• Identification of Caspian Gull: Phenotypic variability and the field characteristics of hybrids (2011)
• Tables 1 and 2 show scoring for possible adult and first-winter birds respectively. Fig 6 shows what these scores mean.
• Identification of Caspian Gull (2003)
• Larus Cachinnans (gull-research)
• Breeding Range Expansion of the Caspian Gull in Europe (British Birds vol. 117, issue 1, pp 26–⁠35; 2024)

What to look for? This is not exhaustive – nor in order – but:

• Dark or specled iris (HG/YLG can also have dark iris)
• Small “beady” / “piggish” eye
• Long bill with gentle curve on upper mandible and low/no gonydeal angle on lower
• Bill less “robust” than YLG
• Striking white head (be wary of Jan+ herrings)
• “Shawl” around the back of the neck
• “High chest”
• Long, thin neck
• “Flat” back
• Darker relative mantle (upperparts?) vs herring
• Long, possibly thin-ish legs
• Yellow or yellow-ish legs, but can be pink-ish
• White tip on P10, also less black than white overall on P10
• Full black bar on P5
• “Grey tongues” on primaries
• “Sloping forehead” / pear-shaped head – this is more of a general impression thing I think – “snouty”
• Probably won’t have a flat crown
• “Albatross pose”
• Quite a few of the characteristics resemble common gull
• Dark tertials and greater covert bases at rest in 1w (like GBBG)
• Birds mature at a different stage in winter vs herring so can be good to note wear and tear and mantle moult
• Sometimes black ring around bill in winter adults
• “Angular” back of head
• Greenish/yellow bill in winter like common gull