Here is a Common Gull that I saw briefly (very frustrating) at Peterhead on 26 February. I've been playing around with criteria to identify heinei Common Gull and it turns out that this bird is a perfect match. Here are the images; some text and analysis follow...
It is the left hand bird in these two flight pics.
It was picked up as being distinctly darker than the canus and lacking their blue tone. Details in the wingtip are key. Note: (i) the deep black band on primary 5 (canus normally has a thin, uneven or broken band, or black only on one web), (ii) black extending all the way to the primary coverts on P8 (canus only 50-75 % of the way), (iii) extensive black on P7 (more than 80% of the way to the primary coverts; canus mostly less than 50%) and (iv) a long black 'bayonet' on the outer web of P6 extending for approx 65 % of the way to the primary coverts (maximum in my canus sample is 53%, with most having black only 20-40%). The overall result of these details is a wing tip that has a lot more black and less white than canus. All of these features evident on the Peterhead bird perfectly match heinei, as far as I can establish.
Can we be sure? Well, I characterised a sample of 55 birds using 8 wing tip traits and analysed them using PCA. The diagram at the top of this post is the result. The Peterhead bird is the red dot: it falls outwith the area occupied by canus and smack within the space occupied by heinei. This is fairly strong evidence, based on multiple traits analysed simultaneously, that this bird better fits Russian Common Gull L. canus heinei than nominate canus. Food for thought...
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