He escrito a Kirk Zufelt, de Canadá. Tiene un interesantísimo blog, Larusology, y le he comentado la información y fotografías de la presunta Larus thayeri que tenemos en el norte de Lugo.
Esta es su opinión y estas las fotos que me envía.
I have looked at some of these pictures previously and the great links you provided just reinforces my impression that this is a "light-eyed" Thayer's. It really has to be a 4th winter bird for all the reasons eluded to in the comments(not quite fully developed white primary tips, tiny amt. of black on tail, bill markings). I have read that up to 10% of adult Thayer's can have a light eye but I am not sure what data that is based on. I think in the east part of its winter range it may be less than 10% as I have only ever seen one truly light -eyed Thayer's and a couple in between birds. I think the structure of this bird is very good for Thayer's. I think there is a little more black on the primaries than the "average" Thayer's but only by a small degree -I think well within acceptable limits. If I saw the bird here I would not hesitate to call it a 4th cycle Thayer's. I have enclosed some pics of a light -eyed adult Thayer's from Sault Ste. Marie, Ontario from a couple years back.