Leviticus 5:7-10

7‘But if he cannot afford a lamb, then he shall bring two turtledoves or two young pigeons as his guilt offering for his sin to the Lord, one as a sin offering and the other as a burnt offering.

8He shall bring them to the priest, who shall offer first the one for the sin offering, and shall nip its head at the front of its neck, but shall not sever it [completely].

9He shall also sprinkle some of the blood of the sin offering on the side of the altar, and the rest of the blood shall be drained out at the base of the altar; it is a sin offering.

10The second [bird] he shall prepare as a burnt offering, according to the ordinance. So the priest shall make atonement on his behalf for the sin which he has committed, and it will be forgiven him.