QUESTION: The Mishnah (2:5) teaches that when one brings an offering of two birds, both must be either two Bnei Yonah or two Torim ("Ein Mevi'in Torin k'Neged Bnei Yonah"). The MEFARESH explains that one must bring the two obligatory Korbanos from the same species of bird. The Toras Kohanim (Vayikra 12:8) derives this from the verse, "One for an Olah and one for a Chatas," which implies that the Olah must be of the same species as the Chatas.
The principle of the Mishnah that "Ein Mevi'in Torin k'Neged Bnei Yonah" is mentioned again later in the Mishnah (3:6, 24b). The Mishnah there teaches that if a woman, before giving birth, pledges to bring two birds as a Korban if she gives birth to a boy, she is obligated to bring two pairs of bird offerings -- two Olos ha'Of -- for her vow when she gives birth to a boy, in addition to the standard Chatas ha'Of and Olas ha'Of that every woman brings after birth. The Mishnah there says that if the Kohen to whom she gave the birds mistakenly thought that each pair of birds was comprised of a Chatas and Olah (and not one pair comprised of a Chatas and Olah, and the other pair comprised of two Olos) and offered them accordingly, she must offer another bird as a replacement Korban for an Olas ha'Of, of the same species as the one offered incorrectly, if all of the birds that she brought were of the same species.
If two of the birds were Torim and two were Bnei Yonah, then she must bring both a Tor and a Ben Yonah as replacement Olos, because we are unsure about which kind of bird was the one which became Pasul (which was brought as a Chatas instead of as an Olah). The Mefaresh there (24b, end of DH mi'Min Echad; see also RASHI to Zevachim 67b, end of DH Tzericha) explains that this is because of the rule mentioned in the Mishnah here (2:5) that "Ein Mevi'in Torin k'Neged Bnei Yonah" -- a bird which is being brought as a replacement must be of the same species as the bird that it is replacing.
This Halachah is difficult to understand. It is clear that when a Yoledes brings the pair of birds that the Torah obligates her to bring, she must bring either two Torim or two Bnei Yonah, because the verse states, "One for an Olah and one for a Chatas," implying that the two must be from the same species. However, when she voluntarily pledges to bring a pair of birds, why should she have to bring them from the same species? In this case, she should be obligated to bring only what she pledged -- two Olos ha'Of. Why should there be any connection between the two birds, such that they must be from the same species? (See RASHASH to 3:6, and CHIDUSHEI HA'GRIZ, Hilchos Ma'aseh ha'Korbanos 14:2, DH v'Hineh.)
ANSWER: The BRISKER RAV in Chidushei ha'Griz answers based on the Gemara in Kerisus (9a). The Gemara there cites a Beraisa that quotes the verse, "v'Hikrivo" -- "and he shall bring it" (Vayikra 1:15), which refers to the voluntary Olah offering (see Rashi there, DH v'Hikrivo). The Gemara explains that because the previous verse -- which refers to Torim -- says, "v'Hikriv," one might have thought that one who declares, "I pledge to bring an Olas ha'Of," must bring at least two birds. The verse therefore repeats "v'Hikrivo" to teach that even one bird is acceptable to fulfill his vow. ("v'Hikrivo" means that "he shall bring it," in the singular.) Rashi (DH v'Hikriv) writes that the first verse, which does not stress that even a single bird may be offered, implies that one must bring a "complete Korban." We see from Rashi that a "complete" bird offering is not less than two Torim or two Bnei Yonah.
Consequently, there was reason to think that when one makes a vow to bring a free-will bird offering, he must bring two birds. Even though the verse of "v'Hikrivo" teaches that one bird suffices, the principle that a "complete Korban" is comprised of two birds remains true.
The Brisker Rav explains that Rashi derives the law that a complete bird offering is comprised of two birds from the law taught there in Kerisus (8b) that when a Ger converts he must bring two bird offerings. The Gemara there (9a) says that it is impossible for him to bring only one, because "we never find anywhere in the Torah that one bird is sufficient." The Gemara then questions this from the verse, "v'Hikrivo," which teaches that one bird is sufficient, and it answers that one bird is sufficient only for voluntary offerings but not for obligatory offerings.
Based on this explanation, we understand the source for the statement of Rashi in Zevachim (67b) that one who vowed to bring a "Ken" must bring two birds of the same specifies, and he may not bring one Tor together with one Ben Yonah, even though this Ken is not a Korban that the Torah obligates her to bring. Even though she may bring only one bird if this was her vow (as derived from "v'Hikrivo"), if she made a vow to bring a Ken, then she has committed herself to bring a "complete bird-offering," which is a minimum of two birds. Moreover, the two birds that she brings must be either two Torim or two Bnei Yonah, because she did not commit herself merely to bring "two birds" (that have no connection with each other), but rather she committed herself to bring a "Ken." Even though the law of "Ein Mevi'in Torin k'Neged Bnei Yonah" is derived from a verse which discusses obligatory Korbanos, Rashi maintains that because she called it a "Ken" in her vow (as in the Mishnah later, 3:6), she must bring a Ken that has the same laws as the Torah gives for ordinary Kinim that a Yoledes is obligated to bring. Therefore, as fulfillment of her vow she must bring either two Torim or two Bnei Yonah. (D. BLOOM)