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I wondered about that subsp. yarrellii when iSpot suggested it, but it wasn't mentioned on the RSPB page. What is the key to identifying it?
Male yarrellii are much darker on the back (jet black, almost)in summer. All ages are darker grey/black than alba, with a blacker rump and, often very obvious, heavily marked, dark flanks.
Having said which...not all are as obviously different as that may suggest!
My Flickr photos...
Lat/Lng: 52.0, 1.3
OS grid ref: TM2749