Though it is only the widespread Eurasian nominate form that occurs in Britain and Ireland, various previous authors have described up to five subspecies in its eastern range in the past, mostly based on subtle biometric variations. Some of these forms have long been suspected to be cryptic species by both taxonomists and field observers, largely due to differences in vocalisations.
Saitoh et al (2008, 2010) have already established the presence of three distinct clades (groups sharing the same common ancestor) in the Arctic Warbler Phylloscopus borealis complex using mitchondrial DNA and morphometric comparison, and identified the likelihood of these differentiations having developed due to pre-Ice Age divergence in the late Pliocene to early Pleistocene periods (2.5 to 3 million years ago).
The new study (Alström et al 2011) used recordings of the birds' songs and calls taken from throughout the birds' breeding ranges to further confirm their biological separation and establish scientific name precedence. The songs of the three genetic clades were discretely and consistently different when sonograms were compared, though there was also some variation between the western and eastern populations of the nominate forms and the Alaskan subspecies kennicotti. these differences were easily the equivalent of several already split species pairs in the same genus and in the Seicercus leaf warblers (see Out of order warblers).
The Arctic Warbler complex now consists of Arctic Warbler itself P borealis (including the previously named forms kennicotti, talovka, transbaicalicus and hylebata, none of which are supported by DNA differences, though there may still be differences in morphology) ranging across northern Eurasia into Alaska; Kamtchatka Leaf Warbler P examinandus in southern Kamtchatka, Sakhalin, Hokkaido and the Kurile Islands (though an apparent recording of this form from Vlas'evo on the Pacific Russian mainland in June suggests that this form may exist elsewhere - see Xeno-Canto website link below) ; and Japanese leaf Warbler P xanthodryas in Japan except Hokkaido. This evolutionary (and indeed geographical) pattern is also circumstantially supported by being present in other Palearctic songbirds, like the Great Tit Parus major complex.
In the field, Japanese Leaf Warbler shows a longer wing than the other forms, along with brighter green upperparts and more yellow on the underparts, though field characters are subtle and the species are best identified by call or breeding range at present.
Winter quarters are yet to be fully established for the split forms, but nominate calls have been recorded in Thailand, Malaysia, the Philippines and Borneo, and Japanese Leaf Warbler has been collected in Bali. both these forms breed in lowland deciduous forest, while Kamtchatka Leaf Warbler prefers mixed subalpine coniferous and birch forest between 1,500 and 2,500 m in elevation.
Alström, P, Saitoh, T, Williams, D, Nishiumi, I, Shigeta, Y, Ueda, K, Irestedt, M, Björklund, M and Olsson, U. 2011. The Arctic Warbler Phylloscopus borealis – three anciently separated cryptic species revealed. Ibis 153: 395-410.
Saitoh, T, Shigeta, Y and Ueda, K. 2008. Morphological differences among populations of the Arctic Warbler with some intraspecific taxonomic notes. Ornithological Science 7: 135-142.
Saitoh, T, Alström, P, Nishiumi, I, Shigeta, Y, Williams, D, Olsson, U and Ueda, K. 2010. Old divergences in a boreal bird supports long-term survival through the Ice Ages. BMC Evolutionary Biology 10: 35 doi:10.1186/1471-2148-10-35.
Recordings can be heard at Xeno-canto (accessed 17 March 2011).