Here, a single photo of a large male that gave great black backed gulls a tough match for food. A bit "fishy" perhaps?..
Then these three birds where standing side by side. Three is about the total number seen in Norway annually...From left to right: ad (or near ad = 4cy+), 3cy and 1 cy. The 1cy is a bit darker than the typical 1cy CG, so we've added quite a few photos of the bird...In our opinion it's within the variation of CG, but maybe a hybrid cannot be excluded? Comments are welcome.
Here are photos of the underwing
Note the strongly marked breast and flanks
Upperwing
Tailbar
Nothing to worry about with the next one, though...
Here are a few photos of the 4cy+. Note that the iris, on a closer look, is really amber, not brown or black. Note very typical body shape and stance. The "back line" - from wing tip to nape - forms an almost straight line, like it was made with a ruler...
As usual, when you see a caspian gull next to a herring gull, you are struck by how much whiter the caspian seem to be, and it's not just the head. Note also difference in leg length, especially above the tibia. Pale yellowish legs seem to be common in this age group and note also the greenish bill.