Adam Clarke on Leviticus 14:7

Shall let the living bird loose - The Jews teach that wild birds were employed on this occasion, no tame or domestic animal was used. Mr. Ainsworth piously conjectures that the living and dead birds were intended to represent the death and resurrection of Christ, by which an atonement was made to purify the soul from its spiritual leprosy. The bird let loose bears a near analogy to the scapegoat. See Leviticus 16.