In Parshas Metzora the Torah details the halchos of tzaraas. When a person has a negah tzaraas that is confirmed tamei he becomes a metzora. At that time he becomes tamei and must leave the machane Yisrael. When he becomes pure, he must bring two living clean birds, cedar-wood, scarlet, and hyssop (Vayikra 14:4).
One bird is shechted right away. The other bird and the cedar-wood, scarlet, and hyssop are dipped into the blood from the first bird. Then the blood is sprinkled on the metzora seven times and the other bird is sent away into the wilderness. There are several other steps in the tahara process of the metzora that the pesukim detail in this parshah. The same process mentioned here is repeated by the tzaraas found on a house.
The Gemara in Chullin 140a says that any bird that killed an animal or a person and is therefore condemned to death and forbidden to eat may not be used as the bird that will be sent away. We know this, the Gemara says, because the Torah commands that the bird be sent into the wilderness, and how can we send a forbidden bird into the wilderness? What if someone finds it and eats it? How can we cause a takalah – stumbling block – for another man?
The Mishnah Lemelech asks an interesting question. Once the forbidden bird is sent into the wilderness, it should become permitted to consume to one who is unaware of its true identity. This is because it should become batul in the majority of birds in the wilderness that are permitted. Why then does the Gemara deduce that we cannot send a forbidden bird away creating a takalah, when it would not be a takalah for the finder?
This question is based on the opinion of the Rosh in Chullin (7) that says that when a forbidden item is batul b’rov the forbidden item changes into a permitted item. It is for this reason that he says that if one piece of non-kosher meat is mixed up with two pieces of kosher meat, one can eat all three pieces at the same time. It should be noted that in Shulchan Aruch Yoreh Deah 110:1 we pasken that all three pieces cannot be eaten at once, rather one after another. And the Rema rules that only two of the pieces can be eaten.
It is the opinion of the Rosh that the Mishnah Lemelech based his question. The Chelkas Yoev (chelek 1 mahadura tinyana 15) quotes the Beis Meir that points out that the question is also specifically that the bird should become batul b’rov, and not halach achar harov. Halach achar harov is when there is a group and one item is separated from the group. The halacha of rov dictates that the item that was separated came from whatever the majority of items were. In this case one could reason that by applying halach achar harov we could permit the forbidden bird as well. However the Beis Meir says that by applying this form of rov the item will remain a forbidden item. It will be permitted for the finder to consume; however it will remain a forbidden item in essence.
I believe that whether the item that is separated form a group and is permitted via the halacha of halach achar harov remains essentially forbidden or permitted is debatable. However I believe that there is another reason that the Mishnah Lemelech only asked that the bird be considered batul b’rov and not halach achar harov. When applying the halacha of halach achar harov we do not permit every last item to be used. For example if there are nine pieces in a group that are kosher and one that is non-kosher, and one piece is separated, that piece is permitted and we assume that it came from the nine kosher pieces. The same will apply to the next eight pieces that are separated from the group. However when we are left with two pieces in the group, we can no longer apply halach achar harov. Acharonim suggest that similarly we cannot pasken at once that every item can be consumed.
By sending the forbidden bird to the wilderness, we are paskening on every bird at once that they can be eaten. Since we cannot pasken that every bird can be eaten we would not be able to utilize the halacha of halach achar harov in regards to sending the forbidden bird away.
The Mishnah Lemelech answers his question by citing the opinion of the Raavad that mi’d’oraisa one may not be mevatel an issur lechatchila. This means that one may not mix a forbidden food into kosher food in order that it should become permitted.