This stunner visited the city park of Bergen, western Norway 6 November 2013. At first I took it for a variant of what we label hybrid Glaucous x Herring Gull. I posted a picture on Twitter, and got a comment from fellow birder Alf Tore Mjøs. He had been digging into some Nearctic literature, and suspected one of them might be involved in this creation... When browsing though the picture load of the bird, and comparing with birds on the web and in my literature, several features seem to fit Slaty-backed Gull better than our hybrids (especially the greater coverts, tertials, tail and the row of markings on primary tips) - and some do probably not... I have no experience with Slaty-backed or its hybrids, so comments are much appreciated. Probably a long shot, but hopefully something to learn from :-)
As a reference: quite a few young putative Slaty-backs are illustrated on Gulls in Hokkaido and in this blogpost from Chris Gibbins.
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