NB. Further observations of this bird through to Feb 26th have shown that an identification of Thayer's Gull hasn't been as straight forward as I initially thought. I have since withdrawn the claim of Thayer's but keep an open mind about the identification. It's still a bird of great interest to me and at the moment I don't have particularly clear idea of what it could be in terms of a hybrid combination. I'm also still not sure sure if I'd totally rule out Thayer's altogether. Subsequent images can be seen in my update here.
Posted on Feb 15th - "How nice it was to finally find a Thayer's Gull at Turner's Falls. The bird was on the ice with a reasonably sized (c.500) late afternoon gathering of large gulls at Barton Cove. I discovered it viewing from the Unity Park side of the cove at Turner's Falls. Coincidentally, Al and Lois Richardson showed up at Unity Park and were able to share the observation. The views from Unity Park were way superior to the views we later had from Riverview Drive in Gill. In fact, the light was so bad from this spot, that it took quite a long time to relocate the bird. All of the images in this selection were taken from Unity Park, Turner's Falls looking North-west.
This was just the sort of first-cycle Thayer's that I would hope to have found at Turner's and exhibited a number of important features when compared to first-cycle American Herring Gull, many of which are now looking faded enough to cause some problems.
1) Uniformly light-tan grayish plumage with soft, velvety appearance compared to coarser patterning of American Herring Gull. Overall appearance looked light and 'frosty'.
2) In contrast, the bill looked completely black though I didn't get close enough to detect any pale at the base of the lower mandible. In mid-February, a bird with such an obviously all black bill was quite eye-catching as just about all the first-cycle American Herring Gulls that I've seen lately have shown bi-colored bills to varying degrees.
3) Broad, noticable 'mask' surrounding the eye visible only at certain angles - similar to this bird from East Gloucester last winter.
4) Tertial centers darker than the rest of the upperparts, but on this bird I thought the tertial centers were relatively light compared to some Thayer's that I've seen.
5) In contrast to the tertials, the folded primaries appeared much darker, in fact the darkest part of the plumage. The primary feathers, especially the inner primaries were very neatly and tightly fringed whitish most notably at the feather tips - a good feature for Thayer's at this time of year.
6) I only saw the bird in flight once and it roosted during most of the observation. However, it showed the clear two-toned pattern to the upper surface of the primaries (dark outer webs, pale inner webs), whilst from below the underwing looked much closer to a 'white-winged' gull with pale primaries with dark tips forming a neat, dark trailing edge. I didn't see the secondary bar and or the tail band particularly well but didn't see anything to contradict my thoughts about this being a Thayer's Gull.
7) Legs, seen in good light only once, were quite deep raspberry pink."
Other birds of note this eveing at Barton Cove; 1 male Red-breasted Merganser (distant shot below), 2 Kumlien's Gull (1st and 2nd cycle), 1 Nelson's Gull (1st cycle), 2 Lesser Black-backed Gulls (adults).