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.
The first Slaty-backed Gull I saw. This individual has a relatively bright bill
No2: looks dark-eyed but in the field the iris was pale-ish, albeit heavily speckled
No3: a typical individual with a pale eye, dark (Lesser Black-backed-coloured) mantle and strong bright pink legs
No3 in flight: same bird as above. Mantle appears lighter at the different angle.
No4: from below
No5: from above. The mantles of some birds look relatively pale in strong light. Note no mirror on p9.
No5: Same bird.
No5: close up of head and underwing pattern. Eye is speckled and could appear dark at distance.
No6: a dark bird with heavy head streaking. 'String of pearls' obvious.
No7: an immature bird but note very pale underwing in strong light.
No7: Same as above. Note how the underwing looks darker in different light.
No8: the legs appear relatively pale and thin (taken in strong light)
No9: eye appears dark. mirrors on p10 and p9.
No9: Same individual as above. Eye looks paler in strong light.
No10: Eye looks dark on this individual. Bill pale yellow with slightly brighter tip.
No11: 'String of pearls' clear on this bird. Heavy streaking on the head.
No12: A typical individual in terms of mantle colour, pale eye, primary pattern and leg colour.
No12: same individual as above next to a Vega Gull (with Black-tailed Gull behind) for comparison.
No13: Pale eye, dark mantle and relatively heavy head streaking.
No13: Same bird as above. Note no mirror on p9.
No14: Typical. Pale yellow bill, sturdy bright pink legs.
No14: same bird as above in flight. 'String of pearls evident, large mirror on p10 with smaller mirror on p9.
No15: Note no mirror on p9.