Negaim, Chapter 14, Mishnah 6. Having surveyed the purification of the metzora in general terms, the Mishnah now turns to the individual components of the process. It begins with the pair of birds: one is slaughtered and its blood is placed into a new earthenware vessel together with mayim chayim, and the second is set free.
"Shtei tzipporim mitzvasan sheyihyu shavos bemar'eh uvekomah uvedamim, ulekichasan ke'achas": the mitzvah of the two birds is that they match one another. Their coloring should be the same, their height the same, their cost the same, and the two of them should be acquired in a single purchase. In short, they ought to look like a pair of twins.
Where this requirement comes from
The Rav explains the derivation. The Torah could have written simply "tzipporim," and since a plural noun implies a minimum of two we would already have known that two birds are needed. Why then does the Torah write "shtei tzipporim"? To teach that the two must be as equivalent to one another as possible.
The meforshim note that this brings to mind the two se'irim of Yom Kippur, which likewise had to resemble each other as closely as possible. There too the pair works as a unit at the heart of a kapparah: the sa'ir whose blood is brought lifnim purifies the Beis Hamikdash and its kodashim from tumah, and the sa'ir bachutz effects atonement for Klal Yisroel. There too one is offered and one is sent away, the sa'ir la'azazel. There is a difference, though. The se'irim are korbanos, while the bird that is slaughtered here is not a korban at all: the procedure is not performed inside the Beis Hamikdash and the bird carries no kedushah.
When the ideal is not met
The Mishnah then eases the standard: "Af al pi she'einan shavos", and even when the pair does not correspond to one another, they are "kesheiros", fit for use. The matching, then, is only a requirement lechatchilah. The same holds for the timing of the purchase: "Lekacho echad hayom ve'echad lemachar, kesheiros", if he acquired one bird on one day and its partner on the next instead of buying the two at once, they remain fit.
What a tzippor dror is
The Mishnah continues: "Shachat achas meihen venimtza shelo dror, yikach zug lashniyah." He slaughtered one of them and it turned out not to be a dror. To understand this we must return to the opening Mishnah of the perek, where the requirement of tzipporei dror was stated.
In that first Mishnah the Rav characterized a tzippor dror as a bird "shedaros babayis kevasadeh," one that lives indoors just as it lives out in the open field. At first glance that sounds like a creature that is half tame. The Tosfos Yom Tov reads the Rav's words in a different direction entirely: this bird is skilled at darting off and tucking itself into the nooks of a house, in any direction it pleases, and so it manages to make its home indoors without ever being caught. Tame it certainly is not. And the Tosfos Yom Tov adds that the name "dror" derives from dirah, a dwelling, since this bird will settle wherever it happens to be. The Tiferes Yisroel traces the name instead to the sense of liberty, as in the verse "ukerasem dror ba'aretz," you shall proclaim freedom throughout the land.
The Tosfos Yom Tov cites Rashi, who defines such a creature as one "she'einah mekabeles marus," that submits to no master, in the language of the Gemara in Beitzah. Yet Rashi makes clear that although the bird earned its name through its freedom, one cannot conclude that any kosher bird resistant to taming qualifies for the metzora's taharah, for the Gemara there raises measurements as well, that the bird be not too big and not too small. The Tosfos Yom Tov then relates that he came across the Ramban on Chumash, who holds that the designation tzippor dror is reserved for a bird of small dimensions. The Tiferes Yisroel takes it a step beyond that and pinpoints an actual species. His commentary frequently reveals a striking command of plants, of living creatures and of the sciences of nature altogether, and here he records, "Venir'eh li," in my view, that the bird is the sperling in lashon Ashkenaz.
With this we can appreciate our Mishnah. If the tzippor dror is defined by species, or by size, or by particular characteristics, then a person can bring a bird in good faith, slaughter it, and only afterwards discover that he was mistaken, that this bird was of a domesticated kind and not a dror at all.
The disqualified bird and its partner
In such a case: "Yikach zug lashniyah." The bird that was slaughtered is disqualified, but the second bird, the one designated to be set free, retains its fitness. This is unlike the se'irim of Yom Kippur, where the disqualification of one affects the other. Here the second bird is simply held in the meantime while a new partner is taken to be slaughtered in place of the first. And the first bird, "muteres ba'achilah," is permitted to be eaten, since it was properly shechted.
"Shechatah venimtzas treifah, yikach zug lashniyah, vehaharishonah muteres bahana'ah." He slaughtered the bird whose blood was to go into the new earthenware vessel with the mayim chayim, and it was found to be treifah, with a defect in the lungs or the like. Since a bird is small, such things are quickly visible. Here too he takes a new partner for the second bird, and the first is permitted in hana'ah, in benefit, even if not in eating: shechitah removes an animal from tumah even when it is a treifah, and this bird, unlike kodshim, carries no prohibition of benefit. Contrast this with the bird designated to be sent away, which is assur behana'ah until it is released.
Why does a treifah disqualify the bird whose blood is used? The Rav explains, "dechiyus kesiv," the Torah requires vitality, "perat literefah," which excludes a treifah, since we assume a treifus will not survive long.
When the blood is spilled, and when the second bird dies
"Nishpach hadam, tamus hamishtalachas." If the blood was spilled on the ground before it reached the water, so that there is no blood to place into the vessel, both birds are now disqualified, and the bird that had been designated to be sent free must die. The Rav explains that one does not destroy it: "yanichenu ad shetamus me'eilehah," he puts it away, in a cage, and leaves it there until it dies of itself.
"Meisah hamishtalachas, yishafech hadam." If the bird designated to be sent away died before it was released, the blood of the slaughtered bird is poured out, even if it had already been placed into the vessel, and the process must begin again with fresh mayim chayim and fresh blood in a kli cheres.
The Tiferes Yisroel makes the boundary of this halachah precise: "aval bemeisas hameshulachas achar zerikas hadam, shiluach eino me'akeiv." If the bird designated for sending away died after the sprinkling had already been performed, the sending forth is not me'akeiv and the taharah stands. It is only when it died before the sprinkling that the blood must be poured out and the entire procedure repeated.