יורה דעה פ״בYoreh De'ah 82

Se’if 1א׳

סִימָנֵי עוֹף טָהוֹר לֹא נִתְפָּרֵשׁ מִן הַתּוֹרָה, אֶלָּא מָנָה מִינִים טְמֵאִים בִּלְבַד, וּשְׁאַר מִינֵי הָעוֹף, מֻתָּרִים. וְהַמִּינִים הָאֲסוּרִים, עֶשְׂרִים וְאַרְבָּעָה הָאֲמוּרִים בַּתּוֹרָה (ל' הַרַמְבַּ"ם פֶּרֶק א' מהמ"א די"ד).

The signs of a kosher bird are not explained from the Torah, rather it only enumerated the non-kosher types, and the permitted types of birds. And the prohibited types, 24 were mentioned in the Torah (See: Rambam Forbidden Foods 1:14)

Se’if 2ב׳

כָּל מִי שֶׁהוּא בָּקִי בְּאוֹתָם מִינִים וּבִשְׁמוֹתֵיהֶם, הֲרֵי זֶה אוֹכֵל כָּל עוֹף שֶׁאֵינוֹ מֵהֶם, וְאֵינוֹ צָרִיךְ בְּדִיקָה (שָׁם). וְעוֹף טָהוֹר נֶאֱכָל בְּמָסֹרֶת, וְהוּא שֶׁיִּהְיֶה דָּבָר פָּשׁוּט בְּאוֹתוֹ מָקוֹם שֶׁזֶּה עוֹף טָהוֹר. וְנֶאֱמָן צַיָּד לוֹמַר: עוֹף זֶה הִתִּיר לִי רַבִּי הַצַּיָּד, וְהוּא שֶׁיֻּחְזַק אוֹתוֹ צַיָּד שֶׁהוּא בָּקִי בַּמִּינִים הַטְּמֵאִים הָאֲמוּרִים בַּתּוֹרָה וּבִשְׁמוֹתֵיהֶם. מִי שֶׁאֵינוֹ מַכִּירָם וְאֵינוֹ יוֹדֵעַ שְׁמוֹתֵיהֶם, בּוֹדֵק בַּסִימָנִים: כָּל עוֹף שֶׁהוּא דּוֹרֵס וְאוֹכֵל, בְּיָדוּעַ שֶׁהוּא מִמִּינִים הַטְּמֵאִים. וְאִם אֵינוֹ יוֹדֵעַ אִם דּוֹרֵס אִם לָאו, אִם כְּשֶׁמַּעֲמִידִין אוֹתוֹ עַל חוּט חוֹלֵק אֶת רַגְלָיו, שְׁנֵי אֶצְבְּעוֹתָיו לְכָאן וּב' אֶצְבְּעוֹתָיו לְכָאן, אוֹ שֶׁקּוֹלֵט מִן הָאֲוִיר וְאוֹכֵל, בְּיָדוּעַ שֶׁהוּא דּוֹרֵס. וְאִם יָדוּעַ שֶׁאֵינוֹ דּוֹרֵס, יֵשׁ שְׁלֹשָׁה סִימָנֵי טָהֳרָה: אֶצְבַּע יְתֵרָה, וְזֶפֶק, וְקֻרְקְבָנוֹ נִקְלָף בַּיָּד, לַאֲפוּקֵי אִם אֵינוֹ נִקְלָף אֶלָּא בְּסַכִּין. (ל' הַמְחַבֵּר) הָיָה חָזָק וּמְדֻבָּק, וְהִנִּיחוֹ בַּשֶּׁמֶשׁ וְנִתְרַפָּה וְנִקְלַף בַּיָּד, הֲרֵי זֶה סִימַן טָהֳרָה. וְאַף עַל פִּי שֶׁיֵּשׁ לוֹ ג' סִימָנִים אֵלּוּ, אֵין לְאָכְלוֹ, לְפִי שֶׁאָנוּ חוֹשְׁשִׁין שֶׁמָּא הוּא דּוֹרֵס, אֶלָּא אִם כֵּן יֵשׁ לָהֶם מָסֹרֶת שֶׁמָּסְרוּ לָהֶם אֲבוֹתֵיהֶם שֶׁהוּא טָהוֹר. (ל' הַרַמְבַּ"ם שָׁם די"ח).

Anyone who is an expert in those types and their names, behold he eats all of the birds that are not from them, and he doesn't need checking. (Ibid.) And a kosher bird is eaten according to tradition, and that is the clear thing in that place that it is a kosher bird. And the hunter is believed saying: This bird, my hunting rabbi permitted for me, since that hunter is known to be in an expert in non-kosher types that are mentioned in the Torah and their names. Someone who does not know them and does not know their names, he should check the signs: every bird that is a bird of prey and he eats [it], and it is known that it is of non-kosher types. And if he does not know if it is a bird of prey or not, if, when they stand it up with wire, split along its legs, two fingers here and two fingers here, or it is [a bird that] clutches from the air and he eats [it], it is known that it is a bird of prey. If it is known that it is not a bird of prey, there are three signs of [its] kosher [status]: an extra finger, a crop [digestive pouch near gullet], and its gizzard is peeled by hand, excluding if it was only peeled with a knife. (See the Mechaber) If it hardened and was attached, and he placed it in the sun and it healed and then it was peeled by hand, behold this is a sign of [it being] kosher. And even though it has these three signs, it should not be eaten, because we are concerned lest it is a bird of prey, rather, unless they have a tradition that was transmitted to them by their ancestors that it is kosher. (See Rambam Forbidden Foods 1:18).

Se’if 3ג׳

יֵשׁ אוֹמְרִים שֶׁכָּל עוֹף שֶׁחַרְטוֹמוֹ רָחָב וְכַף רַגְלוֹ רְחָבָה כְּשֶׁל אַוָּז, בְּיָדוּעַ שֶׁאֵינוֹ דּוֹרֵס, וּמֻתָּר בַּאֲכִילָה אִם יֵשׁ לוֹ שְׁלֹשָׁה סִימָנִים בְּגוּפוֹ. הַגָּה: וְיֵשׁ אוֹמְרִים שֶׁאֵין לִסְמֹךְ אֲפִלּוּ עַל זֶה, וְאֵין לֶאֱכֹל שׁוּם עוֹף אֶלָּא בַּמָּסֹרֶת שֶׁקִּבְּלוּ בּוֹ שֶׁהוּא טָהוֹר (בְּאָרֹךְ כְּלָל נ"ו ובתא"ו נט"ו), וְכֵן נוֹהֲגִין, וְאֵין לְשַׁנּוֹת.

There are those who say that all birds that have a wide beak and the palm of its foot is wide like a goose, and it is known that it is not a bird of prey, and is permitted to eat if it has the three signs on its body. Gloss: And there are those who say that we don't rely even on this, and one should only eat a bird with an accepted tradition that it is kosher (in the Arukh...) and we are accustomed to this and it should not be changed.