Monday, March 16, 2009

Quiz #291 (2009-1-12) Answer

Click the picture for a larger view.

Answer by Tony Leukering

Ooh, what a beast of a quiz photo! We've again got gulls to deal with, in a variety of plumages. To start things off, I'm going to start with trying to figure out what time of year the picture was taken, as that will help me age various of the birds and, thus, identify them. If we look at the left wing of gull #1, note that the outer primary is shorter than the next one in (the 9th or p9). Also note on gull #9 that its right wing also sports a short tenth primary (the tenth -- outermost -- primary on most gulls is the longest one). This tells us that the scene was photographed in mid- to late fall or early winter, as p10 is the last feather to complete growing as part of a gull's pre-basic molt. Since it's so late in the year, we probably are not dealing with any species in juvenal (first basic) plumage, unless we have one or more of the arctic-breeding species that typically delay their pre-formative molt until mid-winter (e.g., Iceland or Thayer's gulls).

So, I will start the ID process with gull #4, as it may be one of the easier ones to identify and, of course, I'll start with ageing it. The strong black-and-white tail and lack of gray mantle tells us that the bird is in its first plumage cycle and is a "four-year gull," that is, a species that usually reaches definitive plumage in its fourth basic plumage (when it is about 3.5 years old).

Before I continue, a quick aside about tail patterns in white-headed gulls. I've mentioned this before in the quiz, but it's important enough to bear mentioning again. Those species that are American and/or Pacific Rim species have solid-colored (or virtually so) juvenal tails, with the odd exception of Ring-billed Gull. The species list with this trait, then, is comprised of Mew, California, American Herring (currently, subspecies smithsonianus, but published data suggest that our bird is specifically distinct from Old World Herring Gulls), Kelp, Thayer's, Yellow-footed, Western, Slaty-backed, and Glaucous-winged Gulls. Old World species have white tails with dark tail bands (of varying widths, some bands occupy more than half the length of the tail) and these are Old World Herring, Yellow-legged, Lesser Black-backed, and Great Black-backed Gull (the species has colonized the New World in recent times). Iceland and Glaucous gulls, obviously, have pale tails.

Back to gull #4. As this bird has a white base to the tail with a distinct black tail band, the bird must be an Old World species (Ring-billed Gull was ruled out because in that species' first winter, it sports a gray mantle). This bird is already fairly white-headed and its greater coverts, particularly the inner ones are noticeably paler than are the secondaries and don't contrast with the color of the median and greater coverts. Said another way, on the bird's inner wing, the secondaries are darker than the rest of the wing, which is of generally uniform tone. The bill is all black and quite large, being both long and fairly deep-based (much larger than the bill of gull #3). Both Yellow-legged and Lesser Black-backed Gulls have darker greater coverts creating a pattern of a dark rear half of the wing (secondaries and greater coverts) and a paler front half (median and lesser coverts). Additionally, the bird's inner primaries are paler than the outer primaries forming a pale window that Yellow-legged and Lesser Black-backed Gulls lack in first-cycle plumage.

That leaves us with Old World Herring Gull and Great Black-backed Gull. The bird's large bill suggests the latter, but we should check other features to be sure. Old World Herring Gulls tend to have the outermost rectrix with much less of a black tip than does Great Black-backed Gull, with many having quite little black on that feather. Again, our bird seems more in line with Great Black-backed Gull. Finally, Old World Herring Gulls tend to have more white checkering on the outer greater coverts, with most having two or three rows of such on these feathers, versus the single one typical of Great Black-backed Gull. So, all these features point to Great Black-backed Gull. The final straw that I'll mention to support this ID is that birds #3 and #7 have tails that are all dark, suggesting that the picture was probably taken in the New World.

Looking up and right from gull #4, we see that gull #5 looks remarkably similar and that it, too, seems larger than the browner gull near it (as did gull #4). I'm pretty happy to call this one a second first-cycle Great Black-backed Gull. With these IDs, we have a very helpful hint to ID the rest of the birds: the picture was probably taken in eastern North America. Moving on to gull #6, we can see an adult-like bird with quite dark upperparts. That bird, too, looks large with a large bill so calling it an adult Great Black-backed Gull is not much of a stretch. Looking back to gull #2, we can see a similar beast, but we also have the additional field mark of the extensive white tip to p10, a great mark for Great Black-backed Gull (and one that easily rules out Lesser Black-backed Gull). Finally, on this bird, we can see that the white trailing edge to the wing is fairly narrow, which gets rid of the Pacific Rim dark-mantled gulls from our contenders list. Since I see no other good candidates for Great Black-backed Gull in the picture, that makes a total of four of that species.

So, with the fairly safe assumption that our picture was taken in eastern North America (there aren't many places in western North America where one might see four Great Black-backeds together!), let's go back to gull #1. This bird is apparently an older immature gull, as it shows a gray mantle and gray wing coverts, though there is black on the primary coverts and more black in the wingtip than found on most adult, gray-mantled gulls. Looking at the small piece of that bird's tail that we can see, there is also some obvious dark markings on the rectrices, another fine indicator that our bird is not a full adult. This pattern of plumage tells us that it is probably in its third plumage cycle (in third basic plumage; 2.5 years old or so). The fact that the outer primaries do not have obviously pale inner webs rules Thayer's Gull out and the bird's pale eye removes California Gull from consideration. Though one might think about Ring-billed Gull -- there is a dark mark on the bill in about the right place, the mark is not a ring and the bird's bill looks far too large to be that of a Ringer. Just to cover the bases, Glaucous-winged Gull is also ruled out by the wingtip and tail patterns, and various hybrid gull combos (particularly Glaucous-winged x Western and Glaucous-winged x Herring) are probably also ruled out by these characters. That should leave only Herring Gull (the New World form) for our bird's ID, and this bird is fairly typical of a third-cycle individual of the form.

Birds #9 and #10 are quite similar to bird #1, though have more adult-like wingtip patterns (including sporting p10 mirrors), but they're otherwise similar enough for me to be quite happy to slap them with IDs of Herring Gull. That leaves just three birds to go, all with all-dark or mostly-dark tails.

Bird #3 has an overall brown appearance that is typical of first-cycle American Herring Gulls. And of first-cycle Mew, California, and Thayer's gulls. However, the first two of those species are quite rare where Great Black-backed Gulls are common and Thayer's Gull is not exactly a common beastie in those areas. However, it's easy to eliminate Thayer's by our bird's upperside wingtip pattern, though the apparent paleness of the underside of the far wingtip might very well suggest that species. Note, however, that the bird's left p10 is much brighter than the other outer primaries, an artifact created by the fact that it just happens to be lit by the weak sun where the other outer primaries are not. Finally, the bill looks too big to be that of the smaller-billed Thayer's Gull. And, of course, if the bill is too big to be that of a Thayer's Gull, there's no way that the bird could be a California Gull. Additionally, California exhibits darker great coverts (more like those of Lesser Black-backed) and shows little or no window in first-cycle plumage (our bird sports an obvious one like those on the first-cycle Great Black-backed Gulls). Western Gull would have a bigger bill and would show a whiter and wider trailing edge to the wing. That leaves us with American Herring Gull for the bird's ID.

Bird #7 is quite similar to bird #3, though with an obvious pale base to the bill that extends quite a way out the maxilla (at least). Note that one the bird's left wing, we can see the window and that the greater coverts are obviously paler than are the secondaries. These various bits rule out all but Herring Gull, and this appearance is quite typical for American Herring Gulls of that age.

Finally, on to gull # 8! This bird seems to show darker upperparts than does gull #7, though it does seem to sport an all-dark tail like that of #7. The tail pattern certainly rules out Great Black-backed Gull. Looking closely at the bird's left wing, we can see a mix of brown and dark gray feathers, a pattern typical of large gulls in 2nd- and 3rd-cycle plumages, but the pattern -- most feathers being brownish with the median coverts being gray -- is much more suggestive of 2nd-cycle. The nearly all-dark tail, extensive brown in the wing linings, and extensive dark on the bill also suggest the younger age, so I think that we're pretty safe to think of the bird as a 2nd-cycle.

The bill on gull #8 seems smaller than that on the Herring Gull that is gull #7, so that should rule out all those large-billed, dark-mantled Pacific Rim species (Slaty-backed, Western, Yellow-footed) and the similarly large-billed and dark-mantled Kelp Gull. Perhaps this is a vagrant California Gull, as if it's not an Old World species (tail pattern), then that's all that's left. However, the bird is just too much darker than gull #10 and 2nd-cycle California Gulls typically sport absolutely distinctive soft-parts coloration -- an quite odd grayish-blue. Our bird's bill and legs are certainly not that color!

Therefore, something is hinky about this bird. If we can ignore the tail, the bird is a reasonable match for a 2nd-cycle Lesser Black-backed Gull, though that species usually sports a darker bill at that age. However, bill pattern is quite variable in subadult gulls, with, probably, no species being more variable in this regard than is Lesser Black-backed Gull (I have seen 2nd-cycle LessBacks with bills this pale and with bills that were still virtually all black). In fact, this bird IS a 2nd-cycle Lesser Black-backed Gull, but one with a somewhat abnormally wide dark tail band and with an abnormally pale bill for its age. I found this bird on 18 November 2008 behind the Cape May - Lewes ferry on the Delaware side of the trip and studied and photographed it extensively because of the oddness of its appearance. I provide below a better picture of the individual on which one can note the dark brown greater coverts and virtual lack of a window typical of 2nd-cycle Lesser Black-backed Gulls. This bird provides an abject lesson about variability in appearance among individuals of a single species. One cannot really become an expert in bird ID without fully understanding that fact and there is ALWAYS more to learn about the appearance of even the commonest species.

Click the picture for a larger view.

Twelve players provided responses without incorrect answers but without enough correct answers. One person only provided a response with all three species correctly identified, but that player provided for a fourth species not present.

Tallies of incorrect species provided in answers:
Western Gull - 8
Laughing Gull - 1
California Gull - 1
Ring-billed Gull - 3
Thayer's Gull - 2

The 0 of 24 providing the correct answer:

Answer: Herring Gull, Lesser Black-backed Gull, and Great Black-backed Gull