A large gull that breeds mainly in Hokkaido within Japan.
The back, shoulders and upper wings are dark slate-grey. Brown head flecks appear in winter plumage.
Observations on Teuri Island, Hokkaido, in 1984–1985 showed that Slaty-backed Gull pairs differed in the food they brought to their chicks. Some males mainly brought fish; others also caught chicks of Black-tailed Gulls and Rhinoceros Auklets and carried them to their own broods. Females rarely delivered seabird chicks.
Pairs whose food loads contained seabird chicks in more than 10% of observed loads raised about twice as many fledglings as pairs below that level, and their chicks grew about 1.5 times faster. Fledgling numbers also varied with territory size and hatching date, while chick growth was associated with egg volume. On Teuri, gull nests occupied cliff ledges near nesting areas of other seabirds.
Japan Red Data Book (5th edition) · Endangered (EN) · Japan, national species assessment
Japan’s 2026 fifth Red Data Book assesses the Slaty-backed Gull as Endangered (EN). At six major breeding sites with recent count data, breeding numbers had fallen by 13–99% over about ten years, averaging 67%. The assessment also estimates declines of at least 65% across major colonies over three generations. It records continued pressure from predation or disturbance by feral cats, rats, foxes and White-tailed Eagles; reduced fish and discarded fish; and human entry or removal of nests and eggs. New nesting sites have also been found at ports.