This beautiful leucistic Black-headed Gull Chroicocephalus ridibundus showed up on the beach of Katwijk aan Zee this morning.
Note the reduced pigmentation in the scapulars and upper wing coverts, reducing the gray to an almost white, whereas the lower wing coverts have been reduced to a complete white. As a result, the black in the outer primaries really stands out.
The head pattern is also almost non-existent.
Black markings can be seen on the alula and greater primary coverts which is normally an indication for a first-calendar year bird. However, this conflicts with the dark-red bill and legs which makes me believe that this is an adult.