Numbers 6:11

And the priest shall offer the one for a sin offering, and the other for a burnt offering, and make an atonement for him, for that he sinned by the dead, and shall hallow his head that same day.

Cross-references

Leviticus 5:8-10 gives the same ritual of two birds for sin and burnt offerings — directly parallels the Nazirite's offering.

Leviticus 14:31 completes the bird offering ritual for lepers — parallels the two-bird offering here.