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Pure or hybrid Caspian Gull?

November 13th 2008, Dosthill Lake, Warks


Caspian Gull or hybrid, Warks, Nov 2008

Caspian Gull or hybrid, Warks, Nov 2008

Caspian Gull or hybrid, Warks, Nov 2008

A. R. Dean

A fourth-winter type photographed at Dosthill Lake, Warks, on November 13th 2008. The slender, parallel-sided and pointed bill, with little evident gonys, suggests Caspian Gull, as does the greenish yellow base to the bill, with a brighter tip and a neat black-and-red subterminal band. Other Caspian-like features include the blackish iris (with 'bullet-hole' appearance) and streaking confined to base of the neck. On the image of the bird preening, the pattern of black on the primaries is visible, including black on the tips of two primaries falling well inside the position of the tips of the tertials. The lowest image indicates extensive white at the tip of p10 (see under surface visible in lower photo). Judging from these images combined indicates that there is black on the outer seven primaries. The head-shape is not quite convincing for Caspian Gull, with a quite pronounced angle at the forehead and a rather flat crown. The legs are decidedly short for Caspian Gull, with little visible tibia, and they are quite bright 'Herring Gull' pink. On a good Caspian the legs are long (with evident tibia), rather spindly, and a paler 'washed out' greyish-pink hue. Thus this bird appears to display some strongly Herring Gull-like features which do not seem compatible with Caspian Gull. Certainly, this individual does not look like a 'classic' Caspian Gull.  Is it perhaps a hybrid Caspian x Herring Gull. Caspian and Herring Gulls (and occasionally Yellow-legged Gulls) are now regularly hybridising in parts of eastern Europe and colour-ringed hybrids have been recorded in the UK:

See  for an example of a Caspian x Herring Gull.

However, perhaps more likely is that it is simply an argentatus Herring Gull 'mimicking' some features of Caspian Gull.

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