Dusky  Warbler Phylloscopus fuscatus     photos Sid Francis

Both a passage and breeding bird - usually picked up on its repeated and constant  "chick" call, which assists with an easy identification - but on breeding grounds also has a song. Lacks wing-bars, it looks similar to a bush warbler - especially when typically skulky and located in low vegitation. 

Similar species - superficial resemblance to other warblers without wingbars - from yellow-streaked by darker drabber plumage and general behaviour differences - skulky dusky vs.the shy. but more open perching, yellow-streaked is an initial give away - the distinctive trilling song of yellow-streaked, often the first thing that puts you onto a bird, is definitive. Buff-throated and a washed out Tickell's are also candidates for possible confusion but these birds are always brighter and  open perchers - and of course neither give a  tic call..  

Habitat - during passage anywhere - breeding NW Sichuan - wet alpine pasture with bushes on the plateau near rivers around 3,300 m. 

Vocalizations - the chick call and a song on the breeding ground that consists of a short phases of around five, rapidly repeated, high pitched notes.

Song here - xeno-canto     Frank Lambert, XC161379. Accessible at www.xeno-canto.org/161379

Note: that there is speculation that the dusky breeding in Sichuan could belong to Smoky Warbler - but presently Sichuan birds are listed under ssp.robustus, which is still lumped with Dusky.