Possible Caspian Gull x Herring Gull hybrid

This gull was photographed on November 28, 2011 and it is the fifth winter it returns to Aarhus Harbour. It has a lot of Caspian Gull to it. However, the ID adds a little uncomfortable feeling as the gull possesses some argentatus-jizz. Even though none of the features that it shows are as itself exclusive of either one of the species most features seem to support cachinnans as best illustrated in the photos from its previous stay in 2010. A hybrid cachinnans x argentatus might be the best explanation?

Each year it is has had the almost complete white summer head from November shortly after arriving at the site. In October 2007 it was seen standing with argentatus, in November 2007 it was also seen standing with argentatus and in December 2007 it was seen with open wing here and here and standing with argentatus. In November 2009 it was seen in flight, standing with argentatus, swimming and standing with argentatus, swimming with argentatus and standing alone.

Glaucous Gull (Larus hyperboreus), 2cy, 12.12.2011, Grenaa Harbour

Herring Gull (Larus argentatus) with extreme bare part colours

On November 28th, 2011 at least 15 different Herring Gulls of different age classes were seen with abnormal bare part colours among some 700-1000 large gulls checked on Aarhus Harbour. Have they all been eating the same food with high concentration of Carotenoids?