This Caspian Gull is wearing a green colour ring with the inscription matching the Polish projects, but I have yet to receive ring data (hopefully published here later).
Is has relative long pale greyish legs where the thinner appearance with longer tibia compared to argentatus is quite distinctive. The small eye is relative dark and when seen at some distance due to colour and size it stood out as being very different from nearby argentatus of same age class. The bill is relative long and without pronounced gonys angle. It has distinctive dark spotted neck boa in contrast to the more whitish rather unspotted head.
It has a good cachinnans jizz with a high push up breast, belly slightly dropped in a sluggish way behind legs and long slim rear with the unmoulted p9 and p10 just adding to elongated impression. Additionally the upperparts are slightly darker than on nearby argentatus of same age class.
In the stretched wing some pro 3K cachinnans characters are: the black center on outer primary coverts, dark outer edges on secondaries, broad black band on p5, small white mirror on retained 2nd winter p10 (only rarely seen in argentatus but most common in cachinnans). There is a well-defined broad black tail band with the inner tail half unmarked white.