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?

Yellow-legged Gull (Larus michahellis), ad, 19.11.2011, Bønnerup Harbour

Note the slender and long-winged appearance with elongated body profile and long yellow legs.

Note the relative heavy bill with strong colours and red gonys spot reaching lower edge of upper mandible, yellowish iris surrounded by a thick red orbital ring, strong and round head profile.

There is a black band crossing p5 and a white mirror isolated from the white apical spot on p10. 

Note darker upperparts than argentatus in direct comparison.

The primaries have smaller white apical spots than the average argentatus.

Black extending fare down the p10 feather towards the primary coverts on the underwing without a grey tongue on the inner web which suites michahellis just perfect.

Caspian Gull (Larus cachinnans), ad male, 4.11.2010, Feldballe

This large male Caspian Gull male was seen just north of Feldballe on Djursland. The primary pattern and many other characters are spot on for cachinnans, but the eye is much paler than on the classic individual. However, Lou Betalan has several similar pale eyed cachinnans on his excellent webpage “caspian and yellow-legged gulls in Romania”.

This male has paler eyes than the average cachinnans, however, note the extended red orbital in front of the eye.

Note the cachinnans jizz with a push up breast, flat back and slender elongated rear with long primary projection.

The primary pattern is perfect with the outermost p10 having a large white tip and a grey tongue running up the inner web, isolated white mirror on p9 and broad black band on p5.

In flight the hand is relative slender and when seen from the under wing the black on primaries creates a classic cachinnans boomerang.

Note the yellowish legs.

The distinctive black feather shafts on the outer primaries in the upper wing is commonly seen in adult cachinnans and in the stretched wing the grey tongues on the outer primaries are obvious when seen from underneath and even from above.

It has a snow white head without traces of streaks as in argentatus Herring Gull.

The upperparts are slightly darker than nearby argentatus which was obvious in both overcast and sunny weather.