Well, they could not afford a lamb....
Lev. 5 KJV
1 And if a soul sin, and hear the voice of swearing, and is a witness, whether he hath seen or known of it; if he do not utter it, then he shall bear his iniquity.
2 Or if a soul touch any unclean thing, whether it be a carcase of an unclean beast, or a carcase of unclean cattle, or the carcase of unclean creeping things, and if it be hidden from him; he also shall be unclean, and guilty.
3 Or if he touch the uncleanness of man, whatsoever uncleanness it be that a man shall be defiled withal, and it be hid from him; when he knoweth of it, then he shall be guilty.
4 Or if a soul swear, pronouncing with his lips to do evil, or to do good, whatsoever it be that a man shall pronounce with an oath, and it be hid from him; when he knoweth of it, then he shall be guilty in one of these.
5 And it shall be, when he shall be guilty in one of these things, that he shall confess that he hath sinned in that thing:
6 And he shall bring his trespass offering unto the Lord for his sin which he hath sinned, a female from the flock, a lamb or a kid of the goats, for a sin offering; and the priest shall make an atonement for him concerning his sin.
7
And if he be not able to bring a lamb, then he shall bring for his trespass, which he hath committed, two turtledoves, or two young pigeons, unto the Lord; one for a sin offering, and the other for a burnt offering.
Hence:
Luke 2 KJV
22 And when the days of her purification according to the law of Moses were accomplished, they brought him to Jerusalem, to present him to the Lord;
23 (As it is written in the law of the Lord, Every male that openeth the womb shall be called holy to the Lord
24 And to offer a sacrifice according to that which is said in the law of the Lord,
A pair of turtledoves, or two young pigeons.
