Previous experience of the species (JSH): Have located (though never ‘found’) the species in 1w, 2w and adult plumages at Hayle and Par Beach in Cornwall (about 12-15 bird days, mainly adults).

5/3/00 c. 12.30-13.30 at about 60m on opposite bank of Welland with about 20 common and 10 black-headed gulls. Mainly seated: stood once and wandered about for c. 2 minutes, calling once.

11/3/00 c. 09.00 - 11.00 in same location mainly seated, but also standing and in fight several times as it shifted along the bank. (With AFB and AD)

18/3/00 c. 150m away in field. Included prolonged views looking down on the upperwings as the bird flew around picking food from the surface of the ploughed field.

Whilst scanning through a group of about 30 gulls with bins on 5/3/00 I noticed what I took to be an adult common gull with a ‘dodgy eye’ - I could not make the iris out as pale but the expression was sufficiently odd to make it worth a look through the scope. Through the scope it became obvious that the expression was caused by a pale yellow iris and so I concluded I must have an adult RBG. Thinking that, through bins, the bird had not looked as obviously different from CG as it should have, I checked the upperpart colouration and tertial crescent through the scope: the upperparts appeared the same shade of grey as CGs present, there was plenty of white visible as a crescent on the ends of the tertials and there were three white tips clearly visible on the primaries that did not look particularly diminished in comparison with CGs.

I concentrated on the head, therefore, to make sure about the eye which I had always thought a clinching feature for RBG. The bird sat inactive with its head drawn into its body for long periods, but occasionally took a look around and once stood for a couple of minutes and called for a few seconds (which I noted sounded most like herring gull’s usual call). AFB made similar comparison, hearing call several times on a different occasion. I noted that the head was slightly squarer than CG and the bill more parallel, with a broad black band, though this was ill defined at the edges. As the bird turned its head I could see that both eyes had a pale yellow iris and dark orangey-red eyering: there was a dark smudge before each eye that added to a fiercer impression than CGs. It had a few dark blotches on the rear of the lower neck. In brief standing views, I noted it appeared to stand slightly taller than the 3-4 standing CGs. The legs were a deeper and duskier yellow than any of the CGs present. I was content by this time that I had an adult RBG and went to alert AFB and AD. On return at c.14.30, all the gulls sat some 3-400' away in a field and the RBG could not be relocated with certainty, although we thought one a possible candidate.

Approximately the same size and (apparently) colour as CG, with structural differences in head and bill shape very subtle. When sat, no plumage feature noted that obviously set it aside at rest from all the CG present. After prolonged views, the bird could be picked out when scanning through CGs by odd expression and head shape and structure, though I was never 100% comfortable with the ID at rest until I could confirm the pale eye. On 18/3/00 at greater distance, difference in head and bill structure more obvious, looking much more like a mini-HG than at closer range - picked bird out straight away on this basis before it began flying around. In flight the bird showed two quite obvious white mirrors on outer primaries, but these were set in more extensive black, and less white was visible on wingtips than all CGs present. AFB, AD and JSH could all independently pick out the bird in flight from a group of CGs on 11/3/00.

Mainly white, with very faint pale brown streaks on crown. Distinct blackish spots on rear lower neck extending only slightly down sides of breast. Eye appeared set in deeper socket that CG with small dark smudge before eye accentuating more fierce expression. Some CGs showed similar squarish head at times, particularly when drawn into body, but this bird consistently showed squarer head in all postures. Forehead ‘turned the corner’ into crown at about 45⁰ in front of the eye on RBG whereas became more gradually domed in all CGs. Structural differences from CG were very subtle, but expression and head shape was consistent way of picking this bird out at rest.

Iris pale yellow. Eyering dark orangey-red. Mouth seen once on 5/3/00 by JSH and on 11/3/00 by AFB - bright orangey-red, but no opportunity to compare this to CG. Bill - quite a bright yellow with a broad black band towards tip. Borders of band ill-defined, more so towards tip, which just showed as pale (could have been horn or yellow). Some CGs had bills as long or longer, that appeared as deep as the RBG’s at their widest points, and appeared as yellow in some lights and with as broad a black band. However, RBG bill was parallel almost to the tip, where squared off at a steeper angle than CGs. In some lights bill obviously deeper yellow than adjacent CGs. Differences in bill very subtle, however. Legs appeared longer than CG in brief standing views on 5/3/00, but could have been a result of stance as only a few CG to compare with. When standing, legs appeared a deeper, duskier yellow than any CG (the brightest legs of which were a more insipid lemon yellow). In flight on 18/3/00 feet were more obvious and a brighter yellow than any CG.

Never appeared any different to CG, though all views were in bright sunlight when at times even CGs could appear almost as BHG in colour. Tertial crescent, when visible, (sometimes entire uppers washed-out by sun) always appeared rather clear-cut. I twice noted the tertials as appearing more squared off than the rounded CG tertials but this was not at all consistent and so dependent upon attitude and stance that I discounted it as any use on this bird.

RBG had v.small white tips to just one (or possibly two - difficult to tell exactly) scapulars each side, whereas three CG checked had more extensive white here, coming further up the tips of at least two scapulars each side. At rest, wings showed three white tips to visible (innermost) primaries. These decreased in size towards tip. Some CGs had more white, others less: I did not think this a useful feature on this bird at rest (but see flight view). Whilst sat on 11/3/00, white mirrors could be seen on the underside of the two outer primaries (i.e. the furthest wing away): these mirrors only just stretched from the white tip of the 2^nd innermost primary to the 3^rd on the nearest wing. On two nearby CGs, the mirrors on the underside of the furthest wings covered a greater distance, clearly overlapping the white tips on the nearest wings.

Prolonged scope views from above (looking down on field) at about 150' as the bird alternately sat and flew around with about 30 CGs. With prolonged views I could convince myself that the RBG had a slightly heavier flight action, but this was not a consistent method of picking it out from CG. The wingtip pattern was, however, markedly different in flight from all CGs present: the bird showed two quite obvious mirrors on outer primaries, but these were set in more extensive black that extended further up the wing. Less white visible on primary tips than all CGs (limited to tips on RBG, but much more obvious and extending further into the black on CG, particularly on inner primaries). This made the wingtip of CG look black and white, but of RBG look mainly black.

I had always taken the yellow iris as diagnostic when separating RBG from CG and my initial identification on 5/3/00 relied heavily on this feature. Following our views of the bird on 11/3/00 AFB, AD and I all agreed that this bird was less obvious than any RBG we had seen before and thought the features other than the iris sufficiently subtle as to consider other explanations. We have not heard of hybrid CG/RBG, but they must be possible. Both Grant and BWP hint at the possibility of CG with at least some ‘pale’ colouration in the iris but give no useful detail, and a Bird Images video of the birds of Badahe (China) mentions, but does not show, a ‘pale-irised’ Kamchatkan race of CG. Of these, RBG seemed the overwhelmingly likely explanation. Views of the RBG wingtip pattern in flight on 18/3/00 confirmed an adult RBG. The tertial crescent and upperpart colouration remain the most odd feature, though both could be put down to individual variation and/or lighting conditions. The overall size and size of the bill suggests this may have been a female.

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