Through binoculars the paler mantle and apparently abraded coverts reminded me of a Ring-billed Gull similar to this one (right) photographed in California in April 2010.
Through the scope it was clear that the wing coverts of today's bird were not abraded but simply lacking in pigment. This leucism was presumably responsible for the paler mantle shade. So, a partially leucistic Common Gull - not unheard of, but certainly not common. After a short time bathing this rather unusual looking leucistic gull took off and joined the stream of gulls heading for the coast during the late afternoon.