Monday, August 3, 2009

Quiz #311 (2009-3-06) Answer

Click the picture for a larger view.

Photo taken in mid-November. For an extra bonus point: Of the gulls, which is the odd one out, plumage-wise? There may well be more correct solution sets than the one for which I am looking, but any well-justified, and correct, response will receive an extra bonus point, regardless of whether that person's answer is correct for the competition.

Answer by Tony Leukering


I will start with the center bird that none missed, though one respondent did not provide the first word in the name of American Coot. Eurasian Coot is ruled out by that species' lack of a bill ring. Now, onto the gulls.

Where should we start? As I consider it the most distinctive one, I'm going to start with the front right bird. It's obviously an immature of one of the larger gull species (note the size comparison with the coot). It has some adult-type back feathers but not much else of the plumage is adult-like. That should indicate to us that it's either a 1st-cycle individual of one of the three-year gulls or a 2nd-cycle individual of one of the four-year gulls; see quizzes 150 and 257 for more on this subject. Using the primary-shape criterion noted in quiz #150, this bird should be a 2nd-cycle bird. Nicely corroborating that decision is a feature that I find remarkly little-used in the species' ID: its blue legs. Only California Gull sports such and only in its second cycle. The bill, too, is blue and these blue bits are, in my experience, absolutely diagnostic.

So, with one bird identified, we can now use it as a yardstick for the others. I continue on the right side to the rear bird, which looks smaller and paler than our Cal Gull. We can just see that it has some gray back feathers and with its smaller size, it must be a three-year gull and thus a Ring-billed. Mew Gulls have back feathers of about the same tone as does Cal Gull and at our bird's age are overall darker than our quiz bird.

The big gull in the left rear is next on our agenda and we should note that it, like the previous two gulls, has some adult-like (unmarked gray) feathers on the back. There is a three-year gull as big as this one, Yellow-footed, but in 1st-cycle plumage it is considerably darker than our quiz bird, so it must be an individual of one of the four-year gull species. As this bird has, except for the aforementioned back feathers, the same color tone throughout its plumage, including its primaries, we really have no option other than Glaucous-winged Gull. Yes, we might entertain thoughts of a Western x Glaucous-winged Gull hybrid, but such birds really ought to show outer primaries at least somewhat darker than the rest of the plumage, among other distinguishing characters.

Last one, and, in some ways, both the most difficult and the easiest. It seems to be your sterotypical young four-year gull, if there is such a thing. It's brown, with darker primaries, and is obviously smaller than the bird behind it and of similar size to the Cal Gull. If that last is true, it's probably not an individual of the sterotypical gull, Herring Gull, but there are abnormally small individuals in most gulls species; let's look more closely.

Ogling the primaries, we can easily determine that they belong to a 1st-cycle bird, as they are fairly pointed. At this point, the provision of the season in which the picture was taken starts to have some meaning. In mid-November, most 1st-cycle gulls have initiated their pre-formative molt, replacing a variety of feathers among a few tracts, particularly the head, back, scapulars, and coverts. Looking closely at the bird's plumage, there seems to be no suggestion of this molt: the scapulars and wing coverts, particularly, seem to be of one generation without any differently patterned feathers. Only some high-arctic-breeding gulls delay their pre-formative molt so long, so our bird can really only be referable to one of three species: Herring, Thayer's, and Iceland. Note that only the high-arctic breeders among Herring Gulls delay their pre-formative, it they do so at all; most Herring Gulls are well into that molt by mid-November and sport whitish heads and multiple patterns among the scapulars and wing coverts.

Obviously, ruling out Iceland Gull is fairly straightforward as even the darkest of juvenile Icelands would not have such dark primaries. Ruling out Herring is a bit trickier, but we can do it with our bird's combination of delayed molt, all-black bill, thin bill, not-quite-black primaries, and obvious whitish primary fringes. So, this is a Thayer's Gull in juvenal plumage (also called first basic plumage). That feature was my aim in the offering of the extra bonus point -- the Thayer's Gull in the picture is the only gull fully in juvenal plumage, thus has not molted since leaving the nest. The four respondents that earned the extra point took that tack. But, as I noted in the red-letter caveat, there are probably other correct answers and I had thought of another before sitting down to write this quiz solution. The California and Glaucous-winged gulls are both 2nd-cycle birds in 2nd-basic plumage, but the Ring-billed Gull has completed (or nearly so) its pre-formative molt, so is in formative plumage, the only one so feathered.

Five respondents provided answers with no incorrect species, but not enough correct ones. Also, one respondent scored the extra bonus point despite not providing a correct answer.

Incorrect species provided as answers:
Glaucous Gull - 1
Western Gull - 2
Herring Gull - 3
Mew Gull - 2

The 7 of 19 providing the correct answer (* indicates those receiving the
offered extra bonus point):
Aaron Brees *
Robert McNab
Mark Dettling *
Kevin Kerr *
Nick Komar
Andy Dettling
Peter Wilkinson *

Answer: American Coot and Ring-billed, California, Thayer's, and Glaucous-winged gulls