A free hour mid afternoon saw me back at the lake, with c450 large gulls on arrival that later swelled to 1000+ after an influx from the tip. The species composition was fairly typical, roughly 800 LBBs, 200 Herrings and 5 michs (3 ads & 2 2cy), but there were also 72 GBBs, an impressive count. One or two oddities kept the interest going in the absence of anything better, including a 3cy argentatus with big P10 mirrors - said to represent less than 5% according to Gibbins et al.
Finally, the highlight of the day appeared: a superb candidate ad AMERICAN HERRING GULL! Big and gangly with a pale grey mantle similar in tone to nearby argenteus, it was the head and breast streaking that really caught the eye. Seemingly made up of finer, browner markings over the head and neck, the streaks merged into blobs and blotches on the breast sides and across the upper nape, where it resembled the smooth brown colouration of younger smithsonianus. There was a denser patch of streaking around the eye as well, accentuating the staring whitish-yellow iris. The snout was paler, leading into a long and parallel bill with a small gonys, with a pinkish basal 2/3 and yellow tip, the red spot confined to the lower mandible and a black mark running across both. The wings were long with an obvious long white tip to P10, and on some views a sugggestion of a whitish tongue too. The final feature in favour of the ID was the apparent isolated dark mark in the centre of a tertial. Unfortunately however, any clinching details in the primary pattern remained hidden, and I had to leave before I saw it flap or preen (two small children in the car and another needing to be picked up from the cinema!).
So whilst looking the part and ticking several boxes, it will never be possible to confirm the ID.