Breeding Status of the Black-tailed Gull on Bentenjima Island, Okuki, Hachinohe City, Aomori Prefecture, Japan

A total of 1,179 nests (55 with no eggs) of Black-tailed Gulls Larus crassirostris were observed at three sites on Bentenjima Island, Okuki, Hachinohe City, on May 15, 2021, and 489 chicks were counted on June 26. Apparent clutch size (excluding nests with no eggs) and fledging success were 1.9 and 21.7%, respectively. Fledging success was lowest on the rock connected to the mainland by a breakwater, presumably because of disturbance by terrestrial animals. A total of 16 individuals with metal leg bands were observed and were 2–22 years old. These were immigrants from Kabushima Island, 10 km northwest of Okuki. Bentenjima Island is an important breeding site of the Black-tailed Gulls in northern Japan, and needs to be monitored.