Monday, February 8, 2010
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Answer by Tony Leukering
Was that ugly enough for you? I could do far worse!
Yup, we had to deal with a gull this week, and the lower number of respondents may be an indicator that some of you didn't like this one. At least, it's a good pic of a bird in good light that is well-focused.
Many (most?) birders might prefer for nearly the entire Larinae to drop off the face of the planet, but some of us oddballs truly appreciate them for the intricacies of ID and the difficulty they pose. To paraphrase a number of people, both fictional and not, "If it were easy, anyone could play!"
With the starting point of "gull," we need to do two things to really get going on working out the ID: ageing the beast and trying to place it in one of the 3-4 sorts of gulls of the ABA area. The substantial bill with both horn and pink among the colors exhibited enable us to rule out all of the smaller gulls. The pattern of colors on the beak also allow for elimination of all of the medium-sized gulls (Heermann's, Ring-billed, Mew), leaving us with what are termed the "large, white-headed gulls." Of course, that's where most of the truly difficult taxonomic and ID problems lie, so, perhaps, we haven't really made all that much progress!
Knowing that it's a big gull, and knowing that all but one of those species are "four-year gulls" (they go through four cycles of plumage to reach definitive plumage) will help us age the thing; the odd-gull-out here is Yellow-footed, which, despite its size, is a three-year gull. Assuming that the quiz bird is not a Yellow-footed Gull (leg color is wrong -- as it is for both Ring-billed and California gulls, for one), the fact that most of the upperparts feathers are an adult-like gray but that many of them have obvious brown smudging and that the bill is substantially black, we can make an educated guess that the beast is in its third plumage cycle. A younger bird would have had some truly brown feathers on the upperparts and, usually, most of the wings would still be brown. An older bird -- an adult or very-near-adult -- would not show so much brown smudging and would have a cleaner bill -- not so much black.
So, now what else can we do to reduce the number of possibilities? Eye color is good, as species that are pale-eyed as adults really ought to have fairly pale eyes by this age. If we took that tack, we could then argue that there are some third-year individuals of pale-eyed gulls whose eye color has not changed, but I want to start the winnowing elsewhere. Skirts. Anyone heard of skirts?
Skirts are a feature of the four species of what I like to call the "Pacific-Rim gulls;" Slaty-backed, Glaucous-winged, Western, and Yellow-footed -- a group of species that, essentially, replace each other by latitude and longitude from Japan around the rim of the northern Pacific down to Baja. These species have relatively wide wings, with much of the extra width being produced by longer secondaries. Unlike most other large, white-headed gulls, the secondaries on these four species extend beyond the greater coverts on the folded wing. Thus, on adult or adultish members of these species, the white tips to the secondaries form a line at the lower edge of the wing, basically in line with the angle of the rear portion of the body. This is what we call a skirt. And, of course, those are quite long skirts, as the leg joint just below them is the ankle! My, what long toes you have! Sort of.
We can see that our quiz bird sports a skirt (see below), so our quiz bird must belong in this group. Quite useful, skirts.
Check out Mr. Bill Mystery Quiz #203, which shows five species of gulls, none of them sporting skirts. Yes, they have tertial crescents, but no skirts.
So, with this winnowing conducted, we can quickly rule out Slaty-backed Gull, as our bird's upperparts are nowhere near black. Western and Yellow-footed gulls are also dark-mantled species -- not quite as dark as Slaty-backed, but still darker than shown by our quiz bird; even the paler-mantled northern subspecies of Western Gull. Also, third-cycle Western Gulls shouldn't sport this much head streaking -- they should be considerably whiter-headed.
That must mean that our quiz bird is a Glaucous-winged Gull. Sing hosannas!
Wait just a cotton-picking minute. Don't Glaucous-winged Gulls have, well, glaucous wings? Why are our quiz bird's wingtips so dark -- too dark for even a third-cycle Glaucous-winged? And, what about the tail? It, too, is quite dark -- at least, it's got a blackish band in the center/base. Do we need to start from earlier assumptions?
Nope, our bird is definitely among the Pacific-Rim gulls, it's just not a pure individual of any of them. In the winter on the West Coast from Washington south, an incredible percentage of large, white-headed gulls are hybrids, and of a bewildering variety of crosses. The most common of these is the swarm of birds produced by hybridization between Glaucous-winged Gull and the northern subspecies of Western Gull (and, of course, various backcrosses thereof), called, variously, Puget Sound Gull, Olympic Gull, or $#&*^)!*%^.
I took this picture of a third-cycle Glaucous-winged x Western Gull in Seattle, King Co., WA, in February 2006. The palish upperparts, head color, and bill color are features typical of Glaucous-winged of this bird's age, while the tail and wingtip coloration are more like those of Western Gull. Quite a while ago, I altered the rules to allow for hybrids, and this is the first that I've used. At least, I used what is almost certainly the most numerous hybrid combo in the ABA area, though Herring x Glaucous-winged Gull might give it a run for its money!
One respondent thought that both birds in the picture were of different species, hence the discrepancy in numbers, below. A few other folks commented on the second bird, but they all agreed with me that it is not identifiable. Besides, what I really think that bird is is another of the same -- there were some seven of the things of the 15 or so gulls around me at the time.
Incorrect species provided as answers:
Western Gull - 6
California Gull - 1
Glaucous-winged Gull - 3
Thayer's Gull - 5
Ring-billed Gull - 1
Herring Gull - 2
Congratulations to the 6 of 23 getting the quiz correct:
Answer: Glaucous-winged x Western Gull