Recent DNA studies have suggested that Manchurian Bush Warbler (Horornis borealis) is a separate species from Japanese Bush Warbler (Horonis diphone canturians). Both occur in Hong Kong, but the differences between them are not well understood, so it is not yet clear about the true status of each.
I would suggest that this is Japanese Bush Warbler (canturians), based on the brown tone to the upperparts, small amount of contrast between the mantle and the wings, and the long tailed/short winged appearance.
