Examples of pale 1st-cycle, 2nd-calendar year Herring Gulls, believed to be Scandinavian Herring Gulls Larus argentatus argentatus, as seen at Katwijk aan Zee, the Netherlands, 8 March 2014.
Influence of hybridisation with Glaucous Gull Larus hyperboreus is often suspected (Viking Gull) but cannot be sufficiently proven.
Bird #1
8 March 2014, Katwijk aan Zee, the Netherlands. Pale Scandinavian Herring Gull (front) with European Herring Gull of same age (back).
8 March 2014, Katwijk aan Zee, the Netherlands. With European Herring Gull of same age.
8 March 2014, Katwijk aan Zee, the Netherlands.
8 March 2014, Katwijk aan Zee, the Netherlands.
Bird #2
8 March 2014, Katwijk aan Zee, the Netherlands.
8 March 2014, Katwijk aan Zee, the Netherlands.
8 March 2014, Katwijk aan Zee, the Netherlands.
Bird #3
8 March 2014, Katwijk aan Zee, the Netherlands.
8 March 2014, Katwijk aan Zee, the Netherlands.
8 March 2014, Katwijk aan Zee, the Netherlands.
Bird #4
8 March 2014, Katwijk aan Zee, the Netherlands.
8 March 2014, Katwijk aan Zee, the Netherlands.
8 March 2014, Katwijk aan Zee, the Netherlands.
Gulls to the Horizon 
why is the hybrid of herring gull and glaucous gull called viking gull?
I actually don’t know, Neil. It is difficult to find any information about the origin of this name.
Regards,
Maarten