I don't think these are nominate pacificus. They look too small, with relatively short wings and tail for that taxon. Also the tail fork is not particularly deep. The rump is not especially broad and appears to show streaking. Given the location it seems likely that these are locally breeding birds (i.e kanoi). However, there is cline of increasing size in east Asia from kanoi in the south through kurodae (south central Japan) to pacificus in the north. As such identification of individuals in the field to subspecies is very difficult and based on current knowledge only very small birds (kanoi) or very large birds (pacificus) can be safely identified in the field.
It is worth noting that a further taxon (cooki) occurs in south-west China (as close as Yao Shan) and could also occur in HK. This is large, very black subspecies with extensive white tips below.
