Parashas Vayikra · Aliyah 2 · Passuk 14
Chabad.org — English translation + Rashi
Torah Temima points to Chullin 22b (verified; mature turtledoves / young pigeons; intermediate stage unfit).
English: Turtledoves: mature ones and not young; young pigeons: young and not mature… excluding the beginning of the yellowing [of neck feathers] in either, which are invalid.
Custom (species-stage specification).
Only mature turtledoves and young pigeons are fit. The intermediate stage (onset of yellowing) is invalid for both.