Black-tailed Gull
Larus crassirostris

The common summer gull of Japan, they are named for the black band on the tail present in all ages of this species (many other gull species have a tail band as juveniles). They are medium-sized for Larus gulls at 44-48cm and a wingspan of 126-128 cm with males being larger than females. They occur from Southern China and Taiwan, round the Korean peninsular and Japan and then on to the eastern coast of Russia where part of the population breeds in the summer. Vagrants of this species have reached the western US and as far south as Australia, although are rare there. As with many other gulls, their diet consists of fish, crustaceans, molluscs, insects and food waste.
 

1: Ueno Park, Tokyo, 17/05/2012.

Additional references:
M. Chochi, Y. Niizuma and M. Takagi, Ornithological Science, 2002, 1, 163-166.
Black-tailed Gull: a photo essay by Paul Doherty, Surfbirds.