02 December 2014

Cook-Inlet or Thayer's Gull?

I found this first cycle gull in Lake County, Illinois on Friday (28 Nov 2014):

Cook-Inlet (Herring x Glaucous-winged) or Thayer's?
My initial identification was "heavy set" Thayer's, partly because that's what "dark" white-winged gulls default to here, but the more I looked at it, the more I began to wonder. Adam Sell was present at the time, and after pointing the bird out to him, I commented on the semi-muddied look to the scapulars and how they seemed odd.

Jizz-wise, this bird stacked up equally to most of the Herrings around it. Not your run-of-the-mill juvenile Thayer's, although the bill size and structure seemed fine for THGU.
Martin Reid questioned the darker bases to the upperwing coverts and the seemingly short primary projection. This was the motivation I needed to think a bit more about the reservations I had in the field.

Broad arm, dark uppertail coverts and solid, dark tailband seem untraditional for Thayer's.

Barrel body, broad wings and larger head are very reminiscent of Glaucous-winged.
Another thing that bothered me during my observation was what sometimes seemed like an oddly high-placed eye.

It's amazing how a slight change in posture can give a longer look to the primary projection, but the eye-to-face look now appears like a classic Glaucous-winged hybrid.
Notice in the photo above (slightly overexposed) that the bill base looks like it's paling, and it actually was - another strike against a November, 1st cycle Thayer's.

Dark uppertail coverts with wide, solid, distal band. The proximal scapulars look solid brown and not the neatly patterned look I'm used to seeing on 1st cycle Thayer's. This, along with the dark-based upperwing coverts, and darker outer webs to the inner primaries all look like they've been influenced by Herring.
So the final verdict on this one is Cook-Inlet Gull...not the first one I've seen in Illinois and probably not the last!