I first detected movement in long grass at the side of the track on between an oak plantation with deep leaf-litter and somewhat sparse 30-40 centimetre high grass on one side and a tangled overgrown clearing on the other. After waiting a while I began to think I'd imagined it and walked forward but as I approached the spot a small warbler flew a few metres and landed in the middle of the narrow track in front of me under deep shade. At this point I fully expected it to be a Dusky Warbler because of its behaviour and more importantly its small size as it flew. When I got onto it, it clearly wasn't a Dusky. The view was no more than two or three seconds in duration, long enough to see the most striking features but not to take in the minutiae. Those striking features consisted of a surprisingly uniform brown head with essentially plain lores, rather uniform dark ear coverts (thus the pale lower eye ring stood out) and a sharp, straight demarcation bordering a neat, almost glowing white throat. The upperparts were an unremarkable brown but there was a distinctly warmer brown rear flank patch. This latter point was actually the first thing that caught my attention when I focused on the bird. The tail was definitely longer than that of a phylloscopus but it left me with the impression of being neat, rather than bulky as in Japanese Bush Warbler. However viewed at that side-on angle I could make out the true shape, plus I'd been influenced by the flight view. The bird crept quickly to the side of the track and disappeared into the grass under the oaks. I very much regret not getting a record shot of it at this juncture as ultimately, brief as it was, this was the best view I had.
I moved carefully forward not knowing where the bird was but it seemed to be on the move as twitching grass stems showed something was now about 10 metres off the track. I waited and as luck would have it a minute or two later the bird popped up very briefly onto a low branch just above grass height now about 15-20 metres away. Rather than look at the bird this time I was intent on getting the shot and I rattled off a couple of hasty shots before it dropped back down into the grass. I'm therefore unable to add anything to the original description and saw little more than anyone viewing the images will. I waited until I had to leave for the ferry but there was no further sign of it. It was silent throughout the encounter.
The photographs I was able to get are disappointing and don't rule out Japanese Bush Warbler. This possibility didn't occur to me at the time as, in life, the size and structure of the bird really weren't suggestive of it; it appeared distinctly small and it had lacked the characteristic full-tailed appearance as it first flew away from me along the track. The underparts were brown- rather than grey-hued (expected of worn birds at this time of year) and the head pattern lacked the expected dark eye stripe and contrasting pale supercillium.
The two images suffer greatly from strong light breaking through the canopy with camera settings adjusted for the deep shade and unsteady hands due to haste. Hence they aren't sharp, particularly the first, and rather over-exposed.
Why isn't it a Chinese Bush Warbler? It would be remiss of me not to state the obvious. Firstly this bird appeared to have plain not barred undertail coverts, illustrations always show strongly barred utc. Interestingly however, most images I've looked at do not (eg Oriental Bird Club images), they range from uniform brown to uniform creamy-white. In other words some seem to lack the pale tips, which is easy to imagine on worn individuals, but others seem to lack the dark bases which is difficult to explain. Undertail coverts barring is probably more difficult to detect in the field than illustrations suggest, though birds which do have dark utc should still be eye-catching. Secondly, and most importantly, my two images suggest a horornis rather than locustella tail shape. Whether this is the result of an image creating a false impression or it is accurate hardly matters, it's damning either way in the light of my not getting a clear view of the tail. The problem of the plain head remains, I don't recall ever seeing a Japanese Bush Warbler with this head pattern but no horornis that could plausibly occur in Japan shows it either. It's natural therefore to default to a strangely marked common species rather than try to shoe-horn a rarity to fit the description.
This brief view/description and limited images would never pass mustard for a record of a rare bird, and I certainly would never count one on these views, but I'd be interested to hear people's opinions about it as it doesn't seem to closely match any of the very few possibilities within a Japanese context.