Monday 27 February 2017

American Herring Gulls... or imposters?

I'm too busy at work to get out birding at the moment but the scraps of free time I do have allow me to revisit birds I've planned to post about but haven't. A number of interesting puzzles never make it to the blog because they are overtaken by subsequent birding trips.

On 29 January this year I arrived on the beach in Tsu (Mie) at noon while most gulls were still actively feeding along the tideline and hadn't settled into late afternoon loafing groups dotted along the three-kilometre stretch I usually check. As the afternoon progressed birds began to clump together. If proof were ever needed that the grass is always greener, then the worry that it's always the next flock down the beach that's bound have the more interesting gulls provides it. But frustratingly for the addicted guller there are always new arrivals dropping in on each group plus a toing and froing between groups, so the big find is always just one more short walk along the beach. The beauty of gulling on the Yamatogawa in Osaka is that you can settle comfortably in one spot and let the gulls come to you.

Ultimately the effort was well rewarded and the undoubted highlight was a possible first-winter American Herring Gull. My second possible first-winter of the winter!

American Herring Gull is rare in Japan (how many might go unnoticed is another story) and like other North American taxa, or at least records of them, there is a clear northerly bias to occurrence. Theoretically an adult AHG (I don't have firsthand experience) should catch the eye amongst Vega because of the paler saddle, but I wonder whether this might be less obvious on Kansai's Pacific side because of our generally clear skies and bright sunlight throughout winter. In fact it seems there's been an adult in the area for a while now, though I've yet to connect with it. First-winter types might be more outstanding initially but they could be even trickier to confirm because various features can be shown by three taxa commonly found here.

To identify any potential first-winter AHG these all need to be eliminated; the remarkably variable Slaty-backed, Vega (which can look far more like AHG from my European perspective than do European Herring Gulls argenteus or argentatus), and also some Taimyr Gulls at the brown end of their colour range, these can look very like AHG until they take flight. Then there's the hybrid factor to throw into the mix.

Slaty-backed is an extremely variable taxon at this age, amazingly so in fact, and a few birds do suggest AHG in terms of plumage. However a Slaty-backed's structural differences should be apparent. However any SBG x Vega hybrids could potentially present far more of a challenge. The following hybrid (on this same date) suggested a Vega with a coverts bar when I first saw it, but between bouts of preening (and much to my relief) the head and bill proportions were clearly those of Slaty-backed.

I still haven't seen a certain Vega Gull with such a substantial greater coverts bar but this bird gave me a scare. I may even have passed it over as a Vega had it not been preening and showing off that demanding coverts bar. 

Had I not already seen the coverts bar I could easily have passed this over as a Vega if quickly scanning through the flock, but it had already caught my attention and the head/bill structure and proportions are clearly Slaty-backed rather than Vega. In contrast the long, relatively narrow primary projection isn't something I'd expect to see from any Slaty-backed. In fact, apart from the head proportions and the coverts bar this gull looks a standard Vega.

Coming from the UK, but having left before AHG had been recorded, my understanding of the taxon has been fashioned from reports and identification papers heavily weighted towards east coast birds turning up in Europe, stressing overall brownness, uniformity of underparts and frequently an all dark tail and plain tertials to differentiate them from their European counterparts. This probably isn't the best primer when considering AHG from a Japanese perspective. Firstly, though Vega Gull tends to be slightly greyer than the kind of AHG I'd hope to see, it can be uniformly saturated below, anything from cold grey though pale sandy to brown. The greyish variations are commonest. It can also be heavily barred on the rump/upper- and undertail coverts and can more often have an almost entirely dark tail. But reading-up on smithsonianus, I'm led to believe some west coast birds can be less the uniformly brown gull I'd be expecting in Europe; greyer and less uniform overall, some showing more proximal white in the rectrices and  generally edging towards Vega in appearance. Perhaps nailing a first-winter AHG in Japan could be even more difficult than it might be in Europe.

Possibly the most important single feature to separate Vega from AHG is the presence or absence of a prominent greater coverts bar, it's most certainly the feature that's going to grab attention both in flight and at rest. My greatest fear is that some Vega Gulls could show an equally strong coverts bar, I've yet to see an unequivocal Vega with a very prominent bar but such birds could exist. Below are a couple of images of Vega showing a suggestion of a coverts bar, this by no means the worst offender I've seen but, again, they are images of a same-date bird.

It's normal for Vega to have a small group of brown outermost feathers, this bird also has three coverts where the visible dark spots visibly link up to form a solid internal area however there's still a broad white fringe. There's also a certain amount of peppering towards the bases of the outer feathers creating a slightly muddy appearance.

The right wing is distinctly darker but hardly qualifies as a coverts bar. I have seen birds with a more prominent bar than this and the questions really are how substantial can a Vega bar get and how rare are such birds? 

The first of my possible AHGs, and much the better candidate, was two months earlier back on 18 November. I only saw it briefly late in the day and unfortunately missed it fly off while I was changing battery in the camera!

Even head-on, and despite the overlapping flank feathers, dark greater coverts are clearly visible.

Second generation scapulars are already in place and the pale patches on the breast are probably also evidence of moult. The underparts are uniformly dark brown, the vent and undertail coverts densely barred. The solidly dark outer greater coverts and more heavily marked bases inwards, along with the carpal and lesser/marginal coverts create a strong frame to already dark median coverts panel. To me this as much as anything suggests this really is AHG rather than Vega.

A glimpse of the very dark undertail coverts. A few Vega are equally dark.

A 'good' coverts bar, densely barred rump and uppertail coverts plus almost solidly black tail. Because each individual feature can be matched by Vega (apart from a strong coverts bar, I hope) a perfect storm of features on a single gull may make all but the most extreme AHG unidentifiable in Japan. That said, a Vega showing a full suite of AHG features is likely to be much rarer than the real thing!

A closer view and there's no doubt the vent is solidly dark brown and the rump/uppertail coverts show more brown than white.

It looks a particularly long-winged gull at rest, longer at least than the average Vega, but that's pretty subjective. I haven't mentioned the rather uniform blackish tertials as that's yet another feature Vega can match but there's a very obvious difference between this bird and the first-winter Vega in the background. The Vega in the foreground is a sub-adult hence the full primaries at this date (18 Nov), the adult at the rear is typical at this time, as is the adult Taimyr with its retained outer primaries and new inners not yet visible beyond the tertials.

I've significantly lightened this image to try to show the underwing coverts.

And another lightened shot here.

The other taxon to eliminate is Taimyr Gull. Like most, if not all, large gulls they range from overall grey colouration to overall brown. Below is a bird very much at the browner end of the colour range.

On the ground this Taimyr shows a striking superficial resemblance to AHG, it also has a strong brown frame to dark median coverts but even in this partially obscured view the rear belly is contrastingly pale, unlike AHG.

The vent is whitish and sparsely marked.

Once in flight things become much easier. The whiter vent with large brown spots in the longest undertail coverts forming a horseshoe seems typical of the Taimyr I see here.

The rump and uppertail coverts are also mainly white with a horseshoe of bold arrowheads around the uppertails and lines of arrowheads in the middle; no hint of cross bars. The out rectrices are basically white with sub-terminal black, the reverse of AHG. The inner primary window is very subdued with the wings less spread.

The primary window is fairly dark with a strong contrast between the inner and outer webs.

Of course the lack of obvious window and the lack of venetian blinds in the outer primaries also rules out Slaty-backed Gull. So does the long-winged structure of course. If anything you could say Taimyr has the so-called venetian blind pattern on the inner primaries.

So now coming to the recent bird. It would fit in nicely with many images of AHG in California I've seen online. But making a strong case for a bird like this in Japan might be a lot to ask. In the field (29 Jan) I felt it was a strong contender for AHG but checking the images at home doubts crept in. However now that I'm preparing images for this post I'm swinging back towards AHG and feel this meets the criteria for identification as such. Obviously it would be unusual as a Vega otherwise it wouldn't have caught my attention and suggested AHG in the first place. That said, to be 100% certain about the identification is an entirely different matter.

Even at first glance Slaty-backed can be ruled out structure, that long primary projection is wrong. It isn't a Taimyr as the retained scapulars will attest. So Vega or AHG? 

On the pro-AHG side it has a very prominent dark frame to the median coverts panel with a strong greater coverts bar and very dark, uniform marginal coverts. The upper- and undertail coverts are no more heavily barred than many Vega but wouldn't look out of place on many AHG either.

I doubt many Vega (if any?) will have as clear-cut a coverts bar as this. I certainly hope not anyway, put that on a dark Vega and identifying AHG in Japan just became a lot more difficult. 

Even though most  single features wouldn't be out of place on Vega the combination, on top of the bold coverts bar, definitely make this a gull of interest. 

The undertail coverts and vent are heavily marked but the bars are also widely spaced.

The vent is an even brownish. The tertials could go either way but those of Vega are usually far more notched than these. Likewise the primaries aren't convincing; though AHG is meant to have a longer, blacker primary projection I'd defy them to be any blacker than some Vega I see.

A good view of the forewing, the marginal and many lesser coverts are uniformly dark unlike Vega.

My best flight shot; more AHG than Vega on this view. Nevertheless, all I can say with certainty is that the uniform primary window (no contrast between webs) is quite unlike any Taimyr I've ever seen.

So, how many American Herring Gulls? Two...  one...    or none?

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