In the Old Testament, turtle doves were used as a substitute for the offering of a lamb. If the worshiper could not afford a kid he could offer the two birds instead.
Since the lamb itself was offered as a substitute for the sinful man, the turtle dove became a substitute-substitute, illustrating the compassion of God in making sure everyone could worship Him, regardless of class or wealth.
To us today, the Lord is our substitute, offered for us at Calvary. He does not ask for us to sacrifice ourselves the way He did, but we can offer our lives as a living sacrifice, fulfilling the role of metaphorical turtle doves.
Thank God for Him!