I still disagree on some points, if I may.
Regarding the age of the gull, a 3rd winter large gull already resembles an adult. On its wings, it should show the white mirror(s), which in this gull is clearly not the case. I'm sure the original photos are much better, but I believe it is obvious that this gull shows fresh primaries. Gulls start with the moult of the innermost primary and end with the outermost primary. All the primaries show very small whitish tips here, both from above and from below. This strongly indicates fresh primaries.
If these would be third winter primaries, than they would show white mirrors, at least one on P10 (outermost primaries). This is not the case, hence this indicates a 2nd winter gull.
The greyish saddle can show some dark feathers, but can be uniformly grey in a 2nd winter.
The tertials still show dark centres, again an indication for a younger age than a 3rd winter. The tail is still largely dark, which would be less so in a 3rd winter.
The bill looks okay for a 2nd winter as well. I think a 3rd winter bird might have a bit more colour near and above the gonys and a larger yellowish tip. But there can be a lot of variation. Some first winters can develop a large pale basis to the bill.
Regarding the species. I have just returned from the Middle East where I have seen and studied thousands of Heuglin's Gulls and many hundreds of Caspian Gulls (a different race than in HK). The mantle colour does not match Heuglin's Gull for sure. Bleaching is always something to keep in mind, indeed. But there is not less sun in the Middle East than in HK, yet no Heuglin's would look this pale.
If Europeans, such as myself, travel to East-Asia, one could consider Heuglin's Gull to look like Lesser Black-backed Gull race intermedius and Vega Gull to look like Herring Gull race argentatus.
In this bird I do not see either European species, but much more 'our' Caspian Gull. However, Caspian Gulls in Europe look different from Mongolian Gulls (mongolicus). But there is some resemblance.
The head looks rounded and the bird has a gentle facial expression. In Vega Gull this tends to look a bit more aggressive, but this can depend on the angle and I feel it is not always safe to judge this on a couple of photos. Direct comparison might indeed reveal such character a bit more obvious, or not at all. I see a small eye that looks black. Small and black eyes are an indication for Mongolian Gull, but do not exclude Vega Gull. These points are indicative, rather than decisive.
But the appearance of the bird, with the rear body pointing to the ground is something that we see more in Caspian Gulls than in Herring Gulls. Maybe another vague indication for Mongolian Gull rather than Vega Gull.
The head and body show hardly any dark markings. I only notice a vague and scattered breast band, something that you hardly can call a breast band. This matched Mongolian Gull perfectly. All Caspian Gulls show this in many plumages too, as if the bird is washed.
Vega Gulls generally show much more streaking or dark markings on the head, (especially) the hind-neck, the sides of the breast and on the flanks. Agreed that there can be variation and there can be Vega Gulls with less dark markings, but I am not sure that they can be as pale and lacking the markings as in this bird.
The seemingly brownish belly is mud, from when the birds was resting, in my view.
Lots of photos (146) of 2nd cycle Vega Gulls can be seen here:
https://picasaweb.google.com/115 ... Cycle?noredirect=1. Yes, it's Japan and not HK and March instead of January, but the differences are mostly pretty big between these Vega Gulls and the gull at Mai Po.
All together, I still feel that most characters point to a 2nd winter Mongolian Gull. Is it therefore certainly a 2nd winter Mongolian Gull? Gulls are so variable (and than we don't talk about hybrids...)and (sub)species are not all fully understood yet, that you cannot argue that this is absolutely for sure. But it shows a lot of indications in that direction, in my view. (I gladly would like to be corrected if I would be wrong!).
Bart
