Heuglin's Gull: Do they occur in Japan, and have I seen one?

Across twenty-odd years of gulling here, I've seen a very small number of gulls which looked good contenders for Heuglin's Gull L. (fuscus) heuglini. However, can we ever be a hundred percent sure about this identification in Japan? Might not a Taimyr Gull L. (f) taimyrensis with a 'perfect storm' of heuglini-like features be impossible to eliminate without DNA confirmation? This has been a perennial frustration, a question to be pushed to one side for however many winters before another suspect re-centres it. This winter has seen another.   

It isn't even possible to ascertain how often Heuglin's Gull is claimed in Japan, let alone their true status if they do occur. Images of some claims inevitably come to attention online, some look good, others are less convincing. Although judging online images adds another layer of incertitude. 

If it seems surprising that Heuglin's could turn up with any regularity at all, given their expected wintering range, no more so than Iceland Gull L. glaucoides kumlini (also?) being a somewhat regular vagrant. And unlike the distinctive and eye-catching kumlini, Heuglin's runs the risk of being either over-recorded on one hand, because of optimism and/or confusion over taxonomy, or under-recorded on the other due to the ease with which this taxon could slip under the radar. The latter not least because of the relative paucity of gullers in Japan; you're far more likely to see non-birders taking pictures of 'seagulls' on the beach than find a birder going scrutinizing a flock. 

On 22 November, an adult gull knocking on the dark-end door of the Taimyr grey-scale range stood out amongst a flock of about 80 gulls, numerically, Vega just edged a near 50/50 split with Taimyr. After securing a couple of distant record shots, I approached to scope it from about 50 metres. 

Possible adult Heuglin's Gull

The first thing to catch the eye was the dark grey saddle (Fig 1). Once focused on the gull, the combination of long, sloping forehead and long bill, the slim build and flat back seemed to justify the attention, definitely suggesting this had the potential to be something other than the usual Taimyr. Additionally, the rather aggressive facial expression struck a jarring note with the gender expectation normally associated with small size and slim build. The saddle colour was also of interest as wear on the scapulars and coverts created a variably brownish, grey-brown or even silvery wash depending on the angle of view and lighting, but basically giving a 'dirty' appearance. This is something which is never obvious on Taimyr. Yes, worn Taimyr do share this same effect produced by wear, but there's far less contrast against the paler ground colour, making it apparent only when looked for. The brown wash was immediately noticeable on this dark-saddled gull.

Size and structure

The bird was small compared to most gulls present, but more striking was the slim build, accentuating the attenuated appearance. It also gave the impression of being long-legged compared to Taimyr, which tends to be a short-legged taxon. Indeed, some Taimyr look remarkably short-legged but this isn't always the case so I wouldn't put too much emphasis on this point. The slim appearance was in part due to its flat back and absence of tertial bulge, but it must be pointed out the upper tertials had been dropped which may have enhanced this impression. When the bird adopted an upright stance, the angle from hindneck to scapular crescent was very steep, and then, with no tertial bulge, there seemed a gentle concave sweep through to the primaries tips (Fig 4). The forehead was long and sloping, peaking behind the eye, the long bill with shallow gonydeal expansion added to a long-headed look (Fig 2); Taimyr is typically short-billed and rounded-headed, especially the smaller individuals, while large individuals often have more angular heads and strong bills with a deep gonydeal expansion.

Plumage

Head white. Fine, pale grey flecks across the forehead and lore coalescing in a dark smudge before eye. Flecks becoming fine dashes around eye highlighting a white upper 'eye crescent'. Crown and hind neck finely streaked pale grey. Base of hind neck more heavily marked with grey, creating collared effect. Conspicuous grey spots spilled onto sides of breast and smaller spots across centre of breast. Saddle grey (I'd judged similar to graellsii albeit without a graellsii for comparison). Wear on scapulars and coverts producing brownish, grey-brown or silvery wash to these tracts (Figs 6-7). P10 mirror restricted to the inner web on left wing, extending onto outer web on right wing (Figs 8-9). Upperside p9-10 black to the coverts (no mirror p9), p8 black to coverts on outer web, grey tongue (probably) extending for just over half the feather length (how much of the worn tip is missing would affect this slightly), p7 black almost to the coverts. P7-8 both with very narrow 'fingernail' white separators between grey tongue and black tip (Fig 9). P5 with complete broad sub-terminal band extending up leading edge. Still growing, it was slightly shorter than p4. P4 with prominent black notch on outer web. P10 underside tongue about a third length of feather, angled diagonally across, p7-9 tongues 'U'-shaped (Fig 8). Underparts white. 

Bareparts

Bill yellow, not washed out but becoming white at the tip. Red gonys spot extensive, covering over a third of lower mandible (Fig 5). Legs orange-pink to pale yellow or straw, depending on light and angle. Eye very pale yellow with limited speckling. Orbital ring deep red.

Moult

Slightly late for the typical Taimyr, but not significantly. Primaries slightly behind average but within expected variation, and coverts a little further behind. The inner four primaries are new, p5 is almost in place. Only a 5-6 dropped greater coverts on the right wing, even fewer on the left, plus a few primary coverts are the only obvious signs of moult, Taimyr normally looks obviously 'patchy' at this time of year, with exposed white feather bases dotted around the wing; very often the white trailing edge of the secondaries is exposed at rest due to missing greater coverts. The uppermost tertails have also been dropped. 

While none of these features are diagnostic of heuglini, nor are any contrary to what might be expected either; collectively they make a good case for this bird being a Heuglin's Gull. I believe this bird would not suggest anything other than Heuglin's within the taxon's normal range and would not stand out within a flock, whereas it certainly does stand out within a flock of Taimyr Gulls.  

Fig 1: The bird at the rear right. Even viewed at a distance the slim, flat-backed appearance is different to all the other gulls present. The difference in head shape can also be guessed at even in this view. The dark gull on front right is Black-tailed.  

Fig 2: Though the foreground Taimyr is out of focus, this shot nevertheless shows the difference in head and bill shape, the slimmer, leggier look and the steeper-angled, flat back to good effect. A grey-brown wash can be seen on the scapulars and coverts. Though mostly obscured, there is more black abutting the coverts at the base of the primaries compared with the Taimyr. A large stretch of the white secondary trailing edge is exposed by missing greater coverts on the front bird, this is typical of Taimyr at present. The bird in question, with very few missing greater coverts, does not expose the secondaries to the same extent and looks much neater. 

Fig 3: Slim, elegant gull compared to bulky Vegas.

Fig 4: Very straight upper bodyline. Long, sloping forehead peaking behind the eye. Bill long, without prominent gonydeal expansion.

Fig 5: Bill yellow with white tip. Gonys not deep but with extensive red. Eye very pale yellow.

Fig 6: The 'dirty' wash across scapulars and coverts (except fresher looking medians...) quite obvious.

Fig 7: Scapulars and coverts variably washed brownish or grey-brown creating 'dirty' appearance.

Fig 8: P10 tongue about a third length of feather, diagonal across inner web. Single mirror restricted to inner web on left wing, extends to outer web on right wing. 

Fig 9: Black of outer webs extends to coverts p8-10. P6-7 are missing, p5 is almost as long as p4; note extent of black up leading edge of what appears to be p4 actually belongs to the growing p5, nevertheless p4 has a substantial black notch.

Fig 10: This image conveys something of the overall appearance of the bird, the slim and attenuated shape and the long head with relatively long bill lacking a prominent gonydeal expansion.   

Fig 11: A long-winged gull. The extent of missing greater coverts on the right wing is easily seen here, the left wing has dropped fewer, and there are no other areas of white exposing feather bases in any of the other covert tracts.

This is point I normally push the record aside. I've got a better set of images of this gull than any previous individuals that caught my attention but I'm still no closer to answering the question posed in the title of this post.