cgeoff

Ho Fai,

Yes, Caspian Gull is what we call mongolicus (whether the latter is a ssp of vegae or not is debated).

As far as we know, only 'mongolicus' occurs regularly. Caspian Gull L. cachinnans is a possibility, but it is likely to always be rare. There are no records currently.

As for separation, I would defer to the literature; e.g. Gulls of Europe, Asia and North America by Olsen and Larsson.

Regards,

GeoffC