Mid-november 2014, at about 2.30 p.m. a single gull drifted down river past Bãi Đá sông Hồng (Hanoi) until it was out of sight, in clear weather.
At almost 3 p.m. the gull drifted upriver, and several photographs were taken. The bird continued upstream until it was out of sight.
Photographs were circulated by know experts on birds/gull identification including Jonathan Eames (Cambodia), Philip Round (Thailand), George Carey (Hong Kong), Nial Moores (South Korea) as well as Osao and Ujihara Michiaki (Japan). Most of these authorities concurred that the bird in question was a first calendar-year mongolicus than a tamiyrensis / barabensis or a really advanced vegae.
Although it looks browner-toned, darker and more marked on the head for a straightforward ID as a mongolicus, the main things that suggest mongolicus in these three images are:
1. In November, most taimyrensis and vegae are still largely in juvenile plumage, while mongolicus that breed in Korea are already moving into 1W plumage in August / September. This bird has several elements that are much more indicative of a First-winter than juvenile.
2. The bird looks largely pale-headed with a darker hind-collar and some streaking round onto the breast sides. Most mongolicus have constrasty fine streaking on the hind-nape and a few have some streaking "coming round" to the breast sides too. Most taimyrensis in Korea at least are in juvenile plumage in November. They tend to have strong face-masks, diffuse streaking and brown tones on the head, and quite a dense, sprawlingly dark look to the nape, not suggested by your images.
3. The bird has a strong blaze across esp. pp 1-4. The darker shafts end in a club-like darker pattern near the tips. This is quite typical of mongolicus and seems too extensive and too mongolicus-like for taimyrensis or vegae.
4. The bird lacks a darker, more solid area across the greater coverts which you would expect on taimyrensis. Various other smaller details of what we can see on the upperwing seem mongolicus-like (including the strong contrast between the dark alula and the paler primary coverts).
5. The underwing shows extensive dark on the undersides of the primaries, contrasting with paler axillaries and light-brown patterned underwing coverts etc. This looks pretty mongolicus-like in these three images.
6. The tail band is complete-looking but narrow and black, with already much white on the rump and uppertail coverts. Most taimyrensis and vegae in Korea have wider dark tail bands and more mottling / marking across the rump and uptcvts than this. However, many mongolicus look quite like this.
7. The bill is already showing some paler to the base. This is typical of mongolicus in November but would be a few weeks earlier than typical in most taimyrensis or vegae.
This is thought to be the second record for Vietnam, following a sighting of a first-winter Mongolian Gull at Dat Mui in the Viet Namese Mekong back in 2000 (Nial Moores, per comms).