Hope this helps:
1. I'd agree on Vega, as it has pale mantle, dense head streaks and large white spots on the wingtip.
2. I'd bet on Heuglin's. The mantle is a bit too pale but "Heuglin's" in Hong Kong can show mantle colour close to vegae; the small mirror also suggests Heuglin's.
3. I'd go for Mongolian (Yellow-legged) too, especially with those smallish eyes and large head and bulky jizz.
4. I think both adult large gulls are Heuglin's - don't think adult Slaty-backed Gull has yet been spotted in Hong Kong!
Would be interesting to hear some expert opinion! =)
