לזכות שושנה בת דבורה גיטל
ולזכות ר' יעקב דב בן דוד הכהן
ולזכות ר' יעקב דב בן דוד הכהן
The Torah says that as part of the purification process of the metzora, two birds are taken, one is shechted and the other is sent out on to the open field.
The gemara in Chullin  searches for the source that enables us to rely on a רוב. How come the gemara doesn't derive the principle from here? Rashi says that the birds may not be טריפות and the only way you can know that is true is if you rely on the רוב that most birds aren't טריפות. The shechted bird can be examined but for the sent away bird we must rely on רוב.
The Maharil Diskin offers a number of answers.
1] After we send it away, we can find it and examine it for טריפות. So we don't in fact have to rely on the רוב.
2] The fact that the person is a metzora is contingent upon the kohen declaring him as such. But how do we know that the kohen is a kohen??? Maybe his supposed father [Mr. Cohen] isn't really his father because his mother was naughty??! It must be that we rely on a רוב that רוב בעילות אחר הבעל. So we see that in this area we are already relying on a רוב and a ודאי is not necessary, so we can also follow the רוב when it comes to the birds. But we still don't know that we can follow a רוב in other instances where we need a ודאי! That is why the gemara didn't use this as a source for רוב. [Problem: The Torah tells Aharon and his sons to be מטמא - they were anointed and are 100 kohanim. So we see that it works to send the birds even with kohanim that aren't based on a רוב.]
3] The kohen can be מטמא the person on condition. If the birds aren't טריפות he is טמא and then the birds bring him טהרה. But if the birds ARE טריפות then he is not טמא in the first place. So we are not relying on the רוב.
Not so fast, Speedy Gonzalez!!
The Toras Kohanim darshens the word לטמאו to mean that one may only be מטמא if it is definitely טמא but not out of doubt. So being מטמא על תנאי doesn't seem to be an option.
Who says that the kohen may be מטמא conditionally, thereby nullifying the mitzva to be מטמא definitively ["מדין ודאי"]?? Now even though when he is מטמא definitively, this means that he will be nullifying the mitzva to bring his birds [because he has no ודאי that they aren't טריפות], maybe he should still do his mitzva now even though later it means that he won't be able to bring the birds for which he is obligated.
This depends on the question of a person who wants to eat matza but won't be able to bentch afterward. Do we tell him to eat the matza even though it will result in him not bentching??? Or maybe we say "don't start problems" and he should avoid the matza in order to avert the problem of missing bentching. If we pasken that he should eat the matza, we should also pasken that he should be מטמא the metzora מדין ודאי even though this will mean that he can't bring the birds later.
However, Tosfos in Chagiga [4a] says that if a person can't perform סמיכה on his קרבן ראיה - he shouldn't offer it in the first place. So maybe this means that he shouldn't be מטמא בודאי since this will result in not being able to bring his birds מדין ודאי.
The Maharil Diskin asked how this metzora can shave off his hair - maybe he is not a metzora and he is transgressing the איסור of not shaving the sides of his head [בל תקיף]?? Saying a Goy will do it doesn't help because there is not only an איסור to shave someone else's head [מקיף] but an איסור to have one's own head shaven [ניקף].
But now we have an interesting question: If he is מטמא מדין ודאי then he can't properly send away the bird [because he doesn't know if it is a טריפה]. If he is מטמא conditionally, then he has no problem with the mitzva of sending away the bird [because he is only obligated if the bird is not a טריפה] but he will transgress the איסור of בל תקיף passively. Which is more severe, a לאו שאין בו מעשה [the איסור of בל תקיף] or being מבטל an עשה [not sending the birds]?? Both require one to give up a fifth of his money, so on that end they are equal, but it would seem that a לאו שאין בו מעשה is worse because it is a לא תעשה as opposed to merely not fulfilling an עשה. So that reinstates the problem with the Maharil Diskin's answer which suggested that he be מטמא מספק, which would result in a לאו שאין בו מעשה which we can avoid by being מטמא מדין ודאי which would result in a mere ביטול עשה.
Another problem that the Maharil Diskin brings up is that if the kohen is מטמא מספק, then the metzora could not have his korbanos brought because they would be possible חולין בעזרה. He resovled this by latching on to the opinion that חולין שנשחטו בעזרה is not דאורייתא [see Tos.Pesachim 22a ד"ה חולין].
WHEEEEEWWWWW!!! That was close.
The Maharil Diskin answered [the issue of shaving and bringing korbanos when he is only a ספק מצורע] that this whole discussion is only according to the Rambam who holds that ספק דאורייתא לחומרא is a only a rabbinic edict, so מדאורייתא we can be lenient and allow him to shave and bring his korbanos.
The problem with this approach is that the Rambam agrees that we follow חזקה and this ספק מצורע has a chazaka that he is pure and therefore he may not shave!
To this you can respond that we don't say that one has a חזקה when a future event will deactivate the חזקה, as we see regarding a man who gives his wife a get that is to take effect one hour before his death [Gittin 28]. She attains the status of a ספק אשת איש because he might die in the next hour and she will have been divorced already now and we DON'T SAY that she is a DEFINITE אשת איש because he has a חזקת חיים. So too, since in the future we are not sure if he will bring kosher birds, and it is a ספק. Therefore, we DON'T SAY that he has a חזקה that he is definitely a מצורע and he can't shave rather we view him as a possible metzora because he might bring kosher birds. This would enable him to shave [assuming we go with the Rambam who says that ספק דאורייתא is לקולא according to the Torah [and we are only מחמיר מדרבנן].
However, there is a fundamental difference between the two cases. In the get case, we don't know whether or not he will die. It can really happen at any second [as we speak people are dying...]. Hence, she has a din of ספק אשת איש because we really don't know what will happen. But in the metzora case, by golly, we already know for a fact that he will bring birds that are only doubtfully kosher, so we should establish him in his original חזקה that he is טהור and not allow him to shave.
[מאוצרות הג"ר אברהם גנחובסקי זצ"ל ולית דין בר נש!!]
Note - Who was the Maharil Diskin? What wiki doesn't mention was that he was a GAON OLAM.