More pics here: http://peteralfrey.blogspot.com/2010/12/beddington-mystery-gull-december-2010.html
There were several features of this first winter bird that were anamolous and suggestive of some kind of Nearctic and also Glaucous Gull influence. These features included the extensively dark rectrices, the heavily barred upper tail coverts, the marbled tertials, the dark wash to the nape and underparts and pale tips to the primaries. It is quite possible that a bird showing these characteristics could be generated from Nearctic gull populations- perhaps something like a 2nd generation 'Nelson's Gull' (Glaucous Gull x American Herring Gull).
However if there is a plausible answer that is closer to home than it is reasonable to assume this higher probability explanantion. Birds showing these features can be found in Icelandic populations and perhaps in some argentatus from northern western europe. Here are a couple of pics by Hans Larsson of argenteus showing strong uppertail covert barring and dark rectrices from Iceland (The Herring Gull populations of Iceland are believed to have originated from argenteus Herring Gull rather than argentatus Herring Gull)
http://www.pbase.com/hans_larsson/image/123551459
http://www.pbase.com/hans_larsson/image/123551457
Lonergan and Mullarney in the 2004 DB article noted a high degree of American Herring Gull traits in the Icelandic birds and suggested that it could be due to the high incidence of interbreeding with Glaucous Gull or that American Herring Gulls may have entered the 'Icelandic Herring Gull' gene pool.
Here are some more pics of argenteus in Iceland
http://www.pbase.com/hans_larsson/argenteus_type_gulls_sw_iceland_28243_2010&page=all
Hybridisation between Herring Gull and Glaucous Gull is very extensive on Iceland:
Here are some pics of Herring Gull x Glaucous Gull (Viking Gull) from Iceland
http://www.pbase.com/hans_larsson/viking_type_gulls_sw_iceland__28243_2010&page=all
So back to the Beddington bird- a bird showing features of American Herring Gull and Glaucous Gull- it could be an Icelandic bird.
Here is another bird found by Chris Gibbins in Scotland which shows several American Herring Gull features and the suggestion here is that the bird could be from Iceland:
http://chrisgibbins-gullsbirds.blogspot.com/2010/12/another-exceptionally-dark-2w-herring.html
Another possibility remains that the Beddington bird is some kind of argentatus variant with or without Glaucous Gull influence.
So the most likely conclusion so far
a) An 'Icelandic Gull'- An argenteus with Glaucous Gull influences (and also possibly AMHG).
b) A unusual argentatus variant
Birdforum thread here: http://www.birdforum.net/showthread.php?t=188204
Thanks to Peter Adriaens, Julian Hough, Richard Millington and Brian Small for comments and links. The conclusions here are a personal interpretation based on comments.
