Japan’s Mikurajima Feral Cats Kill 30,000 Near Threatened Streaked Shearwaters Annually, Putting World’s Largest Breeding Colony at Risk

The Yomiuri Shimbun

16:00 JST, August 4, 2025

The streaked shearwater, a seabird classified as near threatened on the International Union for Conservation of Nature (IUCN) Red List, is being preyed upon by feral cats on Mikurajima Island in the Izu Islands at an estimated rate exceeding 30,000 birds per year, according to a study released by a team that includes the Forestry and Forest Products Research Institute. Almost the entire island is designated as a national park, and the researchers urge the national and Tokyo metropolitan governments to strengthen control measures.

Mikurajima Island, about 200 kilometers south of central Tokyo, hosts the world’s largest breeding colony of streaked shearwaters. The population on the island was estimated at more than 1.75 million in the late 1970s, but by 2016 it had plummeted to roughly 100,000. The IUCN consequently listed the species as near threatened in 2018.

Although it was already known that the birds’ low vigilance made them susceptible to predation by feral cats that had become established on the island, the exact scale of the losses was unclear. From January to March last year, the team analyzed cat droppings; based on the quantity of feathers and bones, they estimated that a single cat kills about 330 shearwaters annually. Because 106 feral cats were trapped by the village and other authorities in fiscal 2022, the researchers calculated that at least 34,980 birds are being taken each year.