This is particularly galling as I had considered Alpine Leaf at the time but didn't give any thought to Buff-throated as I was totally unaware they looked so similar; the OBC images show a far more similar bird than Brazil's Birds of East Asia suggests. Why did I discount Alpine Leaf in the field? Well, partly because as far as I knew there aren't any autumn records from Japan; the OSJ list includes only one record of Tickell's Warbler (spring 1995), and this not identified to subspecies before Tickell's and Alpine Leaf were split. More importantly I discounted the possibility because I remembered the Willow Warblers I'd seen on Hegura as being brighter than the commonly held western birders' perception of the taxon and in that light this bird didn't appear too yellow by comparison. In fact there have been a number subsequent sightings of "Tickell's" in Japan but they've all been in spring as far as I know. Both Alpine Leaf and Buff-throated certainly seem more likely to occur as spring overshoots in Japan given their respective ranges whereas Willow Warbler is fairly regular in low numbers autumn on Hegura. I had an amazing four in a single day in October 2006.
I considered Alpine Leaf in the field because this warbler struck me as strikingly bright yellow below and distinctly green above. Very different to the perceived appearance of yakutensis held by birders in Europe, or at least what I infer from online info and discussion. That said, Willow Warblers I've seen here in the past have consistently been 'more colourful' than European / Middle eastern descriptions and images I've seen online. And even more so in my fallible memory I now have to admit!
Suggestions from people who saw the images I originally posted prompted my to dig out old notebooks still packed away after moving house this month. I was surprised not only how unlike the sketches and descriptions in my notebook these images look but also how my sketches still differed from online descriptions of yakutensis.
I'm left in something of a quandary. If this bird is a Willow Warbler it's so colourful as to be too great a disconnect with western perception of yakutensis. Though I'd unquestionably call it extreme even compared to my previous experience it doesn't alter the fact that Willow Warblers here don't look like the bird I'm led to believe I should be seeing if they are yakutensis. Western focus on very grey and white birds perhaps wouldn't flag these 'Japanese' Willow Warblers as suspicious. If they are part of a south east Asian wintering population, which has been suggested in the past and which regularity in Japan as well as records from Taiwan and the Philippines might support, could they even be something other than yakutensis? Also if it isn't a 'Japanese' Willow Warber then what is it?
It's a great pity I couldn't get better shots of this bird. It was very active and fast moving in a belt of low, dense pine and broadleaf wind-break trees and was difficult to follow with the bins let alone the camera. There were a couple of Yellow-brows, several Goldcrests, Japanese White-eyes and Coal Tits in the same trees and tracking a single bird amongst all the toing and froing amongst the tangle of foliage was difficult. Switching from bins to camera I was often uncertain whether I'd even picked up the right bird and twice photographed a Yellow-browed Warbler by mistake.
The following series of images are the best of a bad bunch.
In shadow with strong surrounding light. There seems to be a greyish wash around the hind neck and hind crown in this image otherwise the fore-crown and upperparts are distinctly greenish. If anything this slight greyish wash emphasises how green the upperparts are. The underparts are a dull but clear yellow in this image except for the greyer sides to the breast. The bill is fine compared to Kamchatka Leaf and appears all dark in this shot but the lower mandible has a yellow base and cutting edge. The legs are consistently dark in all the images, blackish in shadow or dark brown in stronger light. The soles paler, perhaps yellowish.
Another shady shot. Even though first autumn birds will be yellower than adults there's no getting around the fact this is a very yellow warbler compared to most (all?) other phylloscopus taxa I see here. Very different to the typical yakutensis as I understand it.
This may be a useless shot as far as showing detail is concerned but I think the overall colouration is extremely difficult to reconcile with descriptions of yakutensis.
Another image with very little visible detail but at least this one is in focus and still shows a bright warbler, not even remotely similar to a tristis Chiffchaff.
With the bill open here, the lower mandible is yellowish at the base and along the cutting edge. 
Another awful shot but as luck would have it the underwing coverts and breast are caught reasonably well and appear concolourous. In my understanding this shouldn't be the case with a yakutensis.
It looks much darker and duller here but to my eye it's still an essentially greenish/yellowish warbler rather than greyish/whitish.
In this view it does look greyer above than some of the previous shots but it's nevertheless extensively yellowish below except for the undertail coverts.
The angle isn't helpful and doesn't show the comparative length of the primary projection however but the visible 'wing-tip' looks way too long to be the longest tertial and therefore I'm left to speculate on whether this is actually the longest primary which would make it very short, as well as the primary projection very blunt, for Willow Warbler. The seemingly white fringe to the outer tail feather and tips to others may be an artifact caused by light shining through the feather or a feature of the bird. A couple of earlier images also suggest whitish fringes to the outer tail feather.
I looked for this bird again when next on the island two days later but there had been a big clear-out and it had probably left along with most other birds. Unfortunately none of these images give more than a glimpse of the bird, none provide a clear unobstructed view where features can be assessed, but there's certainly enough for me to look forward to my next encounter with Willow Warbler in Japan.
As a postscript the consensus amongst experienced Japanese birders is that this is definitely a Willow Warbler which raises questions about European birders' expectation that a potential yakutensis in Europe would be a grey bird.
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