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Seen at Lake Merritt Oakland CA today. Torn between Thayer's and Glaucous-winged x Herring Gull, thought the neck and head seemed perhaps a bit bulky for Thayer's, and the texture of the brown smudging looks like Glaucous-winged to me, but I'm not sure. Definitely too slender and long-winged for Glaucous-winged x Western, and too small of bill.

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That's a darned good question! Being something of an ID risk-taker, and being quite partial to hybrids, I'll take a stab at it.

First off, the off color of the bill proximal to the gonys is a feature that I associate with Thayer's Gull (THGU). However, if this is a THGU, then it is a really black-wing-tipped individual, which, in my experience are rare-ish. That exceedingly black wing tip would also seem to mitigate against an f1 Herring (HERG) x Glaucous-winged (GWGU), as I would expect such first-generation hybrids to be more-intermediate in wing tip color. However, that black wing tip would not rule out a back-cross (with HERG, obviously).

Were this a THGU, I would expect it to show larger white tips to the outer primaries than it has, though there are pix in eBird of apparently correctly-ID'ed THGUs with tips this HERG-like. Additionally, the black strap across p10 (seen in 3rd pic) seems a bit excessive for THGU, but that, too, is variable in the eBird pix.

As you noted, that scaling on the lower and side breast feathers extending from the neck is a good GWGU feature, and it seems to be somewhat conserved in hybrids, at least in my experience. This feature is the sole reason that I think that your decision on the bird in eBird may well be correct -- that it is a HERG x GWGU hybrid.

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On 1/9/2020 at 1:06 PM, Tony Leukering said:

That's a darned good question! Being something of an ID risk-taker, and being quite partial to hybrids, I'll take a stab at it.

First off, the off color of the bill proximal to the gonys is a feature that I associate with Thayer's Gull (THGU). However, if this is a THGU, then it is a really black-wing-tipped individual, which, in my experience are rare-ish. That exceedingly black wing tip would also seem to mitigate against an f1 Herring (HERG) x Glaucous-winged (GWGU), as I would expect such first-generation hybrids to be more-intermediate in wing tip color. However, that black wing tip would not rule out a back-cross (with HERG, obviously).

Were this a THGU, I would expect it to show larger white tips to the outer primaries than it has, though there are pix in eBird of apparently correctly-ID'ed THGUs with tips this HERG-like. Additionally, the black strap across p10 (seen in 3rd pic) seems a bit excessive for THGU, but that, too, is variable in the eBird pix.

As you noted, that scaling on the lower and side breast feathers extending from the neck is a good GWGU feature, and it seems to be somewhat conserved in hybrids, at least in my experience. This feature is the sole reason that I think that your decision on the bird in eBird may well be correct -- that it is a HERG x GWGU hybrid.

Oh my gosh, that’s a lot. It’s not even my bird, but I’m gonna use that information for future reference. Very, very cool. You, sir, are a birding genius.

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