Given the recent putative Slaty-backed Gull in London, I paid close attention to the Slaty-backed Gulls in Choshi. I saw around 45 examples of what I identified as Slaty-backed and photographed several. As with most gulls, this species is variable and, although there are a suite of features (eg pale eye, dark ‘intermedius LBBG-coloured’ mantle, shortish raspberry pink legs, pale yellow bill etc) that distinguish a typical Slaty-backed, there appear to be many examples with one or more of these features lacking. Below is a selection of my images of Slaty-backed Gulls with some associated personal comments. I am in no way qualified to comment on the London bird, except to say that if I saw it at Choshi I wouldn’t have hesitated to identify it as a Slaty-backed.
Please contact me if you spot any inaccuracies in my commentary or have any additional comments.