Hi Katy & all.
I have seen this on Calbirds and the one Katy linked. Also here:
http://ronstorey.com/Lucchesi/index.html
Interesting gull which doesn´t strike you as a Mongolian Gull. Firstly - the age of the gull is essential to establish.
Being in Marsh a 1st winter would have pointed and more worn primaries and slightly different upperparts, on the subject gull there are some grey feathers which is more in line with 2nd winter (now 3cy), but there´s some variation (as in Herring Gull), some 3cy mongolicus have nearly fully grey upperparts:
http://www23.tok2.com/home/jgull/051118/mongolicus2w.htm
The tail band is more clear-cut and narrower in 1st winter mongolicus with barring above tail band and usually all dark bill, some are pale based and slightly more blunt-tipped:
http://www23.tok2.com/home/jgull/060124/mongolia.htm
http://starling.dyndns.org/~birdkitahiro/hokkaido/gulls/gullindex/mongol/1w08.htm
The inner primaries are pale as the subject gull but note the dark subterminal markings near the tip on all inners in the linked mongolicus.
Have a look at different ages here:
http://www2.kongju.ac.kr/srcho/pintail/2007/feb/2007020501.htm
I agree with Alvaro Jaramillo except I think it´s a 3cy.
http://groups.yahoo.com/group/CALBIRDS/message/6077
JanJ