The Connecticut, USA, SLATY-BACKED GULL

Lee, I saw the images of Britain's first probable SLATY-BACKED GULL on Surfbirds - congratulations on that.

Speaking from a bit of experience here, you may receive questions about the bird's mantle color (dark gray rather than blackish...which may cause folks to scream 'hybrid.'). This bird appears to have a similar mantle color/shade to a sub-adult bird I found here in Connecticut, USA. The idea of a hybrid was looked into, and we looked for any other features (eg, size, structure, and plumage) that might indicate a hybrid with Vega or Glaucous-winged Gull. We could not find any...the bird seemed spot on for SBGU. On top of that, we sent the images to the Ujihara's in Japan for analysis, and they felt the entire bird was just perfect for SBGU, with the mantle color falling within range for pure SBGU (as far as they know it to be).

Some of the problem with the CT bird were that a few of the first photos (some of mine) were a bit over-exposed/'bleached out'...but later photos by folks like Mark Szantyr and Julian Hough were higher-quality images (but the mantle was still not blackish to be sure).

Here is that CT bird, including those original photos: http://www.shorebirder.com/2008/11/slaty-backed-gull-nov-28th.html

Here is a very nice write-up by James P. Smith regarding SBGU mantle shades:
http://pioneerbirding.blogspot.com/2008/12/murky-shades-of-slaty.html

The Ujihara's website in Japan is a fine resource for images of NW Pacific gulls, with a huge selection of Slatys:

http://www23.tok2.com/home/jgull/gullidentifi_.htm

Nick Bonomo, Wallingford, CT - www.shorebirder.com