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.
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.
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.