Monitoring Breeding Performance of the Black-tailed Gull <i>Larus crassirostris</i> on Kabushima Island, Japan, from 2012 to 2017: A Record of Intruder Red Foxes <i>Vulpes vulpes</i> in 2017

We monitored the breeding performance of Black-tailed Gulls Larus crassirostris on Kabushima Island, which has been connected to Honshu Island by a man-made isthmus since the 1940's, from 2012 to 2017. The mean clutch size from 2014 to 2017 (2.01–2.16) was higher than in 2012 and 2013 (1.57–1.64), but the productivity (no. fledgling per egg) from 2014 and 2017 (0.01–0.09) was lower than in 2012 and 2013 (0.23–0.29), especially the lowest productivity in 2017 (0.01). Poor productivity in 2017 might be attributed to disturbance by Red Foxes Vulpes vulpes that were observed at Kabushima Island during the breeding period in 2017.