Phylloscopus kansuensis

The Gansu leaf warbler is a small passerine known only from China, 10cm long and slightly larger than Pallas's leaf warbler. The head has a long white supercilium and a pale stripe along the centre of the crown (Wikipedia, general species account).

Its song is very different from Pallas's leaf warbler's, consisting of a thin, high-pitched note followed by accelerating notes and finally a trill. It is known to breed only in Gansu and Qinghai provinces in northern China (Wikipedia, general species account).