It looks like a juvenile Dark-sided to me.
- The underparts appear to be mottled with dark, rather than showing defined streaks
- The bill looks relatively short, and mostly dark
- The eye-ring broadens behind the eye
- White spots on the upperparts are remaining juvenile plumage. Dark-sided are most often seen in Hong Kong as juveniles, whereas juvenile Grey-streaked seems to be less common (most presumably moult before migrating).
Compare these two juvenile/first winter birds:
Dark-sided:
http://orientalbirdimages.org/se ... ize=1&Location=
Grey-streaked:
http://orientalbirdimages.org/se ... ize=1&Location=
For me, Grey-streaked usually looks somehow larger and more powerful compared to similar flycatchers, whereas Dark-sided somehow seems a little smaller and more delicate.
