This study covers two issues: (1) the language shift process relating to two highly endangered aboriginal languages of Taiwan and (2) the correlations between some variables and their language shift. Both Kanakanavu and Saaroa peoples underwent two waves of migration: (1) a massive in-migration of another Formosan ethnic group (Bunun people) in the 1930s and (2) a massive out-migration of the Kanakanavu and Saaroa villagers to cities in the 1960s. Due to the lack of social compartmentalisation brought by the migrations, three linguistic varieties--the heritage languages (Kanakanavu and Saaroa), Bunun and Mandarin--compete for realisation in the Kanakanavu and Saaroa speech communities. Our study shows that a basic diglossic environment with Mandarin as H-variety and Bunun and Kanakanavu/Saaroa as L-varieties cannot be maintained. Instead, Mandarin "leaks" into the domain previously reserved for vernaculars, while Bunun overrides the heritage languages by carrying a higher prestige. In the multivariate statistical analyses, three changing factors (intermarriage, education level and migration experience) presenting social decompartmentalisation are proved to be correlated to the language shift from the heritage languages to Bunun and/or Mandarin. The increasing intermarriage rates with the more populous Bunun people have resulted in a sub-diglossic situation among the vernaculars where Bunun becomes more prestigious than Kanakanavu and Saaroa.