Monday, October 24, 2011

Quiz #422 (2011-4-04) Solution

Click on picture(s) for a larger view.

Solution by Tony Leukering

Usually, when the quiz picture is a difficult one or when there's a gull involved, the number of responses for the week is relatively low. So, one can imagine my surprise at the good number of responses to this week's quiz when not only is there a gull involved, but it's quite difficult! Though it's quite difficult, nearly any ABA-area gull addict would have gotten it correct. But, that is because the important characters are ones that lariphiles know and most birders don't.

We'll start with the individual in the picture that caused very little consternation, the bird of which the image was intended, until the gull flew in front of the camera at Goleta, Santa Barbara Co., CA, 28 February 2010. The seasonality of the picture may very well be determinable from the subjects. The cormorant is in high condition, what with its bright orange supraloral and gular patches. The apparent four-year gull is not molting, at least, not apparently so. Both features are consistent with early spring. And that will come into play with the gull.

As noted by those respondents submitting rationales for their ID decisions, the head pattern on the cormorant is definitive for Double-crested. Neotropic, which can show orange in the supraloral region, never has so extensive a gular patch (it extends past the chin onto the upper throat on our quiz bird). The separation of the bright color by a dark loral line (and the extensive gular patch) rule out Red-faced Cormorant. All the rest of the cormorants have dark bills.

Features that at least some of the respondents used to ID the gull included tail color, bill pattern, darkness of the greater coverts, and (for one) the pale trailing edge to the secondaries. While the bill has an extensive pale base, it is not nearly pale enough nor demarcated enough for a nearly one-year old California Gull. Any Cal Gull with that little pale would be much younger and exhibit extensive juvenal (=first basic) plumage on the upperparts, which our quiz gull does not. Additionally, the greater coverts appear a bit too pale, relative to the darkness of the secondaries and to the median coverts for our bird to be a Cal Gull, which sport greater coverts closer to the color of the secondaries, rather than the median coverts, not the more-intermediate color expressed by the quiz bird. Yes, California Gulls of this age sport a pale trailing edge to the secondaries, but so do a couple other options.

Herring Gull was the overwhelming choice for the ID of the gull, perhaps due to its large range and relative familiarity to most ABA-area birders have with the species. Additionally, the vague dark-and-pale bill pattern is practically perfect for Smithsonian Gulls (the New World form of Herring Gull) of this age. Unfortunately, formative-plumaged Smithsonian Gulls have pale greater coverts that contrast not at all with the median coverts, and very strongly with the secondaries. They also have a pale wing panel, created by the paler inner primaries that are not exhibited by our quiz bird. Finally, they do not sport such a pale trailing edge to the secondaries; at least, not so wide, not so white, and not so obvious.

Thayer's Gulls of this age have nearly entirely black bills and sport secondaries with dark outer webs and pale inner webs, unlike the apparently all-dark secondaries of the quiz bird. Additionally, many Thayer's Gulls of this age are still sporting at least some juvenal feathers among the scapulars; our bird seems not to have any such scaps.

So, as far as big, dark gulls go, we have only two remaining options. Yellow-footed Gull is, despite its size, a three-year gull, thus at this bird's age, it would have a gray saddle. By process of elimination, then, we are left with the correct answer: Western Gull. But, what about the bill pattern? All the field guides show first-winter Western Gulls as having all-dark bills. Well, if you're talking about "regular" field guides, the ones that treat all of the species of a given region, then yes. However, those guides don't have anywhere near enough space to show much of the variation in appearance inherent in any species, nor the various intermediate stages between the appearances that they do illustrate. "Second-winter" Western Gulls are depicted in those guides as having strongly bi-colored bills, and those bills don't just suddenly become that way. As in smithsonianus Herring Gulls, Western Gull bills transition, slowly, from all dark to bi-colored, and this week's quiz gull is somewhere in the middle of that process. And that pale trailing edge? That is what creates the skirt in Pacific-rim gull species.

Gull ID is relatively straightforward (ignoring, for now, hybrids), however it is a more complex game using features that are not part of the ID process for most other taxa (such as wing panels and the obvious pale trailing edge of Cal, Yellow-footed, and Western gulls) and requires a lot of practice and being able to focus on more than just one or two features. I note that one of only two respondents getting this week's quiz correct lives in coastal California, where Western Gull is the gull. The other is a Brit, and we always expect better things out of them!

One response provided only one species, taking a pass on the gull.

Incorrect species provided as answers:
California Gull - 3
Herring Gull - 19
Red-faced Cormorant - 1
Thayer's Gull - 1

Congratulations to the 2 of 26 getting the quiz correct:
Robert McNab
Peter Wilkinson

Answer: Double-crested Cormorant, Western Gull