鷹柱 -- a pillar of hawks at Lung Fu Shan
The Delifrance cafe at Hong Kong University (later rebranded as Alfafa) has a garden which is convenient for watching raptors over Lung Fu Shan. There are usually Black Kites, often Crested Goshawks, in winter sometimes Eastern Buzzards, and once in a while, at migration times, a possible Grey-faced Buzzard. This morning I was having breakfast there when a multitude of hawks began to appear: first one, then a wave of 5 or 6, then a flock of at least 30. They circled briefly to gain altitude before moving on eastwards along Hong Kong island. They had all moved on within half an hour. At first I assumed that the mass migration of Chinese Sparrowhawks, expected in mid to late April, had arrived early. But on closer examination they proved to be Grey-faced Buzzards, butastur indicus, which in both appearance and phylogeny lie between the accipiters (sparrowhawks and goshawks) and the buteos (hawks or buzzards). These are among the raptors that migrate in flocks, notably at Kenting in Taiwan where they congregate on their way to or from the Philippines. In Japan such an assemblage of circling buzzards is called 鷹柱, たかばしら or takabashira: a 'pillar of hawks'.
The book The Avifauna of Hong Kong notes that Hong Kong sees 'occasional large flocks in spring', with peak numbers in the fourth week of March. Given how quickly this flock passed through, it seems quite possible that the passage of large flocks is regular but often goes unobserved.
Postscript, 2021: On the afternoon of April 11, 2021, Kelvin Ng encountered 'about 40' Grey-faced Buzzards over Mount Davis, as recorded here: https://ebird.org/checklist/S85295423
Following the same route on their northward migration as in 2019, these birds would have made landfall around Mount Davis before proceeding eastwards over Hong Kong Island.
