Parashas Vayikra · Aliyah 2 · Passuk 14
Chabad.org — English translation + Rashi
Torah Temima points to Zevachim 65a (verified; singular v’hikrivo allows a single bird).
English: Our rabbis taught: Since it is stated “from the turtledoves or from the young pigeons,” I might think one who donates a bird may not bring fewer than two. The Torah states “and he shall bring it” — even a single bird he may bring.
Custom (singular vs plural resolution).
A voluntary bird offering may be a single bird; the plural names the species classes, not a minimum of two.