Hi Steve.
I think our main worry was that the mantle seemed too dark (On our very limited knowledge). We were unsure as to the variation in RBG mantle shades, the extent of streaking on the head, a slightly larger than expected tertial crescent, and the fact I was wary due to the string of hybrid records over recent years.
In the field it seemed darker than the adjacent Herrings. Prolonged watching as it swam around in the water confirmed this impression (at least to me) but it didn't approach LBBG in colour at any point, but our 'group knowledge' (Sadly of only Waldo, which is very pale) quickly picked it out as darker in mantle shade to that bird, which perhaps is not the most helpful of examples.
Size wise is was perfect, noticeably smaller than both LBBG and Herrings surrounding it. Structure was good and as I would expect, perhaps a little elongated but I have never seen a RBG sitting on the water before.
The primary pattern is spot on for adult RBG (confirmed by multiple gullers) on both the upper and underwings, and the worries about the head streaking were ruled out as 'pure' birds show similar, which really only leaves the mantle shade.
Can you try to obtain photos/video of the bird at Belvide if it roosts again, particularly of the wing formula?
We have now had multiple high profile gullers who have contacted us, and the same answer is being repeated. At least 2 BBRC members believe it is a RBG, multiple 'big names' and the author of a rather topical book.
As for me, I'm undecided as of yet
