This 1st winter gull was seen at Odense Kanal north of Odense on December 30, 2010. All photos were taken by local birder and finder Tim Hesselballe Hansen.
There certainly is some cachinnans in it but many things point to at least f1 or f2 parentage of argentatus especially as the dark markings on uppertail coverts are very coarse as in argentatus. The tail band has a lot of thin additional transversal bands towards the base of rectrices which is a kind of a cachinnans "trademark" but the extensiveness in this individual is together with the heavily patterned uppertail coverts and rump much darker than the average cachinnans so, even if we don't really know it things point to a possible hybrid.
Other individuals with quite similar uppertail here, here and here.