Possible Caspian Gull x Herring Gull hybrid

In most features this gull from Aarhus East Harbour seen on December 2-3, 2010, looks a lot like a pure Caspian Gull even though such an ID adds a little uncomfortable feeling as the gull possesses slight fore-head streaking and streaking on top of its head together with 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. It might be safe to say that it is probably a cachinnans but with certain off-typical features.

It is the fourth winter that we see this gull and 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.

The bare part colour in the opened bill is spot on for cachinnans being orangey-yellow at the base of the mandibles where the skin colour in michahellis is typical blood red and light pink in argentatus.

P5 in the right wing has black only on the outer edge whereas the same feather in the other wing has a narrow though complete black band. Additionally there is little black spot on p4 in both wings, extensive grey tongues on the outer primaries, all white tips on p10, small white mirror on p9.

It has a good cachinnans-jizz with high push up breast, flat belly slightly dropped behind legs. Additionally the upperparts are slightly darker than on nearby argentatus.

Once in a while it gains some argentatus appearance due to e.g. body structure and head/bill profile. The short bill has a rather blunt tip which could fit a cachinnans, but is more argentatus-like. 

The eye color is relative dark with a slight green peppering in the iris and when seen at some distance it stood out as being darker than in argentatus. There is a thin red orbital ring seen at close range where argentatus normally has a yellowish ring.

The bill is rather short and blunt at the tip; could fit a cachinnans, but is more argentatus-like. 

When feeding on ground or seen on water it mostly gives a cachinnans-jizz with rather small head, slender elongated rear and flat back, high pushed up breast.

The distinctive black feather shafts on the outer primaries in the upperwing are commonly seen in adult cachinnans. 

Once in a while it gains some argentatus appearance due to e.g. body structure and head/bill profile.

In the stretched wing the grey tongues on the outer primaries are obvious when seen from underneath and the black on primaries creates a classic cachinnans boomerang.

P5 in the right wing has black only on the outer edge whereas same feather the left wing has a narrow though complete black band. Additionally there is little black spot on p4 in both wings, extensive grey tongues on the outer primaries, all white tips on p10, small white mirror on p9.

In the stretched wing the grey tongues on the outer primaries are obvious when seen from underneath and the black on primaries creates a classic cachinnans boomerang.