As a Brit birder who has otherwise never seen Thayer's or kumleini, knowing the identification headaches they can cause and having locals tell me we're too far south for Thayer's isn't encouraging. I wouldn't be in the least surprised to be told my tentative identifications are wide of the mark and that I still haven't seen Thayer's in Osaka.
Checking blogs and websites in Japan almost all Thayer's are reported from mid-winter onwards, early winter reports seem rare. The earliest sighting I've had of a possible Thayer's was on the very early date of 26 November (2010). At the time I though the bird was probably a hybrid of some sort but I'm no longer so sure, if it were a hybrid it doesn't match the typical hybrid combinations we see in Japan. I have seen Glaucous x probable Vega hybrids that are superficially similar but they invariable have some tell-tale signs as well as being significantly larger. It isn't a "classic" Thayer's either, particularly from a Japanese perspective where delicate, faded females seem to be preferred, but within its normal range it might not raise quite as many questions. The following images are of that bird.
From below in neutral light the primaries look pale with a distinct trailing edge of dark tips on the outer feathers. Even on this view the dark outer web of p10 contrasts with the inner web. The body is uniformly mottled greyish-brown and the underwing coverts are brownish with darker brown tips. The vent and undertail coverts are white with brown barring throughout. Even without any comparison it has the feel of a smallish, more slightly-built gull than Vega.
Fine dense barring across the rump and heavier barring across the uppertail coverts. The tail pattern isn't typical Thayer's, leaning towards kumleini, but is perhaps within the range of variation?
At rest it's distinctly smaller than Vega, which makes a Vega x Glaucous/Glaucous-winged hybrid seem unlikely. Much frostier than 1CY Vega with prominent white fringes to the primary tips. It is also paler than Thayer's as I understand them, certainly the vast majority at least.
A heavily cropped shot showing scapular, coverts and tertials patterning. The outer webs of p6 and p7 and faintly both webs of p8 have pale indentations behind the tip almost producing sub-terminal arrowheads perhaps more typical of kumleini. I suspect the pattern of these feathers is within the range of a pale Thayer's. I wonder how differently these features might be interpreted were it seen on either the east or west coasts of North America.
Blunt, shortish bill and straight forehead create a very different impression to that of Vega. Structurally the gull is much bulkier than Thayer's usually posted on Japanese websites but presumably it could be a chunky male. The primaries and tertials are both browner than Vega, the white-fringed tips were visible at long range.
As I said earlier, I though this was a hybrid when I first saw it even though it doesn't seem to fit any of the local hybrid appearances. With the passage of time my opinion has swung towards Thayer's, any hybrid origin seems more likely to involve North American taxa and perhaps an atypical Thayer's is a simpler explanation.
The bird below was seen much later in the winter, 18 & 20 March 2012. I have encountered a handful of suspicious birds in March but the usual problems with distance and lighting make this the only gull I have any decent shots of. This is a suitably small bird, short-legged with a sloped forehead and rounded crown. The primaries are white below and brownish above with pale inner webs to p10. Again, if not Thayer's, only a hybrid could account for the combination of features but it's difficult to imagine which progenitor pairing could be responsible.
There are some very small Vega out there but they do have typical Vega structure however small. This gull is not only small but has a slightly different head and bill shape. The head streaking is fine and dense producing a "smooth" appearance with saturated lores at any distance. Even the dark eye-line doesn't impart the aggressive mien the feature often does with larger gulls.
The legs are rather short (tarsus and visible tibia) so the wing tips almost touch the ground when standing upright. The visible portion of p10 is surprisingly white given the shaded position over gravel.
In profile it has a short-legged, attenuated appearance.
In neutral light this is clearly a white-winger. The dark outer webs of the primaries and the dark tips of the upper surface of the inner webs are shown to good advantage.
In compared to the previous shot, light coming through the far-wing shows how the pale inner webs contrast with the dull leading edges and still the tips are clearly darker. Also the near-wing looks typical Thayer's here. Again, note the gentle head and slim bill.
The light build, white-tipped brownish outer primaries and pale window with bold sub-terminal spots all look good for Thayer's.
More flight shots at different angles all show Thayer's wing pattern. The tail is brown with white tips and white notches on the outer web of t6, this seems to hit the spot too.
The following two birds, represented by awful quality video-grabs from ancient equipment, were present in March and early April 2006.
I'd always done my gulling on the Yodo River in Osaka but found out that the Yamato River was a better location. Mark Carmody and I made our first trip there on 12 March but unsure of where we needed to be we took a train to a point too far upriver, it was only just beginning to broaden at that location, and distinctly gull-less. While pondering how best to make our way down stream through the city streets a juv/first winter white-winged gull glid by. Unbelievable, the only large gull we saw there! It landed on the exposed sandy riverbed but by the time we'd negotiated a block or two to get a better view the gull had disappeared. What an introduction to the Yamatogawa.
Though it's impossible to know which of the following duo that first lucky encounter had been with, on the 15th I found a gull I identified as Kumlien's down towards the river mouth. Not unreasonably I thought this must have been the bird we'd seen three days earlier. But a few days later I was off to Amami for a week and didn't get another opportunity to get back to the Yamatogawa until 2 April when Mark and I found the other bird. I was amazed to see a second gull and if Mark was sceptical, he didn't show it. Back again on the 9th and it was the turn of the kumleini to be on show again and for Mark to be amazed there were actually two gulls putting in alternating appearances.
Coming back to the awful images, I'll start with the more Thayer's-like of the two gulls. The question being, is it a dark kumleini or a very pale Thayer's? Could it even be an intergrade between the two? I think the structure is more Thayer's-like, also the primaries are very dark for this late date (2 April) and will presumably have been even darker earlier in the winter. It also clearly shows darker tips to the undersides of the outer primaries. On the other hand the tail pattern might be better suited to kumleini.
The extent of visible tibia is variable depending on stance but the tarsus looks rather short compared to local gulls.
The chequered and barred tertials, tail pattern and white nipping behind the dark tips in the primaries all look more kumleini-like than suggestive of Thayer's. The greater primary coverts are much whiter than I'd expect for Thayer's.
The primaries have narrow dark internal markings because of extensive pale fringes to the primaries. This looks different to the two-tone light/dark contrast producing the venetian blind effect of Thayer's.
Distinct dark tips showing through the primaries on the far-wing in this view.
Unquestionably a white-winger.
The other bird looked far less ambiguous in terms of plumage and structure and the local authorities accepted this as Osaka's first record of Kumlien's Gull. This was a small more elegant bird, with a notably long primary extension emphasised by its consistently horizontal carriage.
These two shots attempt to show the long, low carriage and small size compared to Vega.
The gull had a rounded, dove-ish head, the bill short and still largely black at this late date.
Again small gull jizz, the mantle is developing adult grey and the tertials barred.
White underside of primaries and lightly barred coverts.
Barred undertail coverts and mainly white rectrices.
The best shot I have of the upper tail.
Primaries with pale grey centres.
Better views of the primary detail, the barring on the tertials looks an extention of the greater coverts.