishing hook. The Guardian’s Steven Morris reports that the group found bands that were fresh and bright among others that were old and brittle, which leads them to believe the birds have been collecting the bands for decades.

So where does all the junk on Mullion Island come from? Farms in that area of Cornwall grow flowers and use the multi-colored bands to tie together cut flowers, according to the National Trust. The gulls spend much of their time picking through these fields and likely mistake the bands for food.

Even though Mullion Island is set aside as a sanctuary and people aren’t allowed to visit, human activity is still impacting the birds that nest there. For the 70 pairs of great black-backed gulls that raise their young on the island, the nesting season was poor. Even though many people think gulls are abundant in the U.K., many species, including some common ones, are in steep decline. Great black-backed gulls have fallen by 30 percent recently in that country and herring gulls—the species best known for stealing French fries on the beach—are now a bird of conservation concern, losing 60 percent of their population since 1969, reports CNN.

“Despite being noisy and boisterous and seemingly common, gulls are on the decline,” Rachel Holder, an area ranger for the National Trust, says in the press release. “They’re already struggling with changes to fish populations and disturbance to nesting sites - and eating elastic bands and fishing waste does nothing to ease their plight. Places like Mullion Island should be sanctuaries for our seabirds, so it’s distressing to see them become victims of human activity.”

So-called “ghost fishing” gear—nets, lines and traps that have been lost or abandoned at sea—is a huge ecological problem. About 705,000 pounds of fishing gear are released into the ocean each year, according to London-based World Society for the Protection of Animals report released last year. Abandoned or lost gear can trap or entangle fish, marine mammals and turtles for years, or even decades. The problem affects birds, too. While chasing fish, birds can get ensnared in the nets, or, in the case of the Mullion Island gulls, they may ingest bits of line or get snared by loose hooks.

“The human impact on our oceans is evident even in the most remote parts of the U.K.” Chris Thorne of Greenpeace U.K. tells the Press Association. “To discover seabirds on Mullion Island picking up fishing gear having mistaken it for food is sad beyond belief. Our Government needs to take the problem of ‘ghost gear,’ discarded or abandoned fishing equipment, in U.K. waters seriously before the problem becomes even worse.”