With 13 sightings in 4+ days, Arctic Warblers were a frequently encountered migrant on the island. However, what used to be one species has now been split into three: Arctic Warbler Phylloscopus borealis, Kamchatka Leaf Warbler P. examinandus and Japanese Leaf Warbler P. xanthodryas.
The breeding range of borealis stretches from northern Eurasia to Alaska, examinandus breeds on Sakhalin, Hokkaido and the Kurile Islands and possibly on the Pacific Russian mainland, while xanthodryas breeds in the rest of Japan (south of Hokkaido). The wintering range of the three taxa is incompletely known, but borealis is the commonest identified taxon in Peninsular Malaysia, with there being at least one acceptable record of xanthodryas from Penang island (Wells 2007). Mann (2008) records both borealis and xanthodryas as non-breeding visitors to Borneo, with the former being the commoner.
Elsewhere, Paul Leader summarized the status of the various forms in Hong Kong here, with borealis being the only taxon proven to occur there in autumn, but xanthodryas predominating in spring. Clement (2006) suggests that xanthodryas/ examinandus winter in Taiwan, the Philippines and the Sundas.
The full paper documenting the split is here, and, if you just want the main points, useful summaries are here and here. Since the major field differences are in vocalizations, this link is essential listening. It’s worth paying particular attention to the calls, since ‘Arctic type’ warblers are usually pretty vocal in Malaysia.
This is what most birds I saw looked like. In both appearance and call, they matched borealis. 
One bird encountered on 11th Oct looked and behaved quite differently, staying in the top of a mid-sized tree at all times, rather than in the mid to lower storey. The first impression was of lemony-yellow underparts, rather bright olive-green upperparts and an almost entirely yellow lower mandible.
Unfortunately, the bird was completely silent, but from the plumage, I identified it as either a Japanese Leaf Warbler P. xanthodryas or Kamchatka Leaf Warbler P.examinandus.
According to several sources, xanthodryas has a longer 2nd primary (i.e. the outermost visible primary) than the other taxa, but I haven’t been able to track down the precise differences in wing formulae, other than the overall wing length (up to 73mm in xanthodryas, rarely over 68mm in borealis (Wells 2007)). Not sure whether anything can be discerned from the two photos below.
Xanthodryas has a slightly broader-based bill than borealis. Not sure much can be said from the photos below. If anything, the bill looked more ‘bulbous’ or swollen on borealis and slightly narrower on this bird (which would fit examinandus better than xanthodryas)..
borealis (left); xanthodryas/examinandus (right)
The brighter, greener upperparts of xanthodryas/examinandus are apparent on this comparison.
The head pattern of these two individuals was striking different. The rear of the supercilium of the xanthodryas/examinandus was discontinuous and the lower crescent of the eye-ring stood out in more marked contrast against the darker ear coverts (the eye-ring itself seemed markedly thicker). The supercilium of the xanthodryas/examinandus was creamy yellow throughout, whereas on the borealis, it was whiter toward the rear. Finally, the lower ear coverts of the xanthodryas/examinandus were noticeably washed yellow.
This photo shows the yellowish lower ear coverts well.
Some more shots of this bird, none with a clear side on view of the head!
Overall circumstance make it more likely that this was xanthodryas rather than examinandus, but without the call, I can’t say for sure for now. Photographs of an undoubted examinandus are here, a spring bird. It shows a very extensive dark tip to the lower mandible – more so than is typical for borealis, and much more so than on the bird I saw. It also seems to have a slightly darker moustachial area near the bill, making the bird look as if it has a pale centre to the ear coverts.These may be variable features – but they’re something to look out for at least!
I’ll certainly be taking a much closer look at (and listen to) ‘Arctic Warblers’ in future.
Dig deep 
Hi Dave, Not bad in terms of bird variety seen at such a confined island. I never had the chance to get any clear photos of Arctic Warblers like yours. The comparisons are definitely useful. Another bird which is closely resemble the Arctic features is as you know are the Eastern Crown Warblers.
Hi Ronnie
The key to the bird diversity of the island is its position. Most of the migrants I recorded would not spend more than a few days on the island – it’s just a stepping stone to get to the mainland. Actually, Japanese Leaf Warbler may look a good deal more like Eastern Crowned than Arctic (borealis). The latter can be distinguished by the pale yellow vent contrasting with whiter underparts, the central crown stripe, and the pure yellow lower mandible and different ‘face’ pattern.
I really enjoyed this article. The late, Alick Moore once said phylloscopus warblers are either mysterious blobs at the top of the canopy or dishevelled, feathered mites in the palm of your hand. Alick travelled to a lot of places before the crowds got there. Speaking in the time frame of the seventies he also said to me and others that many of the officially accepted, UK phylloscs. were simply, mis-identified, birds. Furthermore, in that same time frame, who but an ageing Rembrandt perhaps, would paint a field guide picture of Greenish Warbler, from a quarter, rear view position, except for the young master, Lars Jonnson, and this in circa. 1975/77? Warblers were even trapped and netted but misidentified due to the poor available, information. Things are changing in SE. Asia as these photos prove This new, old world warbler order, is just as exciting as large gulls seemed to be in the eighties. Some of the migrant warblers in the Southern Thai mangroves, look like Sakhalin Warblers, even taking the “Arctic”, splits into account. In fact Dave’s first photo with the eyestripe appearing not to reach the bill and with the supercilium doing so seems to mitigate against all the accepted wisdom of what seems to be concrete, Arctic, borealis, id. criteria. Or, conversely, I may be mis-reading his photo! Even the “capped” appearance with the supercilium bisecting the crown to the rear of the nape seems to partially, mitigate against, borealis, Arctic. Presumably, Sakhalin Warbler does not really look like a male, Grey-headed Wagtail in colouration like some of the field guides suggest. The calls of some of these phylloscs. are enigmatic and buzzing, and are not so straight forward as it might seem. I personally find the mangroves of Thailand better for sound appreciation, than Malaysia. Birds are to be found lower down in this area of Thailand as well. This summer I recorded an anxiety call of a UK. Chiffchaff and compared it to the call of an Ijima’s Warbler on the web, Ok. the experts would perhaps say, “chalk and cheese” but both calls were of the same basic, phyllosc. ilk. Ist year, Eastern-crowned, shares the same habitat, at times in Southern Thailand with other migrant phylloscs. How many species are there anyway? It is really difficult to see the crown stripe on these Eastern-crowned, young birds except from below and in good light and with top marque binoculars. It is about as subtle as the coronal stripe on Yellow-browed which most descriptions do not mention simply because it cannot be seen. Also with “Arctics”, the orange or orange- yellow, soft parts can look bright yellow in strong reflected sunshine. The semi-translucent open gape can also flash bright yellow at times. The photos seem to show the same beak and leg colouration? The primrose yellow areas as a crescent on the vent and on the middle, upper chest are also very ephemeral and perhaps most easily seen in Eastern-crowned. Perhaps these are the revealed, underneath feathers or a type of pigment? Also difficult, to see, pin like endings to some tail and secondary feathers that are more easily seen on the grasshopper warblers, for instance, need explanation as well. I would urge all interested visitors to the region to take a closer look at the late winter, resident, “Arctics”, in southern Thailand and Malaysia, which can be watched within a day of two of observing Greenish Warbler, in the tree-lined avenues of Chiang Mai.