As Oskar points out, the outer web on p2-4 are paler than normally expected for a juvenile Caspian Gull. Note however, that the outer web of p4 is not entierly pale, but rather normally dark but with a narrow pale 'window' not creating the same effect seen in p2 & 3. Such pale 'windows' are sometimes seen in both Caspian and Yellow-legged and as a rule in Herring gull, usually situated closer to the tip of the primary, but occasionally along the whole web.
