From Leviticus: 12:6---When the days of her purification for a son or daughter are over, she is to bring to the priest...a year-old lamb for a burnt offering and a young pigeon or dove for a sin offering...Verse 8...If she cannot afford a lamb, she is to bring two doves, or two young pigeons...

Luke 2:23---"Every firstborn male is to be consecrated to the Lord" and to offer a sacrifice in keeping with what is said in the Law of the Lord: "a pair of doves or two young pigeons". Mary and Joseph brought the offering of POOR people.