Abstract A large percentage of the world’s languages – anywhere from 50 to 90% – are currently spoken in what we call shift ecologies, situations of unstable bi- or multilingualism where speakers, and in particular younger speakers, do not use their ancestral language but rather speak the majority language. The present paper addresses several interrelated questions with regard to the linguistic effects of bilingualism in such shift ecologies. These language ecologies are dynamic: language choices and preferences change, as do speakers’ proficiency levels. One result is multiple kinds of variation in these endangered language communities. Understanding change and shift requires a methodology for establishing a baseline; descriptive grammars rarely provide information about usage and multilingual language practices. An additional confounder is a range of linguistic variation: regional (dialectal); generational (language-internal change without contact or shift); contact-based (contact with or without shift); and proficiency-based (variation which develops as a result of differing levels of input and usage). Widespread, ongoing language shift today provides opportunities to examine the linguistic changes exhibited by shifting speakers, that is, to zero in on language change and loss in process, rather than as an end product.