Identification: Closely resembles Arctic Warbler Phylloscopus borealis, but marginally shorter-billed (also usually lacking black on lower mandible) and darker-legged, with slightly more prominent wingbars and supercilium, and greener upperparts when in fresh plumage. Lacks the crown stripe and yellow wash on vent of Eastern Crowned Warbler P. coronatus. With good views, the long outermost primary, which extends well beyond the primary coverts in this species, can also be used to rule out Arctic Warbler; however, given the exceptional rarity of this species, vocalizations (song or call) would be strongly desired to confirm the identification, and to rule out the very similar Greenish Warbler P. trochiloides which remains unrecorded in Singapore.