Photographic evidence appears to show more extensive head streaking on the Azores individuals and so my bird looks good as far as that's concerned. A single mirror on P10 only is more common amongst the Azores population rather than from the other islands - mine had just a single small mirror on P10 and no mirror on P9. The mantle shade/colour of my bird also fits Azorean best as far as I can determine.
|Azores Gull, Portimao fishing Harbour, Portugal, October 1st 2013|
In clockwise order from top left: Yellow-legged Gull, my Azorean Gull, Channel Isles Lesser Black-backed Gull and a Scottish Lesser Black-backed Gull. Looking at these images it would appear that the mantle grey gets darker in that order. Yellow-legged Gull is said to have a Kodak Grey Scale value of 5 -7 , Azorean Gull 7-9 and Lesser Black-backed Gull 8-10.
|Azorean Gull L. m. atlantis|
|Yellow-legged Gull Larus michahellis|
There appears to be plenty to learn about the gulls from the Atlantic islands, as much of what I've read points very much to a work in progress.
Click Here to read one of the reference papers.