Midrash su Deuteronomio 14:11

(Devarim 14:11) "Every clean bird you may eat": Because it is written (Vayikra 14:4) "And the Cohein shall command, and he shall take for the one to be cleansed (from leprosy) two birds," I might think that just as the slaughtered bird is forbidden (to be eaten), so, the sent-away bird; it is, therefore, written "Every clean bird you may eat (including the sent-away bird).

[Variantly: "Send": Scripture speaks of a clean bird. — But perhaps it speaks of an unclean bird too? It is, therefore, written (Ibid. 14:11) "Every clean bird you may eat": This is a prototype, viz.: Wherever "bird" is mentioned in Scripture, a clean bird is understood (unless specified otherwise) as R' Yoshia has stated. If after sending he turned his face away from it he is exempt from sending further.]