Images of what I believe to be a fourth-calendar year Herring Gull x Lesser Black-backed Gull. Noticeable was the darker grey-tone and full black band on P5 (influence of a Lesser Black-backed Gull), with the posture and yellow eye ring of a Herring Gull. Also note the faint mirror on P10.
Seen in the gull colony of IJmuiden Forteiland, the Netherlands, 4 June 2017.
Gulls to the Horizon 