This gull was seen on Aarhus East Harbor on October 30, 2007. When seen at distance it appeared long-legged with a rather dark eye in a generally white head, slender bill and saturated dark upperparts all resulting in a cachinnans impression but with a few drawbacks.
The thin winter streaks in head are needle-thin and very distinctive without the typical coarse spotted argentatus appearance; cachinnans can have such streaks in October as also seen on Netfugl but often less developed than in this gull. It appears not to be a pure Caspian Gull as you expect a good cachinnans to look like.
Characters in favor of cachinnans are the relative long legs compared to the adjacent Herring Gulls, the colourless bill is relative slim without pronounced gonys angle, the red spot on gonys appears to just reach lower edge of upper mandible, on the yawning gull the bare part colour in the opened bill appears to be orangey-yellow at the very most base of the mandibles where the colour is typical blood red in michahellis and light pink in pure argentatus, the eye has red orbital ring and is relative dark, there are reminiscence of a cachinnans neck boa with dark spots on lower neck, the upperparts are darker than on nearby argentatus. But as mentioned there are also characters not in favor of cachinnans such as the bill is not really long and not really pointed at its tip (upper mandible curves strongly as seen in the yawning photo) and on jizz it has a certain Herring Gull feel about it
In Poland it has for years been documented that Caspian Gull are moving in to the Herring Gull colonies giving mixed pairs producing offspring. Such a bird with an intermediary plumage detail could originate from a mixed colony, but we won't know that unless it is ringed. A hybrid can look like one of the parents or it can look intermediate and a pure bird can also look "intermediate" therefore an opened wing to see its p10 pattern is to wish for.