Chestnut Bulbuls arrive in Hong Kong in large numbers in November to spend the winter here - you may have read of recent increases and flocks in Tai Po Kau and on Po Toi, no doubt other places also. So probably this is a migrant or newly arrived flock which struck the building.
Bird strikes on buildings have been well researched in north America, where they are considered to kill over 100 million birds annually, second only to cats as killers. See this article in The Washington Post
https://www.washingtonpost.com/n ... 34b1e132_story.html
Unexpectedly, buildings up to 11 stories cause most damage, attracting birds by lighting as well as window reflection.
I believe Canada has a national campaign to reduce tall building lighting and hence bird kill - can't see that happening here, where the lighting is considered a tourist attraction.
