Alstrom and Mild's 'Pipits and Wagtails of Europe, Asia and North America' (which sadly I don't own) would be the best ready reference and starting point to attempt to sort out any yellow wagtails in the field. Oh to find a Western in Japan! But as I struggle to identify the eastern taxa in all but spring breeding plumage, I'm not holding my breath. That last sentence may suggest spring birds are easy... well, up to a point. There's always the question as what is acceptable variation and what might suggest hybridization.
One of the highlights of a winter Kyushu trip is seeing a few non-breeding Eastern Yellow Wagtails. I see plenty of taivana and tschutschensis in spring on Japan Sea islands (the further west the better) and a single putative macronyx. However non-breeding plumages are more of a challenge to say the least. Starting from scratch I had no idea what I was looking at and though I wish I could say I've made progress over the last three winters I can't. Nevertheless patterns are emerging. Not that emerging patterns mean I know what the race/gender/age individuals are with any certainty, just that there are... patterns. And because of this repetition it's safe to say they're Eastern... hardly a revelation.
I'm uncertain about the status of Eastern Yellow in Kyushu in winter, obviously they winter there but I don't know how widespread they are. They have been easy to find each year on the fields at Isahaya but I've only seen one in about six days birding at Arasaki. I didn't see any at Tamana over two days spent there despite it being very good for pipits with three species present on the fields, or on the much smaller area of fields at Yatsushiro despite four visits during the three winters. Not exactly a comprehensive study! Nevertheless Isahaya stands head and shoulders above any other sites I've visited and I wonder what suitable areas along the north coast might be like.
The images below were all taken during the last week of December or first week of January since the end of 2014. All look good for taivana though one bird does stand out as being a little odd.
|Bird 1. On the left side the middle tertial has also been replaced and two or three median coverts have been dropped.|
|Bird 1. A view from the rear clearly shows the difference between the old and replaced central tertial.|
|Bird 1. The cleaner white throat contrasting with brownish-grey breast and flanks seems typical.|
I think that bird is about as straight forward as it gets, beyond this it's not exactly a quagmire but the ground gets progressively less firm under foot.
|Bird 2. Little more than a hint of yellow on the underparts.|
|Bird 2. Asymmetric tertial moult again, the longest feather on the left side has been replaced.|
|Bird 4. Yet another with extensive yellow on the underparts, it looks similar to the first bird.|
|Bird 4. The upperparts look greyer but the rump and uppertail coverts are distinctly green.|
The next bird is the only bird I've seen a little further south at Arasaki. It was more distant, across a river, and because I didn't have my scope at hand to check details I felt this was a strikingly grey and white bird compared to the Isahaya birds. However after checking the images it clearly has distinct greenish tints across the crown and upperparts, even the extensively yellow throat didn't stand out in the field.
|Bird 5. This bird had a strikingly grey and white appearance in the field, even the yellow throat didn't stand out, though the light was already going very quickly when I found it.|
There are some constants here, such as the greenish upperparts and the narrowly tipped greater coverts whereas the extent of yellow below and on the supercillium is highly variable, as is the extent or even presence of pale on the lower mandible. However if my estimation that these are adults is correct, where are the first winter birds? Am I mistaken or are there fewer young birds wintering in Kyushu? None of these birds match the grey and white first-winters that catch the eye as potential Eastern Yellow in a European context.
The next three birds seem a far better fit for first winters; paler grey and white overall, each with a bold greater coverts bar due to prominent white tips. The first of the two seems an obvious example to my untrained eye, I didn't get good views of the second and the third bird is a little odd in that it shows more green on the upperparts than I'd expect and it's the only bird I've seen which is either actively moulting its tail or has suspended moult leaving an untidy state of re-growth.
|Bird 6. A cleaner, colder individual with the merest hint of yellow below the throat and on the vent sides.|
|Bird 6. Very grey and white, there's just a hint of yellowish at the side of the vent and a bold greater coverts bar.|
|Bird 6. Very clean underparts, no yellow in the supercillium.|
|Bird 7. I don't think I should even try to age this bird with the little I know about the subject.|
|Bird 7. The central tail feathers are very worn, the outers look less so but the quality of the images isn't good enough to be sure of anything.|
|Bird 8. The tail is at a strange and untidy stage of growth.|
Finally a couple of very poor images of a bird which doesn't quite sit comfortably within this series but as it also appears to have some yellow in the supercillium when the images are blown up I presume it must also be taivana.
|Bird 9. This bird stood out because of its very dark mask and contrasting supercillium as well as having the greyest and most thrush-like breast of any of the birds I've seen to date.|
|Bird 9. Unfortunately because of distance and angle to the light it isn't possible to be sure of details and it may simply be this that enhances a somewhat different impression.|