Gemara

(Mishnah): The Torah did not give Simanim of Tamei and Tahor birds.

Question (Beraisa): "The Nesher (vulture or eagle)" - just like a Nesher has no extra toe, nor a crop, the skin of the stomach does not peel off, and it is Dores, also any bird with these Simanim is Tamei;

Just like a Tor (turtledove) has an extra toe, and a crop, the skin of the stomach peels, and it is not Dores, also any bird with these Simanim is Tahor.

Answer (Abaye): The Simanim are not written in the Torah. They are part of the oral tradition.

62a (Rav Nachman): If one who recognizes the Tamei species finds a bird that has a Siman, it is permitted;

If he does not recognize the Tamei species, if the bird has one Siman, (perhaps) it is Tamei;

If it has two Simanim and he knows that it is not a raven or a related species, he may eat it.

(Ameimar): The law is, if a bird has a Siman, it is Tahor, as long as it is not Dores.

Rav Ashi: This is unlike Rav Nachman!

Ameimar: Yes! The only Tamei species that is not Dores is Peres or Ozniyah. These are not found in settled areas, so we are not concerned for them.

Rishonim

Rambam (Hilchos Ma'achalos Asuros 1:15): Anyone who is expert in the (24 Tamei) species and their names, he may eat any bird not from them. He need not check (Simanim). One may eat a Tahor bird based on tradition, if it is clear in that place that it is Tahor.

Rambam (16): One who does not recognize them or does not know the names checks the Simanim that Chachamim gave. Any bird that is Dores and eats, it is known that it is Tamei. If it is not Dores and it has one of the three Simanim of Taharah, i.e. an extra toe, a crop, and the stomach can be peeled by hand, it is Tahor.

Rambam (17): The only Tamei birds that are not Dores and have a Siman of Taharah are Peres or Ozniyah, and not found in settled areas, only in wildernesses of distant islands at the end of the inhabited world.

Magid Mishneh (20): The Mishnah and Gemara connote that if a bird has three Simanim of Taharah, it is Tahor even if we do not know whether it is Dores. The Ramban, Rashba and most Meforshim hold like this. Even though Rashi disagrees, their opinion is primary.

Ran (21a DH v'Hu): It is clear from the Gemara that if it has two Simanim on its body, even if they are not the Simanim of a raven (an extra toe and the skin of the stomach peels), rather, other Simanim, e.g. a crop and an extra toe, or a crop and the skin of the stomach peels, it is a species of raven, and it is Tamei according to R. Eliezer if it dwells with ravens, and according to Chachamim if it also resembles them. Rav Nachman requires that one recognize ravens no matter which two Simanim it has. If not, he should have specified 'a bird with the two Simanim of a raven.' However, if one Siman is that it is not Dores, we know that all species of ravens are Dores.

Poskim

Shulchan Aruch (YD 82:2): Anyone who is expert in the species and their names, he may eat any bird not from them. He need not check.

Beis Yosef (DH v'Ika): The Rambam says that one who recognizes the species need not check Simanim. Rav Nachman says that if one recognizes the species, a bird with one Siman is Tahor! Also, the Rambam says that if one does not recognize the species, the bird must have one Siman and it is not Dores. Ameimar says that one Siman suffices. It seems that the Rambam explains that any bird that is Dores is Tamei, but if it is not Dores, we need also another Siman. Rav Nachman discusses a bird with one of the other three Simanim, but requires also that it is not Dores. He did not say that he must check whether it is Dores. Since he recognizes the species, he need not check Simanim. He mentioned it for parallel structure with the case of one who does not recognize the species. Then, if it has one Siman (perhaps) it is Tamei. All the Poskim rule like Ameimar, for it seems that Rav Ashi accepted his opinion. However, we do not find that anyone argues with Rav Nachman regarding a bird with two Simanim. He must know that it is not a raven or one of its species. The Rambam should have specified, for seemingly if we are Machshir with one Siman, all the more so with two (even if he does not recognize ravens)! Perhaps he relies on what he wrote 'if it has one Siman', and did not say 'all the more so if it has two'. Do not infer that likewise, we are not Machshir when it has three. Since we are Machshir only if it is not Dores, it is unreasonable to forbid if it has all four Simanim of Taharah.

Shulchan Aruch (ibid.): One who does not recognize them or does not know the names checks the Simanim. Any bird that is Dores and eats, it is known that it is Tamei. If we do not know whether it is Dores, if when we put it on a string it divides its toes two in front and two in back, or it catches its food in the air and eats, it is known that it is Dores.

Taz (1): Tosfos says that Dores means that it eats its prey alive. It does not kill it before eating.

Shach (3): The Ran and Magid Mishneh in the name of the Rambam say that Drisah is poisoning through its claws. The Beis Yosef did not give another Perush. This shows that it is what he wrote above in Siman 57 (poisoning). The Shulchan Aruch means that it is Dores in order to eat, even if it does not eat while the prey is alive.

Shulchan Aruch (ibid.): If it is known that it is not Dores, there are three Simanim of Taharah: an extra toe, a crop, and the stomach can be peeled by hand, to exclude if it can be peeled only using a knife.

Taz (3): The extra toe is in back of the others. Even though most birds have it, it is called extra because it is not in line with the others. Others explain that the front toe is bigger than the others (Ran).

Beis Yosef (DH v'Hikshu): The Rashba and Ran refuted Rashi with clear proofs. The primary Perush is of R. Moshe b'Ribi Yosef. Twenty Tamei species have a Siman Taharah on their bodies. All of them, and a raven, are Dores. Peres or Ozniyah each have one Siman. For one of them, the Siman is that it is not Dores. Therefore, any bird with three Simanim is Tahor. This is why the Mishnah says that any bird that is Dores is Tamei, and if a bird has an extra toe, and a crop, and the stomach peels off, it is Tahor. These are independent rules. If it has the latter three Simanim, we know that it is not Dores, and it is Tahor. Likewise, if it has one Siman and it is not Dores, it is Tahor, for all are Dores except for Peres or Ozniyah, and we are not concerned for them, for they have only one Siman. Rav Nachman was Machshir a bird with one Siman. If the Siman is on its body, one must know all of the Tamei birds (to know that it is not one of them). If the Siman is that it is not Dores, one must know only Peres and Ozniyah. Ameimar says that in this last case one need not recognize Peres or Ozniyah, for they are not found in settled areas. He agrees that if the Siman is on its body, one must know all of the Tamei birds.

Gra (3): Ameimar taught that if we know that it is not Dores, it is Tahor. This is the one Siman he requires. If it has three Simanim, even if we did not see whether it is Dores, it is Tahor even if one does not recognize the species.

Gra (5): The Shulchan Aruch requires another Siman in addition to that it is not Dores. He does not rely on R. Moshe b'Ribi Yosef, for there are many Shitos, and in any case it does not matter for us, like the Shulchan Aruch says below (that we are concerned lest it is Dores, just we never saw this).

Shulchan Aruch (ibid.): Even if it has these three Simanim, we do not eat it, for we are concerned lest it is Dores, unless he has a tradition that it is Tahor.

Beis Yosef (DH u'Mah she'Chasav Lefichach): On 61a, Rashi says that a bird with one Siman of Taharah is permitted if he knows that it is not one of the 24 species written in the Torah. On 62a, he says that he must recognize Peres or Ozniyah, and know that it is not one of them.

Beis Yosef (ibid.): Some say that perhaps Chachamim said that Peres or Ozniyah are not found in their settled areas, but we are concerned lest they are found in our settled areas.

Shach (6): If one has a tradition, he need not check Simanim. However, if we see it Dores or divide its legs, our tradition was wrong. If we did not see it Dores, we rely on the tradition even if we see that if it has none of the other three Simanim.