© A. R. Dean

The Caspian Gull Larus cachinnans and Yellow-legged Gull Larus michahellis in the above composite image were photographed in the same gathering of gulls at Dosthill Lake, Warks, in November 2009. The Yellow-legged Gull showed an unusually extensive amount of white in the tip of the outermost primary, a less robust head-shape than many michahellis, and a relatively dark-looking iris (though careful observation revealed that it was in fact deep amber-yellow). In these respects, it converged with characters associated with cachinnans. All the features of the Yellow-legged Gull fall within the range of variation of michahellis but the opportunity to make a direct comparison with a well-marked cachinnans was valuable. Note the more upright posture of the Caspian Gull, its proportionately smaller head and longer bill, and its more attenuated appearance overall. In comparison, the Yellow-legged Gull has a stockier, sturdier appearance and more horizontal carriage. Other points of difference are annotated. In flight or during preening, further distinctions would be evident in the pattern of the primaries (see photo gallery of each species for details). Classic individuals of each species are relatively straightforward to identify but occasional individuals raise questions. It should be noted, also, that Caspian Gull, Yellow-legged Gull and Herring Gull interbreed in mixed colonies in eastern Europe. Proven hybrids (colour-ringed) between Caspian Gull and Herring Gull have reached Britain (e.g. in Cambs, Dick Newell in litt.) and hybrids between Caspian Gull and Yellow-legged Gull can also be anticipated to occur here occasionally. Thus, it is not always possible to determine where 'individual variation' ends and indications of hybrid origin begin. When (as here) gulls are observed in full daylight and on land, it is possible to evaluate a full range of characters. At roost sites, with gulls on the water and light conditions less than ideal, things are sometimes more difficult ....

Further images of the Caspian Gull above can be found here.