I think all of the photographs above are of Heuglin's Gull.
1. All birds show fairly extensive, moderately heavy greyish-brown mottling around the head/neck. While mongolicus can show this very early in the winter, by this time usually what is left is remnant faint spotting, and it appears notably more white-headed than other large gulls in HK.
2. John is right to point out the importance of primary moult timing. However, while mongolicus moults a month later than nominate cachinnans, it is still complete before the turn of the year, and usually by end November. Thus, any bird in active primary moult in the first three months of the year is very likely to be Heuglin's. John's photo of a bird still growing the outer 3 primaries is typical of Heuglin's at this time of year.
3. The first three photos in particular show the relatively dark upperparts typical of Heuglin's. Although John's picture shows a rather paler bird, this is because of the light. It's still not as pale as I would expect for mongolicus.
4. The bill shape of these birds is also typical for Heuglin's. In contrast, mongolicus has a heavier, more parallel-sided bill shape.
It's great to see these large gull photos, especially good ones such as these.
Geoff
