Juv/1st.winter Slaty-Backed Gull L.schistisagus
Dec. 17th/27th 2001 Choshi Chiba Japan

Juv/1st.winter Slaty-Backed Gulls show very variable colorations, from "smithsonianus-like" to "Glaucous-winged-like".
Dec.17th 2001
An advanced worn bird. Note the pale and patched colorations overall, worn tertials, coverts and primaries. Compare it with this fresh bird photographed on the same date.
Dec.17th 2001
Another advanced, relatively Glaucaus-winged-like bird. Its molt is more advanced, and shows greyish breast and back. The scapulars are mostly new.

Glaucous-winged Gulls look still more uniform, and usually lack anchors in scapulars. And their outer webs of primaries look paler than that of this Slaty-backed Gull. Compare it with this Glaucous-winged Gull.
Hybrids also seem to exist a little. But, at least those seem to be not usual, unlike Glaucous-wingedXWestern in the New World. Strictly, I don't know whether this is quite pure or not, but I  guess this is probably a variation of Slaty-backed Gull.

Dec.17th 2001
An average bird, but delayed molt.
Dec.27th 2001
A darker, relatively fresh bird. Mainly juvenile plumage except for the new upper scapulars and back.
Dec.17th 2001
Another fresh bird. It looks like almost Juvenile from a distance and  differs extremely from the older bird at the top of this page.