During September I tweeted out that I had 'scored' the Amble Caspian Gull against a paper in British Birds. As with most thing to do with Caspian Gull I got interested fairly swiftly, the paper is written by Chris Gibbins, Grzegorz Neubauer and Brian J. Small. It's a very impressively written and thorough paper in the December 2011 issue of British Birds. It was handily written only recently, as our knowledge of the acceptable variability in Caspian Gulls has grown, it was written to deal with the pitfalls of possible hybrids.
If you have time to sit with a highlighter and read the article I can highly recommend it. Some particularly interesting features were listed in the article and the article has a positive conclusion. So if you are interested in gulls make sure you read it!
'Table 1' in the article is what this entire brief blog post is about, it lays out a scoring system for adult Caspian Gulls. I thought I would run the Amble Caspian Gull through the test, trying to be as objective as possible. The reason for this is that although it was from a 'Caspian Gull colony', it was a bit naughty to fail to point out that the colonies in that area are known for having mixed pair.
Lets be clear, this bird was a '4th winter', if you like, so NOT an adult. However it would appear that all the features listed in the table remain unchanged between this age and adult as they are so similar. Seems as though the species is seemingly so rare north of the Tyne I thought it best to be 'cruel' rather than 'kind', to 'edge on the side of caution'. So when scoring the bird I gave a realistic, if not somewhat mean score in order to be 'sure' on the outcome.
When I scored it in September it came out with a score of 7, but I scored it again this month and came out with 8. Even though I was very cruel with the final score of 8, the article comments; "For adult birds, it is clear (fig. 6) that a bird with a score of less than 9 should be pure Caspian, while one with a score of 9 or more is not safely identifiable as a Caspian." Later the same article goes on to say "Frequency statistics indicate that these scores can be used to separate even the most extreme (atypical) first-winter birds and the vast majority (c. 99%) of adults with confidence."
If you have got the article then you can go away and have a look for yourself, but basically the score I allocated for the black band on P10, overall P5, overall P4, the iris peppering, the eye-ring colour, the bill, and the leg length would likely go uncontested! The only feature that I think could be contested was what I scored the overall black to white ratio on P10, I scored this a 2. As 'having more black than white', however I think this may have been quite unfair! Even so, it still passed the test.
However, this was based on me giving the bill a score of 1, which is "slim, slight gonydeal angle (ratio 2.4–2.79)", whereas 2, "intermediate (ratio 2.0–2.39)", may have been a fairer assessment. If we said the bill was 2, and dropped the black to ratio down to 1 ("approximately equal black and white"), the bird would still come out with a score of 8. Which is still pure Caspian Gull, I think that my original score of 7 was being a bit too kind. I do wonder how the Cromer (Norfolk) bird would score.....
I personally fail to see how the Able bird could be a hybrid, it even sounded well and truly the part in the field. Time and research may eradicate all of this data, but at the moment I think it's safe to shimmy the 'Amble Casp' into the 'Casp column'. Most people who have commented (minus LGRE) have not expressed concern, the bill may appear a little chunky and deep at the base, but a quick Google search can show that this isn't too abnormal for Caspian Gull. I think for now, I'll trust the 99% percentage. So I will say...