Slaty-Backed Gull?

Jeff Acuff

On Friday I saw and photographed an interesting gull at Clifton Court Forebay.  It is probably an American herring gull but perhaps is a slaty-backed gull.

Photos (sorry for poor quality) are here:

The bird is a large white headed gull, first or second
Bird was of a size consistent with Western/Glaucous-winged/Herring
gulls.  Had a very large bill with very noticeable gonydeal expansion.  Bill
color is dark gray smudgey with pale yellowish color throughout, maybe a bit darker at tip. 
Pinkish legs.  Pale and light gray plumage overall, mostly whitish in
the head and breast area.  Feathers on back and scapulars were fairly
pale and plain with two generations of feathers mixed together.  Darkest
feathers were the primaries and tertials, and they were a medium gray,
not black or dark gray.  There is a value contrast between the greater
wing coverts and the tertials and there is a visible tertial crescent.  And, as one friend commented, the bird's "got the mascara thing going on."

Overall size and bill size/shape clearly ruled out California Gull and Thayer's Gull.

The lighter plumage, light-pale greater wing coverts, and quality of the smudgy bill all seem to suggest slaty-backed over herring to me. (???)

Good Birding,

Jeff Acuff, Lafayette

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