According to an article in Birdwatch magazine at the Arctic Warbler has been split into three cryptic species due to differences in genetics. (Note there is no confirmation whether this new arrrangement has been accepted by eg The International Commission on Zoological Nomenclature).
Saitoh et al (2008, 2010) had 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).
A 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.
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); and Japanese Leaf Warbler P xanthodryas in Japan except Hokkaido.