Parashas Metzora · Passuk 14:05
Chabad.org — English translation + Rashi
Torah Temima points to Toras Kohanim.
English: “The one bird” — the clearer / more choice of the two; also, if one dies or becomes terefah he must obtain a mate for the remaining one.
Derash on “ha-achas”
Chazal teach that the bird to be slaughtered should be the choice one of the pair, and that a replacement mate must be supplied if one of the original pair is lost.