Unidentified dark-mantled gull at sea off New York, 11 Feb. 2006

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Discussion

Co-observer Shai Mitra kicks off the discussion with a summary of the leading candidates. These are presented in no particular order but clearly some of the possibilities are better supported than others.

Analysis of ID candidates for a dark-mantled gull observed on a Long Island, New York pelagic trip, 11 Feb 2006


Lesser Black-backed Gull x Herring Gull (LBBG x HEGU)

Known and suspect hybrids of this kind have not shown bright pink feet and legs. If the LBBG parent were graellsii, one would furthermore expect a mantle color much paler than that of the NY bird, and a rather different wingtip pattern (both putative parents generally show mirrors on P9 and less extensive black on, for example, P7 than shown by the NY bird). If the LBBG parent were intermedius or fuscus, one would expect to see at least some structural tendency toward the LBBG side of Herring Gull: smaller body size, slimmer bill, longer and more slender wings—rather than the opposite, as seen in the NY bird.

Great Black-backed Gull x Lesser Black-backed Gull (GBBG x LBBG)

Both of these taxa show red orbital rings, and neither has bright pink feet or legs. The notion that the hybrid young of such a pairing might inherit brightly colored legs from graellsii and pink color tone from Great Black-backed Gull is possible, but it is purely speculative and contrary to the tendency for leg color to fall within the range of variation shown by parental types in the hybrid young of large gulls (e.g., Lesser-blacked x Herring and Kelp x Herring). Both graellsii and (especially) Great Black-backed Gull differ from the NY bird with respect to wingtip pattern. If one parent were intermedius or fuscus, one might expect to see a wingtip resembling the NY bird, but one would then expect a mantle color as dark or darker than Great Black-backed Gull.

Slaty-back Gull (SBGU)

Compared to the NY bird, Slaty-backed Gull shows a very different wingtip pattern: a mirror is often present on P9; long gray tongues penetrate the subterminal black portions of PP6-9; and these tongues are often capped with white crescents on PP6-8 (or even P9). This species typically does not show as pronounced a gonydeal angle as is present on on the NY bird. Winter-plumaged Slaty-backed Gull shows extensive head streaking. Adult Slaty-backed Gull shows a very pale eye that often appears relatively large and centrally placed on the head.

Great Black-backed Gull x Herring Gull (GBBG x HEGU)

The wingtip pattern of the NY bird is unlike both putative parents, regardless of whether one invokes smithsonianus, argentatus, or argenteus as the Herring Gull parent. In particular, both Herring (broad sense) and Great Black-backed Gulls show a high frequency of white mirrors on P9, and, compared to the NY bird, both of these species show obviously restricted black subterminal areas on PP6-8 (and even P9, in many Herring Gulls). Also, Great Black-backed lacks black on P5, Herring Gull tends to show less black on P5 than is shown by the NY bird, and neither Herring nor Great Black-backed is expected to show any black on P4. Furthermore, neither putative parent typically shows bright pink feet. Finally, Herring Gulls at least (and probably Great Black-backed Gulls too) show narrower white trailing edges to the wings and smaller tertial crescents than observed on the NY bird.

Kelp Gull x Herring Gull (KEGU x HEGU)


Descriptions and photos of F1-type hybrids in Dittmann and Cardiff’s (2005) account of “Chandeleur Gulls” come much closer to matching the wingtip pattern observed on the NY bird, but it seems that the various hybrid phenotypes are likely to differ from the NY bird by showing one or more of the following wingtip characters: a differently shaped white mirror on P10, a white mirror on P9, or a long gray tongue capped by a white crescent on P7. Significantly, the various hybrid types described by Dittmann and Cardiff, including presumed F2s and Herring x F1 backcrosses, show yellowish or greenish legs. This is important because even individuals with mantle shades very close to smithsonianus are described as lacking the pink tones typical of smithsonianus—let alone the bright pink tones present on the NY bird. Finally, F1 hybrids (and presumably also Kelp- like backcrosses) are described as showing whitish irides, reddish-orange orbital rings, and large red gonydeal patches—in each case differing from the NY bird.

Western Gull (WEGU)

The NY bird matches Western Gull in many respects. Although some observers felt that the NY bird did not present the large, bulky impression they associate with Western Gull, Western Gulls are roughly the size of Herring Gulls and probably appear especially large in the west because they (and the similarly built Glaucous-winged Gull) are often the largest gulls present. When thinking of the NY bird as a Lesser Black-backed Gull, it seemed too big and stocky, heavy-billed, and not sufficiently long- and slender-winged for that species. It seemed roughly similar in overall size to Herring Gulls (as a Western should). Significantly, the NY bird was frequently briefly mistaken for nearby Great Black-backed Gulls (and vice versa) by many observers—something one would not expect for any kind of Lesser Black-backed Gull or hybrid thereof, but rather plausible for a Western Gull. Western Gull tends to show a small, beady eye placed high in the head and a heavy bill with a striking gonydeal angle. These features seem quite consistent with those shown by the NY bird. Western Gulls show very little head streaking, even in winter plumage. The NY bird showed absolutely no head streaking, nor any sign of dark markings on tertials, primary coverts, or rectrices. A very small dusky spot just distal to the red gonys spot is visible in photos, but does not suggest immaturity.

Given that the NY bird can be confidently regarded as an adult, its similarity to adult Western Gull in terms of soft parts coloration and wingtip pattern is especially notable. For instance, it seems very unlikely that its extensively black wingtips could be attributable to immaturity in an individual derived from Great Black-backed, Slaty- backed, Herring, or British Lesser Black-backed Gulls. The NY bird’s soft parts colors matched those expected for Western Gull as follows:

-- Bill bright orange-yellow with small red spot on gonys (red gonys much more restricted than usual for LBBG).
-- Iris appeared dirty yellow in the field, not clear pale yellow/whitish as in LBBG or “Chandeleur Gull.” We think the photos actually show this, but they are somewhat difficult to interpret in this regard.
-- Orbital ring orange-yellow, unlike condition in Great Black-backed, Kelp, Slaty-backed, and Lesser Black-backed Gulls.
-- Feet deep, bright pink; legs duller pink, perhaps with subtle orange tones.

As do most of the candidates already discussed, Western Gull might easily match the mantle color documented for the NY bird. SSM’s original notes were: “Mantle color like LBBG but perhaps slightly darker than classic graellsii.” [Frank Gallo's notes described the mantle as "charcoal gray".] On posted photos, we agree that several images show a slightly paler mantle tone than our impressions in the field. Like the NY bird, Western Gulls show extensively black wingtips, with readily discernible contrast between the black subterminal areas and the dark gray of the remainder of the upper wing. A white mirror in P10, bisected by the feather shaft, is depicted for Western Gull in Malling Olsen and Larsson and numerous photos. The NY bird’s P10 mirror presents a very close match, whereas this condition is inappropriate for Great Black-backed Gull and many presumed Great Black-backed x Herring Gull hybrids described (in litt.) by Bruce MacTavish. Western Gulls generally lack a white mirror in P9, as does the NY bird. In contrast, Great Black-backed Gull, many Slaty-backed Gulls, smithsonianus, graellsii, and at least some Kelp x Herring Gull hybrids show such a mirror. In both Western Gull and the NY bird, PP7-9 lack long gray tongues capped by white crescents proximal to the black subterminal portions, unlike the conditions shown for one or more of these feathers by Great Black-backed, Stay-backed, and Herring Gulls.

According to Malling Olsen and Larsson, Western Gulls of the subspecies occidentalis show P4 “sometimes with dark subterminal spot near tip,” raising questions concerning the blackish subterminal marks visible on the NY bird. Given that the pattern of P4 is variable within occidentalis, and that wymani tends to show more extensive black on the primaries (including P4) than typical occidentalis, this feature of the NY bird does not appear to be counter-indicative of Western Gull. Like Slaty-backed Gull and the NY bird, Western Gull shows a notably broad, white trailing edge to the wing and a very large tertial crescent when resting. This would not be expected for hybrids involving Herring Gull and either Great Black-backed or Kelp Gulls. (13 Feb 2006)

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David Vander Pluym (Santa Cruz, California, USA) writes: This looks like a good Western Gull to me, from one of the more northern populations. I feel your discussion has already hit the main points, but I wanted to add two things. Having seen both Slaty-backed and Lesser Black-backed Gulls (graellsii) in flocks with Western Gulls, one thing that really stands out is the mantle color. The shade can be very similar between these three but while Slaty-backed and Lesser Black-backed show more of a flat slate gray, Western Gulls show a bluish tone to them. I believe I can see this tone in the photographs especially the one compared with the Great Black-backed Gull. The 2nd point is that the last photo for what its worth seems to show the secondary skirt. An amazing find! (14 Feb 2006)

Bruce Mactavish (St. John's, Newfoundland, Canada) writes: Very exciting gull when you consider the prospects of Western Gull. This didn't occur to me at all when I saw your photos. I don't know Western Gull. But I must say your bird looks excellent for Western. It will be interesting to hear what the west coast birders have to say. In the meantime here are couple photos of a strange GBBG type hybrid thing from St. John's, Newfoundland on Feb. 5, 2006. Difficult to see in the photos was the smaller, slimmer build. The wing tip pattern is interesting. The mantle was paler than a GBBG. Haven't been able to find a 3rd or 4th winter GBBG in this plumage. We joked that this was a GBBGxLBBG hybrid mainly cause we couldn't fit it anywhere else. I am sure there is a better explanation.

I have some other GBBGxHEGU photos. One from this past Sunday was like typical GBBG except paler, same GBBG P10 and P9. Orbital ring was yellow-orange like a Herring. We also get GBBGxGLGU hybrids, in fact more than GBBGxHEGU suspects so it is sometimes it is impossible to be sure if it is GBBGxHEGU or GBBGxGLGU.
(14 Feb 2006)

Alvaro Jaramillo (Half Moon Bay, California, USA) writes: I must say that if this bird was in my local flock, it would be passed over as a regular old Western Gull without any doubt. I was looking at some flight shots I have and the extent of dark to P4 is certainly not the common pattern, but I think I have one photo which shows a dark bit on P4. The only other somewhat atypical issue is that the orbital looks too orange. This may be an effect of the photo and would not kill Western as a possibility, but classically the orbital is pretty much the same color as the bill, a bright yellow. Orange in orbital is not as bad as pink in orbital, which is a clear suggestion of Glaucous-winged genes, your bird does not show that pink I associate with Glaucous-wings. I wonder if orbital ring colors may be inherited in a simple mendelian dominance pattern as Glaucous-winged pink shows up on hybrids much more than Western Gull yellow. But I am digressing. I am one who believes that Western Gull is about the least likely gull to wind up in the east, but I can't see anything on your bird that would allow me to take Western out of the pool of candidates. It indeed looks like a perfectly normal Western Gull to me, as hard as it is for me to believe that. Maybe when I get some more time I can post some of the WEGU flight shots I have from the local flock. Several show the same small pale tongues on P4-5, less so on 6. (14 Feb 2006; posted 16 Feb 2006)


Figure 1 and 2. Two different adult Western Gulls photographed in Half Moon Bay, California showing a dark subterminal bar on P4. Note the very broad white trailing edge to the secondaries and inner primaries. Photo Copyright © Alvaro Jaramillo 2006.

Ottavio Janni (Italy) writes: I looked at the photos of the NY gull with great interest. My comments below are only half-serious, and are not meant in any way to cast doubt on the possibility of it being a Western Gull (I don't have enough experience with this species to make meaningful comments), but I was wondering if Yellow-legged x Great Black-backed Gull was taken into consideration. I’m not sure whether this hybrid combination has been reported before, but the breeding range of these two species overlaps in Atlantic France, and apparently they are quite closely related genetically, so it seems plausible.

Size and structure of the NY bird seem intermediate between the two species, the mantle color described as slightly darker than graellsi is also intermediate between the two, both species would show a bright white head in February, the wing pattern looks superficially quite similar to YLGU (mirror on P10, many YLGU have no mirror or a tiny mirror on P9 like the NY bird, the extent of black on P7 & P6 is perhaps a bit much for most YLGU but the black on P5 looks quite similar to that of YLGU, while black on P4 would be atypical for YLGU but not out of the question). I think many YLGU show an orangey orbital ring similar to that of the NY bird. The bill pattern also seems intermediate between YLGU and GBBG. The leg color doesn’t seem right, although YLGU can at least very rarely show bright pink legs (I photographed one such bird near Salerno in southern Italy last year, on Jan 31 – the date is perhaps not coincidental since late winter is the time of year when soft part colors in YLGU are at their brightest and most saturated) and the same reasoning made for GBBG x LBBG in Shai Mitra's comments could apply to YLGU x GBBG. The broad white trailing edge to the secondaries is another strike against a YLGU x GBBG hybrid. Still, of all the hybrid combinations taken into consideration, YLGU x GBBG might be the one that would most closely approximate the NY gull, and although the comments above are perhaps way too speculative to be useful, I figured this hybrid combination at least deserves to be taken into consideration. (14 Feb 2006)

Floyd Hayes (Hidden Valley Lake, California, USA) writes: Yesterday afternoon I looked at many Western Gulls (hoping to find a Slaty-backed) on the Sonoma County coast in northern California (well north of the Slaty-backed hotspot) and would concur that your bird looks very, very similar to a "pure" Western Gull, although usually they have a darker iris with a beadier looking eye (but many do have a paler iris like your gull). If I encountered your bird here I would have dismissed it immediately as just another Western Gull. You might want to point out that any hybrid combination with Lesser Black-backed Gull, Yellow-legged Gull, Herring Gull or Slaty-backed Gull might be expected to have streaking on the head, unlike your immaculately white-headed gull. (14 Feb 2006)

Bruce Deuel (Redding, California, USA) For what it's worth from an experienced birder but no gull expert (at least outside California), when I looked at the pictures before reading a word of the discussion, I said "Sure looks like a perfect Western Gull to me." Nothing about this bird would be problematical for that species if seen here in California. (14 Feb 2006)

Jay Withgott (California, USA) writes: Wow! Dead ringer for Western Gull. Looks perfect; I see nothing that would contradict that identification. As to whether it's a southern- or northern-race bird, the uncertainty with photo exposure (despite the excellent quality of the photos) leaves that uncertain for me, though it looks maybe northern-central California-ish.  However, if some observers on the boat were confusing it with Great Black-backeds, that suggests to me that it's a southern bird. Northern birds don't seem close enough to GBBGs in mantle color that they would be easily confused. Spectacular find!
(15 Feb 2006)

Dennis Paulson (Seattle, Washington, USA) writes: The second I saw the first photo I thought "Western Gull," and I never deviated from that thought as I looked down the line of photos and read all the comments. I agree with you that it doesn't look dark enough for wymani, and occidentalis certainly is a more likely vagrant to the east coast. I couldn't see how to send my comments to the web site, so the best I can do is post them here. Not that there are very many comments to post. I can't see a single thing about the bird that contradicts the idea that it is a Western Gull. I assume all of you know about the Slater Museum wing collection online; Western wings are at http://www2.ups.edu/biology/museum/gullwings2.html. By the way, we are redoing that web site in different format to make it "slick" as a web site is supposed to be, not the "mom and pop" approach I used for years. (15 Feb 2006)

Phil Pickering (Lincoln City, Oregon, USA) writes: I've been studying this gull and I think it may be OK for wymani [i.e.Western Gull], but I wanted to point out that it appears to be off somewhat for occidentalis, or at least for the northern version. The mantle appears too dark compared to the Herring, the under-secondary bar is so dark and defined that it seems to show almost no contrast with the under-primaries, and most occidentalis around here have clean P4s, or at most show a gray spot on the outer web. My impression is that solid black marks on both P4 webs is uncommon at best in occidentalis, and is probably rare. Of course I cannot speak from experience for wymani, and O & L note solid black on P4 as apparently within range for that form (presumably from study of skins, I cannot find any photos that show it). So the key may be checking the bird specifically against southern Westerns. This might not be a big deal because it can certainly be argued that it may be more likely for a southern Western to reach the Atlantic via Mexico or further south and then wander north, than that an ocean-loving northern Western would somehow cross the continent. Perhaps wymani is the more "expected" form if one is going to show up in the east. Otherwise in pattern and structure it seems like a pretty typical Western, albeit with a proportionately large-end bill. I can't pick out anything off center enough to raise statistical concerns. The black spot distal to the red on the gonys is not atypical in winter, although any orange coloration to the legs should raise concerns (mentioned in the written description, I don't see any in the photos). (15 Feb 2006)

Steven Mlodinow (Everett, Washington, USA) writes: Ahhhh. But can any gull be typical. Per Bell's article in the Condor (98:527-546), eye ring color clearly separates GWGU from WEGU, being yellow in WEGU and pink in GWGU. Indeed, this mark breaks sharply at the species' line, more so than does mantle color and a number of other color or morphology based characters. So, per Bell's article, the NY gull shouldn't be a Western Gull. On the other hand, in La Paz just a couple weeks ago, I saw a bird that looked typical for a wymani WEGU (i.e., clearly not a YFGU), with pink legs, smaller-than-YFGU bill, not-quite-white eye, etc. And it had an eye-ring color essentially the same as the NY bird. So, though Bell's work seems to show that the eyering color is diagnostic and yellow in WEGU, it seems to me that is not strictly the case. Another example where no individual gull is "typical" in all aspects. I agree with everyone else that the NY gull is a WEGU. And, as a last little tidbit, the one IL Western Gull speciman was identified as a wymani, not occidentalis. It was collected on 19 Oct 1947, called a "near adult," and is housed at the Chicago Academy of Sciences (#1663). I do not know the basis for that ID. (15 Feb 2006)

Bob Lewis (Sleepy Hollow, New York, USA) writes: With all the interest in the recent possible Western Gull in New York, it may be a good time to solicit opinions of the North Carolina bird from 1995. Perhaps a comparison would be enlightening. A description and photos of the bird are on my web site, http://www.bway.net/~lewis/birds/gulls.html and
http://www.bway.net/~lewis/birds/Western/western.html
The photos are not up to recent standards, but many key features can be seen. The orbital ring was yellow-orange.
In the intervening years I have occasionally received emails about the bird, maybe half a dozen, and all were to the effect that the bird is Western Gull. Ten years ago at least one respected west coast observer thought it was probably a hybrid. Quoting from the web page: "I took some photos of this bird on March 14 and 15, 1995 at Hatteras. I wrote an article about it that was published in the Chat in the fall of 1996 in which the bird is identified as a Western Gull (Lewis, R. H. 1996. First North Carolina record of Western Gull. Chat 60.4: 149-153). I still think that is what it is, and at least one West Coast expert agrees, but I have never been positive about it. After the article was published, I learned that it had not been refereed in the manner that I expected. I also learned then that the North Carolina Records Committee had not yet acted on the bird. That too was a surprise to me. Later yet, the Committee rejected the record because they did not think hybrids were sufficiently eliminated." (15 Feb 2006)

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