In Strange Parallels: Southeast Asia in a Global Context Lieberman discusses early modern migration into the territory stretching between the Burmese capital of Ava in the north and the Mon capital at Pegu in the south and the ensuing ethnic complexity of the region:
“...tension was rooted in north-south population movements, which pitted newcomers against established populations...Shan raids on Upper Burma...offer the most dramatic example. But of greater long-term significance – especially in light of the fact that dry zone ethnicity remained heavily Burman – was the continuous movements of Burmans themselves into thinly populated areas in the Upper delta and Lower Burma which since the 11th or 12th century, if not earlier, had been dominated by Mons" (Strange Parallels, p. 133).
Lieberman then goes on to provide some hints on how future, more empirically-based research on migration could proceed using indirect linguistic evidence:
“The spread of Burman ethnicity from the 14th century to the early 16th century can be traced in the eclipse of old Mon communities in the north, in growing settlement of coastal cities, and most especially in the Burmanization of rural districts between the 18th and 19th parallels that once contained sizable Mon populations. Although vestiges of Mon place names survived, by the mid-15th century there is little evidence of Mons living north of the latter line” (Strange Parallels, p. 133).
For the last detail on Mon place names, Lieberman cites a personal communication with the great Mon linguist H.L. Shorto who taught at SOAS and died in 1995 and translated large parts of the Nidana Ramadhipati Katha (a text that is nearly impossible to find in its circa 1912 Pak Lat Mon alphabet published version and which the near impossibility of access to manuscripts housed at the Burmese and Thai national libraries precludes unravelling, see manuscripts 18 and 19 in this Pak Lat Press list). Note that similar work using linguistic evidence has already been published for Tai populations: Hartmann, J. 1998. A Linguistic Geography and History of Tai Meuang-Fai [Ditch-Dike] Techno-Culture. Journal of Language and Linguistics. 16(2):67-100 [See this discussion].
There are scattered references in the Burmese chronicle to Shan members of the ruling elite in what are ostensibly non-Shan areas in both Upper and Lower Burma during the fifteenth century. For example, around 1492 the Burmese ruler of Toungoo, Min-gyi-nyo, attacked a settlement on the frontier between Toungoo and Mon Ramanya that had a Tai ruler (See page 304 of Fernquest, Jon (2005) “Min-gyi-nyo, the Shan invasions of Ava (1524-27, and the beginnings of expansionary warfare in Toungoo Burma: 1486-1539, SOAS Bulletin of Burma Research, Vol. 3, No. 2, Autumn 2005, Link). This is probably just the tip of a much bigger iceberg.
One small and excessively simple idea in Harvey's history:
“Possibly it was a war of migration, in that the main avenue of Shan pressure was from the north and the Ava state, thus reinforced, was able to swarm down on Pegu” (82).
Incubating over a 70 year period, now addresses a much more complicated actual historical reality.