1st-winter Slaty-backed Gulls

I'm going back to Japan on this post. And one of the things I really wanted to get to grips with during the trip was the concept of a 'search image' for 1st-winter Slaty-backed Gull. Unfortunately, I found this to be extremely difficult on the basis of both plumage traits and the massive range in structure. Hopefully this post though will illustrate how distinctive they can be, to varying extents, and you'll see like other gulls the variability in moult and plumage wear - all photos taken on Hokkaido from 25th to 31st December 2015.

To summarise shared characteristics, I guess the following will do: -
- pale-fringed, dark brown to near black primaries
- plain brown tertials
- all dark tail
- tightly barred uppertail and rump
- dark, velvet underwing coverts
The problem is though, 1st-winter Slaty-backed Gulls really are variable and to some extent it's an unknown in terms of how much leakage in the gene pool there is with Glaucous-winged Gull (check out the second photo down as an example). Any comments on these birds are appreciated. For sure though, one of these at some stage is likely to wing its way to Britain or Ireland...