Great stuff Andrew
This is what makes patch birding so interesting - I had 64 Little Egrets fly over my place in Lam Tsuen this evening - big news to see so many here!
Another possible explanation (if they are not migrants) is that the Black-naped Terns may have finished breeding somewhere narby and find Sai Kung Harbour a good area to feed.
Cheers
Mike K
Mike KilburnVice Chairman, HKBWSChairman, Conservation Committee