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...