I'm perhaps missing something but are you sure those are Crested Myna? The nest would be unusual for mynas (which usually nest in holes in rocks/buildings) and they seem a bit too grey and certain other features seem wrong (iris colour, tail pattern, etc). Do you have more photos or have you seen adults visiting the nest to confirm ID?
By the way, Oriental Turtle Dove has oversummered in the NT for a few years, and has bred at Lok Ma Chau. I have seen birds in a couple of places in the last couple of weeks.
