[February 2014 addition: John King and I were completely happy that this bird was an adult Caspian Gull for the reasons set out in the above captions (long, thin washed out bill, small unstreaked pear shaped head, small dark eye, reddish eye-ring, darker than argenteus mantle, annaconda neck, dull long thin legs, hanging rear belly and pattern of p10). We felt that it was a classic bird in every respect, a view shared by several others that I have spoken to (some, like me, quite cynical!) who have seen these images posted on my blog. Unfortunately it never stretched its nearer wing for me to photograph so I couldn’t be sure of the pattern on p4 or p5. Without seeing these features I felt that attempting a trait score might be pointless but without seeing enough for an absolutely certain trait score I feel it is conclusive enough to support its identification as a pure Caspian Gull. The following comes from British Birds104:702-742, Identification of Caspian Gulls part 2 (December 2011) and applies to adults:
P10 overall pattern: white to black ratio. Less black than white. Score 0 (applies to all sampled Caspian Gulls, 25% of hybrids and 39% of Herring Gulls).
P10: white tip. Complete sub-terminal bar. Score 3 (applies to 12% of Caspian Gulls, 25% of hybrids and 7% of Herring Gulls). This is highest scoring (worst) trait noted on this bird, although a similar bird is shown on Steve Arlow’s blog (see above for details) as being ‘typical of some Caspians but generally not commonly seen on the birds in south Essex’.
P10: tongue. White or whitish. Score 0 (applies to 76% of Caspian Gulls, 17% of hybrids and 2% of Herrings).
P5: extent of black. Not seen. Score 0-4. Note that only 1% of Caspian and 0% of hybrids scored 4 for this trait so that outcome can safely be excluded while only 9% of Caspian and 25% of hybrids scored 3 making that a very unlikely outcome too. Most likely score 1 or 2.
P4: extent of black. Not seen. Score 0-2. Most likely score 2 as per 72% of Caspian and 83% of hybrids.
Iris peppering. Dark-looking. Score 0 (applies to 47% of Caspian Gulls but none of the sampled hybrids or Herring Gulls).
Eye-ring colour. Dark/deep orange to red. Score 0 (as 21% of Caspians, 57% of hybrids and 6% of Herrings).
Bill shape. Slim, slight gonydeal angle (ratio 2.4-2.79). Score 1 (applies to 87% of Caspian Gulls, 42% of hybrids and 8% of Herring Gulls).
Leg length. Long. Score 0 (applies to 52% of Caspian Gulls, 42% of hybrids and 5% of Herring Gulls).
The total score for this bird is in the range 4-9 depending on the unseen extent of black on p5 (0-3) and p4 (0-2) but almost certainly 7-8. This is comfortably within the range of the known pure adult Caspian Gulls sampled (4-12) and just outside the range for hybrids (9-20). Adult Herring Gulls ranged from 12-20. This supports the identification as a pure Caspian Gull made on the observed features, none of which suggest any hybrid influence at all. ]