Vega Gulls Larus vegae (and some 'taimyrensis'?)
December 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.