Originally pronounced with a /t-/ initial (compare 島 (OC *tuːwʔ, “island”)). In many dialects, it changed to a /n-/ initial to avoid homophony with the vulgar word 屌 (diǎo, “penis”), which may have developed from the sense “bird” eventually. Birds/fowl are characteristically associated with Chinese slangs for genitalia; compare 雀 (“sparrow > penis”) and 雞 (“chicken > penis”).
Alternatively, Schuessler (2007) hypothesised that the /n-/ initial form for “bird” may be unrelated to the /t-/ form. Instead, it may be connected with the Tai-Kadai word for bird; confer Proto-Tai*C̬.nokᴰ (“bird”), whence Thaiนก (nók).
The /t-/ form may be derived from Proto-Sino-Tibetan*daw (“bird”), cognate with Proto-Karen *thoᴮ (“bird”) and Proto-Bodo-Garo *daw/dow (“bird”).