I would agree with Martin; birds like this are best left unidentified. The main point for identification is that all the features of the bird are matched by those in Japan. However, these traits perhaps could be shown in hybrids. Furthermore, well several traits are similar to that of a Slaty Back, I noted that scapular feathers coming in were rather pale gray for a Slaty-Backed. However, if there is any other reason someone thinks it's a Slaty Backed, I'd be happy to hear it.
Carson City NV
On Feb 6, 2014, at 6:44 PM, Martin Meyers <[log in to unmask]> wrote:
> The more I study first-cycle large gulls, the less I understand. I
> posted photos of this bird last fall -- I don't know if it is the same
> bird described by Nick Rosen. And I don't know what it is. At the time,
> several birders with very substantial credentials responded, with
> varying opinions, most suggesting that they did not think
> Glaucous-winged hybrids could be ruled out. Nearly everyone discounted
> Thayer's as a possibility.
> Tristan McKee has been working hard on this issue, and I very much
> respect his opinions based on that hard work. But, personally, I'm
> leaving this (and, for the time being, all first-cycle Slaty-back
> possibilities) as "unidentified large white-headed gull", a "name" I
> seem to be putting on more and more first-cycle gulls.
> There is one NBRC-endorsed record for Slaty-backed Gull in Nevada, a
> third-cycle bird that Greg Scyphers and I found at Walker Lake last
> year. (They're a whole lot easier at that age!!)
> Photos of the bird I saw at Sparks Marina last fall (and have seen
> occasionally since) are on my website (address below.)
> On the tabs across the top of the page, go to:
> ID Issues/Rarities
> Then select the first entry, "Thayer's or Slaty-backed Gull or ???"
> Martin Meyers
> email: Martin (...AT...) SierraBirdbum.com
> Photo website: http://SierraBirdbum.com
> Truckee, CA