Parashas Vayikra · Aliyah 7 · Passuk 7
Chabad.org — English translation + Rashi
Torah Temima points to Keritot 10b / Toras Kohanim on the bird tier.
E-Daf / Tsurras Hadaf
PDF of the daf / source
English: If his means do not suffice for a sheep — two turtledoves or two young pigeons… one for a chatat and one for an olah.
Second tier of the sliding scale — birds replace the animal for a poorer person.
One who cannot afford a flock animal brings two birds: one as chatat and one as olah. The chatat bird is offered first.