Slaty-Backed Gull Larus schistisagus
Sakawagawa-river Odawara Kanagawa Japan, December 20th 2001. Phptos by O.Ujihara
Note the typical short-bodied, short-winged general structure, and the deep pink legs. The white tertial crescent is relatively narrower in this individual for Slaty-backed Gull.
Probable 4th. winter in front. 
The same bird as the 4th winter above. The "string of pearl" extends to P9, in this individual and the adult below. 
2 pieces of the "string of pearl" can be seen in the P7 and P6 distinctly. 
A different bird from the one above. The "string of pearl" reaches at least the P7 in the upperwing of this individual. Probably the "pearl" of P8 is hidden by the P7.
4th? winter. The "string of pearl" reaches the P8. This bird  lacks P9 mirror.
 A pretty  broad  "string of pearl" is shown here. And the white trailing edge is very broad.

 

 Western Gull  L. occidentalis
for comparison
Dec. 2nd 1999
SanFrancisco USA