Yesterday I visited Simrishamn, a great place for gulling in Sweden. Except for four Yellow-legged Gulls I also saw a Herring x Caspian Gull hybrid. I initially saw it in the binoculars with its wings up and I thought "nice, a young Caspian Gull!".
Here, the pale underwing that got me thinking of Caspian is visible. The pale window at inner primaries fits Herring better than Caspian.
After scoping it I found out all the upperparts including tertials were of Herring type.
bird's shape was all Caspian Gull as well as its face.