Friday 22 November 2013

Caspian Gull Review (Not for the Faint-Hearted)

I've just carried out a review of all of the Caspian Gulls I've seen at Shawell this year. I found two amongst them that I no longer consider proven, but I'm happy to say that the others were OK.

Third-winter Caspian Gull

I followed the criteria outlined in the paper published in British Birds: From the Rarities Committee's files - Identification of Caspian Gull. Part 2: phenotypic variability and the field characteristics of hybrids, by Chris Gibbins, Grzegorz Neubauer and Brian Small.

The paper offers a point scoring system based on the authors experiences with known Caspian, Herring and hybrid Caspian Gulls. As I have photographed most of the Caspian Gulls I have found, I was able to point score them. The paper only covers adult and first-winter birds, so I had to define my own criteria for some bits for the other ages.

The first thing I did was measure the bill ratio (length divided by depth) from my photographs. The ratio of all, but the two suspect ones, was between 2.5 and 2.85 (most were nearest to 2.8). It is interesting to note that Caspian Gull bills are not exceptionally long, but because they are quite thin they appear to be long. The paper states that amongst the 100 sampled Caspian Gulls, 87% had a bill ratio of between 2.4 and 2.79 and all had a slight gonydeal angle. Only gulls considered to be hybrids had no gonydeal angle at all.

Almost all of the Caspian Gulls I have photographed showed enough to score them. Some did not show leg length, so on these I scored them as having moderately long legs and so gave them a score of 1 instead of a 0 for long legs. I did the same with any other features that couldn't be seen, so some individuals had a higher score than was perhaps the reality.

35 gulls scored within the range for Caspian Gull and so did one of the two that I am concerned about. The bill ratio appears a bit small on the one that scored OK, so I think it is best to pend it for now. The other I now consider to be most likely a hybrid.

Pure adult Caspian Gulls should score between 4 and 9 and first-winters 12 to 22.

The first-winter gull below had a point score of 20 and so should be a pure Caspian Gull - the most frequent score amongst the sampled first-winter Caspian Gulls was also 20. Interestingly , this individual had a longer primary projection than another that looked spot on because of its longer bill ratio.

First-winter Caspian Gull

The adult below was one I saw recently and that one I scored a 7.

Adult Caspian Gull

This excercise has been well worth doing because it has shown that there are perhaps fewer hybrids than might be expected. 

Once the features that attract a trait score are understood this is a fairly simple task to do.

A set of record shots of Caspain Gulls allows the observer to fully check the identification at home afterwards. As far as I'm concerned photographic evidence is essential if you wish to prove the identification of these challenging gulls.

There is a great deal of studying required to ID these critters confidently, but luckily most of them do stand out in the crowd.

To read the paper on how to point score them click HERE

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