Probable Caspian Gull Larus cachinnans cachinnans
Choshi Chiba Japan, March 11th 2003

This white-headed gull has nice cachinnans characteristics in many respects.
But its primary molt is abnormal. This might be somewhat problematic for identification, from a point of view.

Note the pale grayish legs. It is vague grey that can show some faint yellowish/greenish/fleshy tones. And it is pretty exceptional in Japan, and seems to fit  cachinnans (or some mongolicus?) well. (Taimyrensis tends to show orange yellow or warmer flesh tone rather than grayish tone.)

Note the straight long bill showing black spots and pure white head. (Most Vega Gulls still remain winter plumage.)

The primary molt is quite strange. Terribly old worn primaries still remain to the right wing. (-P10 and 9? ). It is too strange for cachinnans, but is strange for vegae or taimyrensis as well. (In March, molts are supposed to be finished already, or be nearly the end, even later birds  of taymyrensis. )

The P10 of the left wing is not yet fully grown. The P10 pattern fits cachinnans, and  is unusual for mongolicus, taimyrensis, and vegae.  The left wing does not seem to have old primaries.

The left wing P9 seems to be advanced and longer. But the right P9 seems to be short, beneath  the P8. (The old primaries are invisible here.)  I don't know what this strange, unbalanced molt means. But I feel it might be too strange and irregular to be a key identification factor in this case. Could it be caused by a kind of illness or something?