Vega Gulls Larus vegae (and some 'taimyrensis'?)
14th 2006 Miura Kanagawa Japan

Note their incomplete primaries in mid-December and heavily mottled heads-breasts.
Only 2 adults had complete primaries among 52 adults/subadults.

Their large and soft mottling on neck-breast can often be a good sign to distinguish from Mongolian Gull.

More Vega Gulls on the same date