Monday, December 17, 2012

Quiz #479 (2012-4-11) Solution

Click on picture(s) for a larger view.

Solution by Tony Leukering

This quiz was incredibly tricky, with an immature gull and some fuzzy ducks. I'll start the solution with the relatively straightforward individual, the yellow-billed, green-headed thing in the bottom right of the picture. In the ABA area, there is only one species that sports that combo, Mallard. The next-easiest bird is probably the sleeping duck in the bottom left. It has a blackish crown contrasting with a brown neck and white cheek and brown sides contrasting with darker wings and with those darker wings also contrasting with the brown neck. The cheek patch is too large to be that of a female Bufflehead and the brown hindneck contrasting with both the crown and the darker wings is not a feature of any Bufflehead plumage. The brown neck and sides also rules out Horned Grebe, which always has a blackish hindneck.

Now, things get even more problematic. Probably the first problem to tackle at this point is deciding how many birds are in the picture, while the next thing may be deciding how many are identifiable. Here's is my take on both problems. There is an obvious duck behind the Mallard that has a sizable white patch somewhere and there is at least one duck in the shade under the vegetation just right of the gull's right wingtip. Though there might be two ducks together in the shade, I don't believe that anyone can make a convincing case for any more birds and that's if you could convince me that there were definitely two ducks together in the shade. The browny-gray aspect to the back right duck suggests Gadwall to me and that white bit might be part of the bird's speculum, which would be definitive in my mind if I could convince myself that white was, indeed, part of the speculum. I cannot so convince myself, therefore, I consider this bird to be unidentifiable. The bird in the shade, seems to have a large patch of white on what might be the chest, but it definitely has a black vent area contrasting somewhat with what appears to be a gray side. So, though the back end of that bird suggests Gadwall to me, male Gadwalls don't sport anywhere near that large a white patch anywhere. I also considered male Mallard (there is a suggestion of white in the tail) and male Northern Pintail for that bird, either of which could help explain the white bit, but since it seems to contrast so strongly with the apparently gray side, I am unsure of what is going on there. Additionally, I see enough evidence to suggest that there are two birds back there, one behind and slightly right of the one that we can see even remotely well. If that is the case, I can then not even decide to which bird that big white patch belongs. Thus, I consider this bird (or these birds) to be unidentifiable.

Here's the tricky part (Fig Newtons, anyone? Yes, my brain is very weird, I remember this one quite vividly from my childhood), the gull. When I started writing this solution, I was amazed at the large number of respondents that thought that the gull was of a different species than I identified it as. So, I corresponded with others that know both of the suggested species and have come to the conclusion that I labeled the bird correctly in the first place. So, bear with me, this may take a while.

First off, we need to age this thing. The lack of any sort of gray in the mantle and the very pointed outermost primaries tell us that this is a gull in its first plumage cycle. The bill being all black assists us in that determination and also helps us rule out California Gull. The evenness of the color tone of the bird and the solidly dark tail really leave us only a few species to consider: Thayer's, Glaucous-winged, and Slaty-backed. Oh, and various hybrid combinations, of course, but we'll hold off on going there until and unless we rule out all good species. Herring Gull is readily eliminated from consideration due to our bird's relatively pale tail (even though all dark) and the relatively pale wingtips, among other notable features. Of our possible solution set of three species, Slaty-backed is the easiest to rule out, as that species usually has a darker wingtip and dark tips to the greater and median secondary coverts that form two very vague dark wing bars. Additionally, first-cycle Slaty-backeds usually have darker secondaries contrasting more strongly with the inner primaries.

Now, here's the really tricky part, differentiating Thayer's and Glaucous-winged gulls. Given appropriate circumstances, separation is fairly straightforward, as standing and swimming birds, particularly with good yardstick species present for comparison, are easy to differentiate on size, both overall and bill size. Here, we lack both -- the bird is flying and is the only gull present, and it's not even particularly close to the identifiable ducks to give us even a gross impression of gull size. Thus we have to use plumage characters, although I will say that, to me, this bird looks like it has a fairly broad wingbase, but I would in no way rely on that impression for the ID.

The single feature that is most likely to lead observers/respondents to consider Thayer's Gull for this quiz bird is probably the 'Venetian-blind effect' of the dark outer webs of the outer primaries contrasting with the paler inner webs. However, it is important to remember that first-cycle individuals of all large, white-headed gull species exhibit this contrast, with the species differing only in the degree of contrast which is governed mostly by how much of the inner web on a given feather is pale and how widely spread the wingtip is (thus, how much of the inner web is visible).

To my eye, the quiz bird's forehead is too sloped and the bill too big to be those of a Thayer's Gull, but, again, we have no good comparison individuals present. The eye seems to be 3/5 or more of the way back on the head from the bill tip (front of eye to bill tip, which I measured as 60.4%, with a very generous estimation of the location of the back of the head). This again suggests to me that the bird is not a Thayer's Gull, as the values of my sample of six Thayer's Gulls in pictures in Howell and Dunn (2007), birds in pictures 36.6, 36.8, 36.11, 36.12, and 36.13, ranged from 47.9% to 54.4%. Of course, this is a very gross measurement when conducted on birds not in the hand, so we should not have much confidence in the result. But, it does provide yet another weak structural cue to the bird's ID.

However, we don't have to rely on such characters, because there is at least one fairly strong plumage cue: The pattern of individual feathers in the scapulars and inner secondary coverts. On Thayer's Gull, these feathers are typically fringed with white, with the paler individuals having wider and more-extensive fringes; picture 36.8 in Howell and Dunn (2007) provides a good comparison of this feature in darker and lighter first-cycle Thayer's Gulls. One would imagine that this feature would be easy to mis-interpret, because one would expect the paler birds to show less contrast to the white fringes than would darker birds, perhaps making the pale fringes less obvious. But, in picture 36.8, the darker bird has considerably narrower and less-extensive white fringes than does the paler bird and, despite the lower fringe-ground color contrast, the paler bird still has more-obvious white fringing than does the darker bird. Since our quiz bird is in the paler half of the variation of first-cycle Thayer's Gull plumage darkness, we could expect this bird to have fairly substantial white fringes to these feathers. However, our quiz bird has what appears to be white tips or subterminal spots on the scapulars and inner secondary coverts (greaters, medians, and the lower lessers). This pattern is typical of Glaucous-winged Gull, three examples from the same locale in the same month (all on 12 November 2007) of which I've included below.

And here are three examples of first-cycle Thayer's Gull from the same locale and date.

The only sticking point for this bird being a Glaucous-winged Gull, is the darkness of the outer webs of the outer five primaries. While this feature might suggest some gene contribution from Western Gull or Herring Gull, my experience with the species from Washington to Sonora is that it is found in  a variable, but far from insignificant, percentage of individuals that look otherwise perfectly fine for Glaucous-winged Gull, even in older plumages. So, I do not see the need to invoke introgression, but, regardless, the bird is not a Thayer's Gull. I took this picture of a first-cycle Glaucous-winged Gull (one that serendipitously has a couple of identifiable ducks in it) at Lucchesi Park, Petaluma, Sonoma Co., CA, on 23 November 2007.

With the final quarterly competition and the annual competition coming to a close with this quiz, it is time to award a couple of prizes. For the quarterly, Richard Jeffers wound up with the most correct responses (nine). However, since he won the previous quarterly competition, he is ineligible to win this one, so we go to the five respondents that each had eight correct responses to search for a winner. Those five were Tyler Bell, Ben Coulter, Donald Jones, Margie Joy, and Joshua Little. Congrats to all of them, but there can be only one winner. The first tie-breaker is the fewest number of incorrect responses, and one of the five had only one of  those. Joshua Little wins the fourth quarterly competition of the year, winning a year's membership in Colorado Field Ornithologists, thus receiving the great journal, Colorado Birds.

In the annual competition, the winner had the title clinched a few quizzes ago, as Ben Coulter's closest competition had 39 correct responses to Ben's 43. Ben wins a waived registration fee for the 2013 Colorado Field Ornithologists' convention in the southwestern corner of the state in Cortez! The convention will be an excellent venue, providing access to a great variety of bird (and other) species, with some southwestern Colorado specialties being of prime interest, particularly the only known breeding areas in the state for Lucy's Warbler (McElmo Canyon) and Acorn Woodpecker (Durango).

Incorrect species provided as answers:
Thayer's Gull - 11
American Dipper - 1
Horned Grebe - 2
Blue-winged Teal - 1
Bufflehead - 1

Unfortunately, 0 of 15 respondents got the quiz correct, but with most of those getting the two duck species correct.

Answer: Mallard, Ruddy Duck, Glaucous-winged Gull