Immature gull ID, England (1 Viewer)

[Lissagriffin]

I am continually baffled by immature large gulls that were all known as Herring Gulls when I was learning my birds many decades ago. This one doesn't look like a straightforward argenteus to me, but I'm having trouble placing it. Perhaps some hybrid combination? When I first saw it I thought I might be able to pin it down as a Caspian Gull, but structurally it doesn't look quite right. And the bill seems a little on the dainty side, suggesting perhaps some Lesser-Black-backed genes. Comments from gull aficionados would be a helpful learning experience.
The bird was seen on 25 April 2024 at Fairlop Waters, London Borough of Redbridge.

 

[Alexander Stöhr]

Hello Lissa,

I see where you are coming from.
Whil it looks strange/wrong for a 1st cycle Caspian Gull for me too, I must admit I would think its a Caspian x Herring Gull hybrid when seeing your bird in NE-Germany.
I think its underwing is too pale and unmarked for a 2nd cy YLG (I darkened the picture and think the underwing is really that unmarked). Please compare to this 2cy YLG (Schwetzinger Wiesen, SW-Germany, 19.05.2013). But can extremly pale, worn and bleached YLG be excluded, considering the uniform very pale underwing?
Same with the virtually unmarked tail-base. Yes, any markings could easily been burned out by overexposure, strong light and low resolution (do I still have to write my no offense sentence?), but such an unmarked tail with an sharply demarcated tail band is not good for 1st cycle HG
And the white head with the right mascara streak behind tge eye and an indication of an shawl also adds to a Caspian/YLG Impression to me... but yes, 2cy HG are regularly white headed too
(ageing as a 2nd cy bird is correct by replaced tertial with immature pattern, not uniform greyish and very worn and bleached brownish primaries???)
but I better stop before my comment becomes too confident and as always: thanks!