There is a large Red-footed Booby colony (about 10000 birds) in the South China Sea, south of HK. From June to August, the sea current near HK is generally from SW to NE. I think that helps to bring this bird to HK. Let's say, this is just after the breeding Season. After the chicks leave the nest, the parants start moulting even though they are not fully recovered from the tired breeding process yet. Then the sea current bring it here.
The bird that I saw in Dong Ping Chau many years ago is very similar in behavior to this bird.
HF Cheung
