This Caspian Gull has relative long and characteristic thin legs with slight yellowish colour which is most obvious on Århusbirder. The small eye has relative dark colour with a slight green peppering in the iris. When seen at some distance due to the peppering and the smaller size it stood out as being rather dark in contrast to argentatus. The bill is rather argentatus-like, but within female variation. It has a good cachinnans-jizz with push up breast, flat belly slightly dropped behind legs and long elongated slim rear. Additionally the upperparts are slightly darker than nearby argentatus which was obvious in both overcast and sunny weather.
In the stretched wing which was seen on location but without documentation the grey tongues on the outer primaries was obvious when seen from underneath, on p5 there was a broad black band, on p10 there was a isolated mirror and an even smaller mirror on p9 which is atypical for western cachinnans, but more typical in more eastern bird within cachinnans populations. The white apical spots are all relative small on this bird and there is a unique step in distance between tip of p7 and p8 making this individual distictive.
In flight the hand was slender and when seen from the underwing the black on primaries created a classic cachinnans boomerang. More photos from November 12 on Århusbirder
From time to time such birds with more black in wingtip than average are observed among Caspian Gulls all over their winter range and at breeding sites in spring, even in the western Black Sea populations. Together with its blunt tipped bill it could be of mixed parentage with Yellow-legged Gull which could explain the large amount of black, but there are definitely Caspian Gulls with such bills and smaller windows among the ponticus breeding population so on doubt this is rather a cachinnans than anything else.