Possible (probable?) Slaty-backed (photos)

Noah Arthur

I'm increasingly confident in this big guy, which I photographed at Oyster Bay on Tuesday afternoon, for first-cycle SLATY-BACKED GULL: https://www.flickr.com/photos/73989529@N02/sets/72157641987613475/
To me the plain pale greater coverts, pot-bellied shape (like a domestic pet goose), "messy" patterning on upperparts, all-dark bill, and pale inner webs on the primaries all suggest Slaty-backed. I know that most or all characteristics of 1st-cycle Slaty-backed can be matched by some Glaucous-wingedXHerring hybrids (and I also know that I don't know Slaty-backed ID very well!), but this one really seems to show all the characteristics I'd like to see. I may well be missing something, though...
Any opinions on this bird are very welcome!
Noah Arthur, Oakland, CA

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