Spring ocularis White Wagtail (East Siberian, or Taiwan Wagtail as it's known in Japan) is a reasonably regular migrant on the islands in western Japan and common in the southern islands. Like other taxa in spring it's fairly straight forward to identify, the uniformly pale grey back and rump with black restricted to the longest uppertail coverts along with a black eyestripe is a unique and instantly recognisable combination. The wing coverts are less cleanly white than those of lugens but nevertheless whiter than the other taxa we're likely to see here while the flight feathers are distinctly darker than lugens creating a strong contrast in the wing. The following series of shots illustrate just how distinctive it is.
So far so good, but the next bird presents a problem. The median coverts are neatly dark centred with broad white tips and the greater coverts are basically dark with narrow white fringes but again with broad white tips. Thus the coverts are largely dark with two conspicuous wing-bars, totally at odds with the earlier birds. Differing wing pattern doesn't seem to be gender related in other White Wagtails so it's unlikely that this being a female and the others male is the solution. Besides I'd expect to have seen more birds like this if that were the explanation. By early May, when I usually see these birds, they seem to have completed their pre-breeding moult so I don't think it can be age related either as in the field any 2CY should look close enough to adult to be of little significance. Plus images of first winter ocularis I've found on the internet have less well marked coverts than this.
The bird in the following image is far more likely to be a female which seems to lend credence to the previous bird being a dark-winged male.
Though "East Siberian" is regular in spring this doesn't seem to be the case in autumn and the following image is of the only putative first winter I've seen.
I believe this to be a first winter ocularis, due to the grey greater coverts with narrow white fringes. Simply because this bird was on Hegurajima doesn't make it more likely to be a rarer taxon but it is a location where autumn ocularis could turn up. Hegurajima, 7 October 2011.
A more heavily cropped version of the same shot, the only one I got of the bird before it flew off. The sides of the rump are less black than on over 40 first winter type lugens I photographed in Kyoto in October 2014, though I wouldn't rely on that too much given the variability shown in general.
White Wagtail M.a.lugens is a very common year-round bird in this area, it's possible to go out and see well over 100 on any given day in October. However despite significant and confusing first winter/non-breeding female variation at that time of year I've never seen a bird with these obvious piano key coverts. In fact though first winter lugens are reporterd to lack the white coverts of adults there isn't a vast amount of age related difference. One of the reasons for the difficulty in aging some female types in the field. Even the most strongly marked juvenile and first winter lugens coverts are less distinctly patterned grey, most have just a slight greyish wash of limited extent and others are almost fully white.
Below are a number of shots showing the normal range of first winter lugens coverts to compare with both the putative first winter ocularis and the earlier dark-winged bird.
Two shots of a (possibly female) first winter lugens with very white coverts, Kyoto 18 October 2014.
Closer to juvenile judging by the greyer forehead, this bird has more prominent but still limited grey centres to the coverts. The black upper border to the supercillium may indicate male. Ishikawa, 10 October 2014.
This bird still has a grey forehead and is closer to juvenile than other birds present on this date. The coverts are greyer than most birds too but the markings are smudgy rather than sharply defined. Kyoto 18 October.
Much darker bases to two of the median coverts (which look the same generation) compared to other birds, Kyoto 18 October.
I still live in hope of getting a smoother ride out of my oval wheel one day.