My Mystery Gull

A few days ago, I included two photos of a mysterious gull I found while gulling at Fort Baker, Marin county, CA.  Still no consensus on the bird, and it looks like there probably won't be, but I figured it would be good to post the field marks of this bird, and the direction I and others are leaning on this.
Originally I suspected a Glaucous X something hybrid based on bill structure and color, and head structure.  I suspected a first cycle bird based on plumage and dark iris.  After reviewing the photos more, and corresponding with experts, the bird appears to be more likely a second cycle bird.  Gray feathers are coming in on the mantle, and the primaries look rounder-tipped rather than pointy as first cycle birds have.  The overall uniformity in plumage color suggests GWGU X something as well (if not pure GWGU!), though mixed birds can be rather uniform in their second cycle.  Nelson's Gulls generally have more patterning on the greater coverts and primaries.

Now that we know this bird is most likely a second-cycle it allows us to eliminate certain options.  Nelson's becomes exceedingly less likely because they should show a pale iris in their second cycle.  Glaucous-winged Gull (GWGU) X something then becomes exceedingly more likely since GWGU has a dark iris in its second cycle.

Bill shape and size is large in GWGU, and is bicolored in second cycle, but rarely is the coloration this extreme.  On closer inspection, though, the bill is not really all that pink as would be expected in a Glaucous hybrid.  Similarly when looking at the photo above, bill structure seems less suitable for a Glaucous hybrid (that doesn't necessarily mean it isn't).  Originally the bill looked large and heavy, but when taking a closer look, the bill doesn't look all that different from a typical Herring Gull bill.

It seems like the best arguments I've heard are for Herring (really Smithsonian, but let's not go there now) Gull X GWGU.  Still, I've received more votes for Nelson's than anything else.

Any more ideas about this gull are welcomed.  Email us or comment on this post if you have anything to add.  As a side note, check out the single dark feather on this bird's scapulars - very strange.

By Luke Musher