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Feral Cats Prey upon Near-threatened Birds on Tokyo Island

Courtesy of the Forest Research and Management Organization
A streaked shearwater

Feral cats are attacking streaked shearwaters, a near-threatened species that breeds on Mikurajima island in the Izu Islands.

A research team from the Forest Research and Management Organization and other groups has estimated that a single feral cat may prey upon an average of 313 shearwaters each year, warning that their breeding grounds are in danger.

Streaked shearwaters, known as omizunagidori in Japanese, are migratory birds that travel between the southern hemisphere and remote Japanese islands. The bird, which has a wing span of about 1.2 meters, travels to Tokyo’s Mikurajima about 200 kilometers south of the capital in spring for breeding. According to the organization, there used to be between 1.75 million and 3.5 million birds in the late 1970s, but the number has dropped in recent years to about 100,000. The bird is listed as a near-threatened species on the International Union for Conservation of Nature’s Red List of Threatened Species.

The research team collected feral cat feces on the island from 2016 to 2017 and found that about 80% of the 82 samples collected during the breeding season – between March and November – contained feathers and bones of streaked shearwaters. The annual number of prey animals per feral cat was calculated based on the amount of calories a mammal needs each day.

Streaked shearwaters lack vigilance by nature and are not good at taking off. The birds therefore have been targeted by feral cats, which are believed to have increased on the island since the 2000s. The cats have either been caught or removed from the island, but such efforts have not completely solved the problem.

The team is currently investigating the number of feral cats on the island to learn the entire picture.