gary

I thought it is easy to separate Manchurian Bush Warbler and Brownish-flanked Bush Warbler because I though I would hear it before seeing it. But until recently I found the female MBW looks rather similar to BFBW, even in a clear photo like this.
I think these 2 sp. share some features such as contrast between crown and mantle, making it difficult to ID.
Generally MBW has thicker bill than BFBW, but I think this one is not thick enough to rule out BFBW. Actually I think the bill of MBW is rather round and "tube" like.
In addition, some suggest MBW has longer eyebrow and more solid eye strip. On this point, I go for BFBW for this one.
So, I would say it's a Brownish-flanked Bush Warbler, but I am not very sure.

Gary

gary

According to the updated taxonomy adopted in the latest HK bird list. The Japanese/ Manchurian Bush Warbler commonly found in Hong Kong is identified as Manchurian Bush Warbler Cettia canturians, while Japanese Bush Warbler Cettia diphone is still a valid species but not found in Hong Kong, at least up to this moment.

Gary