Folks,
Steve Larson and I spent 2.5 hrs studying gulls at Teller Ponds in Boulder County. The birds are feeding on recently thawed dead fish in the mud and ice of what is left of the nearly dry lake. Conditions were very good for studying gulls with bright sun, low wind, warm temps and the stench of dead fish in the air. We started with 70 or so birds and ended with about 200. We saw 3 Lesser-black backed gulls, all seemingly 4th cycle. These birds look a lot like adult (definitive) plumage birds except for a few differences. They all had some black smudging still on their bills. The iris and leg color of some weren't exactly yellow. A small, dusky dark spot was seen on 1 feather of one bird's tail. Otherwise they are adult-mantled with some degree of white primary spots.
Thayer's Gulls were numerous, possibly 8 or 9 in different stages of plumage including at least 1 adult. These birds are valuable (when identified) so that comparisons can be made to more difficult-to-ID birds nearby. And there were a couple of more difficult-to-ID birds for us to study. One appeared to be a first cycle, Glaucous-winged X Herring Gull based on the following: The bird was noticeably paler than the first cycle Herring and Thayer's Gulls. The bird was large and large-headed. The overall color was grayish brown, and smudgy looking. The bill was large (larger than most of the Herring Gulls), and mostly dark. The dark eye looked 'beady' on the large head. The folded primaries were pale brown, a hair darker than the mantle and edged in a paler shade. In flight, the bird looked more uniformly pale than Herring and Thayer's Gulls, but not enough so for pure GWGU. The outer flight feathers were darker than the rest of the wing, and there seemed to be a blotchy look to the wing overall compared to the next difficult-to-ID bird I am about to describe. Blotchy and grayish-brown are key adjectives I look for and use when describing anything sub-adult, Glaucous-wingy. Just my sense of how they appear.
Finally, there was another 1st cycle, large pale gull in the difficult-to-ID category. The bird was similar in many ways to the bird described above, but structurally smaller, with smaller head and bill. The bird was slightly smaller than nearby Herring Gulls and had a proportionally smaller bill. Now I am talking about fractions of a degree here, so the nearby HERGs and THGUs are handy to have for comparison. The mantle and wings were certainly paler than the GWGU X HERG and were more uniform when seen in the flying bird. The bird was more finely patterned and didn't appear smudgy. All of these things added up well for first cycle Iceland Gull (to me). IF it is an Iceland Gull, it must be a male, because the head and bill I would not characterize as 'dainty'. Both features were larger than SOME nearby Thayer's Gulls, which may have been female birds. The tertials on this bird seemed very worn, so judging the degree of 'pattern' on these feathers was difficult. They were the same shade as the rest of the mantle however, patterned or not. My best guess is this bird is either a male Iceland Gull, or an extremely pale, worn Thayer's Gull. The cool thing is how similar these birds can look to another bird of likely different species. Well, I think it's cool ;- )
If you go to see these birds, study them hard. There is slight differences that you will start to recognize. Photos of the 2 pale gulls and others can be seen at the link below. The adult Iceland Gull did not make an appearance during our visit.
Happy gulling!
Glenn Walbek
Castle Rock, CO
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