Leviticus 5:7-8

7A person may not have money to buy a sheep or a goat. If he does not, he must buy two doves or two pigeons. He must give them to the Lord. One bird will be a sin offering. The second bird will be a burnt offering.

8The priest will bring one of the birds to the altar. It is the sin offering. He will break the neck of the bird, but he will not pull the head off.