The Mishna states that the four physiological signs of a kosher bird are not explicitly detailed in the Torah, but are instead established by the Sages. The Gemara challenges this assertion based on a braita, which indicates that these kosher indicators are actually derived through a comparison of the anatomical details of each type of bird—specifically evaluating the characteristics of the nesher (griffin vulture), which is listed as non-kosher, against those of the tor (turtledove), which is known to be kosher due to its role in sacrificial offerings. Abaye resolves this difficulty by explaining that the analytical comparison of the physical details between the tor and the nesher outlined in the braita is precisely the methodology employed by the Sages to formulate and define these diagnostic signs.

Rabbi Chiya introduces a braita asserting that a bird possessing even a single kosher sign is permitted, based on the premise that only a bird structurally identical to the nesher—which lacks all four signs—is fundamentally excluded, along with the other non-kosher species explicitly enumerated in the Torah. The Gemara raises a difficulty, noting that one could argue the exact inverse: using the tor as the absolute paradigm, one could deduce that a bird is kosher only if it possesses all four signs. Since the other non-kosher birds listed in the Torah possess either one, two, or three kosher signs, the Gemara resolves the difficulty by explaining that if all four signs were universally required to render a bird kosher, there would be no logical necessity for the Torah to detail the other twenty-three non-kosher species, as their status as non-kosher would already be self-evident.

The Gemara then proceeds with a series of analytical queries, asking why the halakhic paradigm should not be derived from those forbidden birds that possess three signs, two signs, or one sign, respectively. In each instance, the Gemara answers using a redundant-case logic: if any of those categories served as the baseline paradigm, the Torah would have had no need to explicitly list the remaining forbidden species that possess fewer kosher signs (simanim).

The premise of the final query assumes that the peres and ozniyah (birds possessing a single kosher sign) could have established a binding paradigm were it not for the counter-example of the nesher. The Gemara immediately questions this, invoking the hermeneutical rule that two biblical verses teaching the same law cannot serve as a general paradigm (shnei ketuvim haba'im k'echad). Since both the peres and ozniyah share the same baseline characteristics, how could it be suggested that they form a general rule? The Gemara answers that they represent two distinct halakhic categories; one of them is completely unique because, within the entire biblical list of non-kosher birds, it is the sole species that possesses a specific, distinct kosher sign not found in any of the others.

In conclusion, the ultimate paradigm is the nesher (representing the principle that a bird is kosher unless it shares all the forbidden traits of the listed species), and we do not derive the rule from the tor. The tor (with all four kosher signs) is explicitly mentioned in the Torah solely to indicate its validity for sacrifices, rather than to serve as a universal archetype for kosher birds.