Reference for Chullin 279:29

למעוטי שלא לזווג לה אחרת קודם שלוחיה למאי

- Rather said R'Nahman B'Isaac, [The expression 'clean'] serves to exclude the birds of a beguiled city.<span class="x" onmousemove="('comment',' A city whose inhabitants were enticed into idolatry was to be utterly destroyed and everything belonging to it was forbidden absolutely; cf. Deut. XIII, 13ff. The term 'clean' thus excludes the birds of this town from being used in the purification rites of the leper.');">²⁰</span>

Jastrow

Jastrow