The black-naped terns (sterna sumatrana) is a member of the family of seabirds Laridae, which includes gulls, terns, skimmers, and noddies. They breed in Hong Kong primarily in August on the rocky coasts of various islands and outcrops.
The best way to see these birds is by boat—particularly by ferry, as the propeller stirs up the small fish they like to eat and so they will often follow the boat for food.
Black-naped terns are unique among terns being one of the only species aside from the snowy crowned tern that lacks a black cap for mating season.
They lay their eggs on bare rocks without nesting material and so there is a small chance that divers and fisherfolk who fish from the rocks may disturb tern nests or worse, so if you are hiking on any islands be careful of tern eggs!
Check out this post in my Hong Kong bird log for more information about this interesting seabird.