Amongst the commoner species were three adult Yellow-legged Gulls including an intriguing dark mantled individual and a new adult or sub-adult Caspian Gull.
I have checked its trait score, which I scored as 7 (see below) and so within the range of pure Caspian Gull. The bill ratio was typical for this species, as was the primary pattern. Its leg length was quite short, but that isn't that unusual.
P10, the longest primary feather, has a long white tip, less black than white and a pale grey tongue on the underside.These three features collect a trait score of 1. P5 has a bar across both webs, but thicker on the outer web, which scores 1. There is no sign of any black on P4, so that scores 2. The Iris is dark coloured, so that scores 0. The eye ring colour is not visible, so I've scored it 1 instead of a likely 0. The bill is slim with a ratio of just under 2.8, so that scores 1. Finally the legs were quite short, but not as short as the nearby LBBG and so I've scored it 1. That gives a total of 7. A total of between 4 and 9 is the necessary result for Caspian Gulls.
This is perhaps not a classic individual, but I can't find any reason to not call it a Caspian Gull. The main challenge with gulls is that there is quite a lot of variation between individuals. Females can have short legs and bills, so perhaps this one is a female - hard to tell, but it looks quite small when side by side with the LBBG. Females are quoted as looking a bit like Common Gulls and I think that description fits this bird?
The value of video is demonstrated by the fact that the hanging rear belly is not always visible, but if you watch it does appear. It is best seen towards the end of the sequence when it walks on to the shore. This feature was not obvious on the still images I took of it on the shore. Therefore it is not safe to assume that Caspian Gulls don't have a hanging rear belly if you only see the bird out of water for a short while.