Parshat Metzora - Two Birds

The week's parsha begins with the procedure for permitting the Metzora to return to his home. Here are some of the verses from Vayikra 14 (Judaica Press translation):

Leviticus Chapter 14
1 And the Lord spoke to Moses, saying,
2 This shall be the law of the person afflicted with tzara'ath, on the day of his cleansing: He shall be brought to the kohen.
3 The kohen shall go outside the camp, and the kohen shall look, and behold, the lesion of tzara'ath has healed in the afflicted person.
4 Then the kohen shall order, and the person to be cleansed shall take two live, clean birds, a cedar stick, a strip of crimson [wool], and hyssop.
5 The kohen shall order, and one shall slaughter the one bird into an earthenware vessel, over spring water.
6 the live bird, he shall take it, and then the cedar stick, the strip of crimson, and the hyssop, and, along with the live bird, he shall dip them into the blood of the slaughtered bird, over the spring water.
7 He shall then sprinkle seven times upon the person being cleansed from tzara'ath, and he shall cleanse him. He shall then send away the live bird into the field.

Verse 4 mentions that "two live, clean birds" are to be used. One bird will be slaughtered (verse 5) and the other bird will be set free (verse 7). We should first note that these birds do not have the status of offerings. In a later stage of his cleansing the metzora will bring offerings. If he has sufficient means then he will bring lambs. If he is poor then he will bring either two turtledoves or two young pigeons (see verse 22). Because those birds will be brought as offerings the Torah specifies only those bird species which may be brought on the altar. However, in the beginning step of his purification the Torah only requires two clean (kosher) birds.

Rashi in his comment to Verse 4 tells us why the metzora brings birds:

Rashi Leviticus Chapter 14
4 clean Excluding an unclean bird. Because lesions of tzara'ath come as a result of derogatory speech, which is done by chattering. Therefore, for his cleansing, this person is required to bring birds, which twitter constantly with chirping sounds.

Rashi's explanation doesn't help us understand why two birds are required. The Baal HaTurim on Verse 7 offers this insight (my translation):

He shall send away the bird: This is a hint that the tzara'ath has flown away from him. And one bird was slaughtered, as if to say, thus the tzara'ath should not come to him again. And one bird was sent away, as if to say, that if he (the metzora) will continue to do evil, the tzara'ath will return to him like the bird that was sent away.

The person who was struck with tzara'ath had engaged in speaking lashon hara. The only way for the tzara'ath to be healed was for him to do teshuva. He has now reached that level. The Baal HaTurim tells us that the two birds teach the metzora that what happens next is up to him. His actions in the future will determine whether or not the tzara'ath returns.

I saw another explanation, but failed to record the source. A person could have thought that after being healed from the tzara'ath that he should never talk again. The Torah tells him that that is not a correct idea. One bird is left alive, that is, still chattering. The healed metzora too is permitted to speak, it is just up to him to learn to speak only in appropriate ways.