Lesser Black-Backed Gull
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I agree entirely with your identification points. In the south of England, we now have to be careful not to confuse this species with yellow-legged gull. You won't find these in many bird books as they have recently spread from Europe. You might not even find them in books on European birds as they used to all be considered as herring gulls. Yellow-legged gulls are like the lesser black-back in the photo but the back and wings are a much lighter grey, between the colour of a herring gull and a lesser black back.
These gulls form what is known as a ring species with Herring gulls, Yellow-legged gulls, American herring gull and up to six other species (depends who you speak to). A ring species is one which is found in a band around the globe and as you travel east-west it gradually changes, sometimes the changes are enough to create a new species and other times just new sub-species. Until you return to the point of origin where you have two distinct species, in this case the herring and lesser black-backed gulls.
The Larsus gulls are considered the classic ring species, however, recent genetic studies have shown that the situation is far from clear cut - all of the seperate species/sub-species can interbreed, even herring and lesser black-backed gulls. Also there is not a clear gradation in the genetic makeup of the species as you go east-west.
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The concept of ring species only works if they stay put. We used to only have these 2 extremes of the ring (actually there's an even darker one in eastern Scandinavia) but recently the yellow-legged gull from southern Europe has spread to the UK. So now we have a pale grey version (herring), a mid-grey version (yellow-legged) and a dark grey version (lesser black-backed).
Lat/Lng: 55.857087557906, -4.2553782463074
OS grid ref: NS589649