These results support that probably no birds from mainland China have crossed the sea and mixed into this population during the last 50 years. So all Japanese RC are descending from only about two dozen surviving birds in the 40s / 50s of the 1900s, which not only generates a problem of inbreeding, but also might have been a source of quicker evolutionary changes.
In result, the two populations may have sufficiently genetically separated so as to be regarded as sub-species. We should not disregard, however, that "language" is strongly based on social structures and dynamics. I assume, that the even stronger effect on the evolution of a different "language" in Japan is based on the social situation there (change between territorial behaviour in spring through automn and bigger flocks in winter with high bird density on the fields) compared tothe one in China / Korea (territorial during breeding season, then flocking for migration, partial territorial behaviour in wintering site with some tendency towards bigger flocks where additional food is supplied).
(Dr. Bernhard Wessling) Bargteheide, March 12, 2000