Several of the species being mentioned are forest specialists arising from feral releases that are dispersing very slowly from the original point of release in the central NT. Species like Velvet-fronted Nuthatch and Blue-winged Minla (and, even more so Yellow-cheeked Tit) are rare or unrecorded in most of the NT (Plover Cove, Pat Sin Leng, Sai Kung), where they can disperse via woodland/forest habitat. I don't see any reason that they should turn up on HK Island which requires them to cross urban areas in Kowloon and then also cross water in the harbour. Rufous-capped Babbler is spreading more quickly in NT, but still I think Kowloon and the harbour are a barrier. Similarly, none of these species are present on any of the other islands (Lantau has suitable habitat). Silver-eared Mesia, Scimitar Babbler and Black-throated Laughingthrush are the result of a separate introduction on the island - also Greater Necklaced Laugher, which I think is a different subspecies from the ones in the NT.
Some of the others are more of a surprise. I don't understand why minivets don't turn up on HK Island, but maybe they also don't like the sea crossing. I did once see a 'Yellow' minivet on Tung Ping Chau, but unfortunately just a flyby and at the time I wasn't sure about ID (in retrospect I think probably Scarlet). I think there are records of Lesser Shortwing and Pygmy Wren-babbler so these might be on the way - as recent colonists of the NT I think you need to give them time to get there.
Crested Serpent Eagle I thought was regular in some parts of HK Island, especially around Tai Tam (but I don't often bird the island, so could be wrong). Certainly it is regular on Lantau, so the sea crossing should not be an obstacle. Maybe the snake populations on HK Island have been too persecuted and cannot now support a large population?
