Re the flock of Grey-chinned Minivets.
Largish flocks of Grey-chinned Minivets like this are regularly recorded in Hong Kong in early autumn through into January.
John Allcock and I recently had a discussion about these flocks, whether they were of Hong Kong origin or winter visitors from outside. We were trying to decide how to describe them in the HK Bird Report.
We eventually decided the current evidence suggests they are post-breeding flocks of local origin rather than from outside, although the possibility that some are true winter visitors cannot be excluded.
This behaviour is rather different to that of Scarlet Minivet which, as you note, tend to be seen in fairly even numbers throughout the year.
