Formerly treated as conspecific with Green-crowned Warbler (Phylloscopus burkii), which also included Grey-crowned Warbler (Phylloscopus tephrocephalus) and Bianchi's Warbler (Phylloscopus valentini), and the populations recently described as Martens's Warbler (Phylloscopus omeiensis) and Alstrom's Warbler (Phylloscopus soror). All of these, however, exhibit constant differences in morphology, vocalizations and mitochondrial DNA. Subspecies nemoralis has been treated as a separate species on basis of plumage differences, but these are rather minor and possibly clinal, and songs appear identical.
The following 2 subspecies are recognised:
whistleri (Ticehurst, 1925) - Himalayas.
nemoralis (Koelz, 1954) - North-eastern India (Lushai Hills, in Mizoram; Naga Hills, in Nagaland) and north-western and western Myanmar. Has been treated as a separate species, Seicercus nemoralis.
BirdLife International Species Factsheet