(CNN) For several years, conservationists have been puzzled by thousands of brightly colored rubber bands showing up on an uninhabited island off the UK's southwestern tip.

Their new theory is that sea birds have been mistaking the bands for worms, according to UK conservation charity the National Trust and the West Cornwall Ringing Group, which has been monitoring bird colonies on the island since 2013.

Scavenging for food, the birds have been scooping them up in their beaks, eating them, and then regurgitating them on the uninhabited Mullion Island, about a kilometer (just over half a mile) off the Cornish coast.

The little rocky outpost is a sanctuary for a number of bird species with vulnerable populations, including great black-backed and herring gulls, as well as cormorants and shags.

Elastic bands and fishing waste collected from Mullion Island.

Experts think they've solved the mystery

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