Certainly much less frequently than Dusky Warblers, but probably not that different from Radde's, or perhaps slightly more frequently.
However, note that vocalising behaviour of the bird at Ping Che is probably unrepresentative of the species, given how much playback has been used to lure the bird out.
Certainly, this type of record, a singing rarity on passage, is pretty unusual in HK as we are so far south of the breeding grounds of many of these species. Many species pass through a month or so before they establish territories in earnest.
GeoffC
