Caspian Gull, which (as the name suggests) breeds on the Asian steppes, was once regarded as a form of Herring Gull (a name now used only for northern Europe's commonest "Gabbiano reale"), as was the Mediterranean's form, the Yellow-legged Gull. They were "split" into separate species in the early 1990s I believe, though this remains a controversial point among ornithologists.

Despite all my experience I find it quite challenging to tell them apart, which probably explains succinctly why they were once regarded as a single species.

The Yellow-legged Gull is the Mediterranean's most common breeding gull, and can be found almost anywhere near the coast and around landfill sites (yuk).

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Note added at 3 hrs (2008-08-27 15:21:46 GMT)
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The Caspian Gull can be seen occasionally in winter in the Mediterranean, especially along the coasts of Apulia and Sicily, most of them being inexperienced juveniles which have for some reason dispersed beyond the species' usual range. It has never bred in Italy

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Note added at 18 hrs (2008-08-28 06:31:35 GMT)
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Andrea Corso is probably the foremost Italian ornithologist, and in your link at http://www.ebnitalia.it/qB/QB001/gabbiano_ca.htm he writes "Uno degli esempi più emblematici è costituito dal Gabbiano reale del Caspio (L. (c.) cachinnans) le cui osservazioni si sono susseguite in maniera vertiginosa, arrivando a dimostrare come da rarissimo accidentale, quale era ritenuto sino a uno o due anni fa, debba invece essere considerato come un migratore regolare; difatti così è già avvenuto per la Germania, la Francia, la Gran Bretagna, l'Olanda ecc. " I count myself fortunate to have spent many hours in his company on bird surveys etc. I think the major problem here is probably Nyteck's source text, which is rather ambiguous, but rest assured that the Caspian Gull is not a breeding bird in Sardinia, whereas the Yellow-legged is extremely common. That said, however, if the text is actually referring to rare migrants rather than common breeders (can Nyteck provide more context?) then my interpretation is wrong