Mongolian Gull L.c? mongolicus
December 2nd 2000 Choshi Chiba Japan
At Choshi, fresh juvenile type individuals are seen in October to December, every year. On the other hand, advanced early-molters that are common in Korea (-easier to identify) are not so regular here. Therefore, such late-molters often look problematic to distinguish from Vega Gulls in Japan. But at least this individual that resembled juveniles at Lake Baikal had very nice characteristics overall, and stood out in the flock. I suppose the late-molters might be northern, or late-hatched birds.
The underwings are whitish and sparsely barred. Note the narrow black tail-band, white base of outer tail feathers, and whitish belly with sparse mottlings. Underwings of Vega Gull for comparison