This is a 1st winter Heuglin's Gull.
The dark greater coverts rule out Vega 1y and mongolicus 1y. It has newly-replaced grey inner greater coverts; moult of the greater coverts in the first winter almost never happens in mongolicus, and is pretty unusual in vegae too. The new feathers are greyish, but not very pale; they have the dull, brownish-grey coloration typical of the heuglini-fuscus complex at this stage.
The tail band is actually relatively broad and also blackish, which is good for heuglini.
The bill is not very substantial really. It has a relatively marked 'pinched-in' effect just short of half-way along, and here it is rather narrow.
I think the whiteness of the head and upper body is exaggerated by the photo. However, heuglini can be notably white, especially later in the winter.
Finally, the all-dark bill fits heuglini better at this stage of the winter than it does other taxa occurring in HK.
Interesting bird.
Geoff
