(ו) סִימָנֵי בְהֵמָה וְחַיָּה נֶאֶמְרוּ מִן הַתּוֹרָה, וְסִימָנֵי הָעוֹף לֹא נֶאֱמָרוּ. אֲבָל אָמְרוּ חֲכָמִים, כָּל עוֹף הַדּוֹרֵס, טָמֵא. כֹּל שֶׁיֶּשׁ לוֹ אֶצְבַּע יְתֵרָה, וְזֶפֶק, וְקֻרְקְבָנוֹ נִקְלָף, טָהוֹר. רַבִּי אֱלִיעֶזֶר בַּר צָדוֹק אוֹמֵר, כָּל עוֹף הַחוֹלֵק אֶת רַגְלָיו, טָמֵא:
(6) The signs of cattle and a wild animal are stated from the Torah, but the signs of a bird are not stated. But the Sages said, "Any bird which attacks [prey] is prohibited. Any that has an extra digit, a crop, and gizzard that peels [away] is permitted." Rabbi Eliezer ben Rebbi Tzadok says, "Every bird which parts its toes is prohibited."
(ג) 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.
(3) 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.
The "Gloss" is a commentary by Rabbi Moshe Isserles (the Rama), who provides a mainstream halakhic ruling for Ashkenazim. The main next of the Shulkhan Arukh was written by Rabbi Yoseph Karo in the mid-16th century, and its rulings are those of the Sephardic community of Tzefat. In this case, Isserles seeks to end an argument regarding a lack of clarity regarding signs of kashrut on birds.
(כד) וּבְדָגִים שְׁנֵי סִימָנִין. סְנַפִּיר וְקַשְׂקֶשֶׂת. וּסְנַפִּיר הוּא שֶׁפּוֹרֵחַ בּוֹ. וְקַשְׂקֶשֶׂת הִיא הַדְּבוּקָה בְּכָל גּוּפוֹ. וְכָל שֶׁיֵּשׁ לוֹ קַשְׂקֶשֶׂת יֵשׁ לוֹ סְנַפִּיר. אֵין לוֹ עַכְשָׁו וּכְשֶׁיַּגְדִּיל יִהְיֶה לוֹ אוֹ שֶׁיֵּשׁ לוֹ קַשְׂקֶשֶׂת כְּשֶׁהוּא בַּיָּם וּכְשֶׁיַּעֲלֶה יַשִּׁיר קַשְׂקַשָּׂיו הֲרֵי זֶה מֻתָּר. וּמִי שֶׁאֵין לוֹ קַשְׂקַשִּׂים הַחוֹפִין אֶת כֻּלּוֹ מֻתָּר. אֲפִלּוּ אֵין בּוֹ אֶלָּא סְנַפִּיר אַחַת וְקַשְׂקֶשֶׂת אַחַת הֲרֵי זֶה מֻתָּר:
And regarding fish there are two signs: fin and scale. And a fin is a thing which sprouts out of it. And a scale is that which stick on all of its body. And all [water creatures] which have scales [also] have a fin. [Regarding a fish that] has no [scale] now but when it grows it has them or it has a scale when it is in the sea, but when it is brought up, its scales are gone--behold, this is permitted. And one who has no scales attached all over, [it is] permitted. Even if has only one fin and one scale--behold, this is permitted.
(א) סִימָנֵי דָּגִים מְפֹרָשִׁים בַּתּוֹרָה: כָּל שֶׁיֵּשׁ לוֹ סְנַפִּיר וְקַשְׂקֶשֶׂת, טָהוֹר. וּסְנַפִּיר, הוּא שֶׁשָּׁט בּוֹ. וְקַשְׂקֶשֶׂת, הֵן הַקְּלִפּוֹת הַקְּבוּעוֹת בּוֹ. הַגָּה: וְדַוְקָא שֶׁהֵם נִקְלָפִים בְּיָד אוֹ בִּכְלִי, אֲבָל אִם אִי אֶפְשָׁר לְקַלְּפָן מֵעוֹר הַדָּג, לֹא מִקְרֵי קַשְׂקֶשֶׂת (המ''מ פֶּרֶק א' דמ''א). וַאֲפִלּוּ אֵין לוֹ אֶלָּא סְנַפִּיר אֶחָד וְקַשְׂקֶשֶׂת אַחַת, מֻתָּר. וַאֲפִלּוּ אֵין לוֹ עַתָּה, וְעָתִיד לְגַדְּלָם לְאַחַר זְמַן אוֹ שֶׁהָיָה לוֹ בְּעוֹדוֹ בַּמַּיִם וְהִשִּׁירָן מִיָּד בַּעֲלוֹתוֹ לַיַּבָּשָׁה, מֻתָּר. (רַמְבַּ''ם ספ''א מהמ''א ורי''ף ור''ן וְה''ה) הַגָּה: וְיֵשׁ אוֹמְרִים דְּאֵין לְהַתִּיר בְּקַשְׂקֶשֶׂת אַחַת, רַק כְּשֶׁהִיא עוֹמֶדֶת תַּחַת לְחַיָּיו אוֹ זְנָבוֹ אוֹ סְנַפִּירוֹ (הַטּוּר וְהָרֹא''שׁ לְדַעַת בֵּית יוֹסֵף וְר''ן וְה''ה בְּשֵׁם הָרַמְבַּ''ן), וְטוֹב לְהַחְמִיר.
(1) The signs of fish are explicit in the Torah: All that have fins and scales, kosher. And fins, it is what it swims with. And scales, they are hardened skins on it. Gloss: Specifically when they are peeled by hand or with a utensil, but if it is impossible to peel them from the skin of the fish, it is not called scales (HM''M 1:41) And even if it only has one fit and one scale, it is permitted. And if it does not not, and in the future when it grows after a time or when it had them while still in water and he removes them immediately upon taking it out to dry it, it is permitted. (Rambam Forbiddne Foods Chapter 1 and the Rif and the Ran and this is the law) Gloss: And there are those who say that one does not permit with one scale, only when it stands on its jaw or tail or fin (the Tur and the Rosh according to the Beit Yosef and the Ran and this is the law in the name of the Ramban), and it is good to be stringent.
(א) כָּל הַבָּשָׂר אָסוּר לְבַשֵּׁל בְּחָלָב, חוּץ מִבְּשַׂר דָּגִים וַחֲגָבִים. וְאָסוּר לְהַעֲלוֹתוֹ עִם הַגְּבִינָה עַל הַשֻּׁלְחָן, חוּץ מִבְּשַׂר דָּגִים וַחֲגָבִים. הַנּוֹדֵר מִן הַבָּשָׂר, מֻתָּר בִּבְשַׂר דָּגִים וַחֲגָבִים. הָעוֹף עוֹלֶה עִם הַגְּבִינָה עַל הַשֻּׁלְחָן וְאֵינוֹ נֶאֱכָל, דִּבְרֵי בֵית שַׁמַּאי. וּבֵית הִלֵּל אוֹמְרִים, לֹא עוֹלֶה וְלֹא נֶאֱכָל. אָמַר רַבִּי יוֹסֵי, זוֹ מִקֻּלֵּי בֵית שַׁמַּאי וּמֵחֻמְרֵי בֵית הִלֵּל. בְּאֵיזֶה שֻׁלְחָן אָמְרוּ, בַּשֻּׁלְחָן שֶׁאוֹכֵל עָלָיו. אֲבָל בַּשֻּׁלְחָן שֶׁסּוֹדֵר עָלָיו אֶת הַתַּבְשִׁיל, נוֹתֵן זֶה בְצַד זֶה וְאֵינוֹ חוֹשֵׁשׁ:
(1) All meat is forbidden to cook with milk, except for the meat of fish and locusts. And it is forbidden to place it with cheese on the table, except for the meat of fish and locusts. One who vows [to abstain] from meat, is permitted to [eat] fish and locusts. "A bird may go on with cheese on the table, but is not eaten," in the words of Beit Shammai. And Beit Hillel says, "It does not go and it is not eaten." Said Rabbi Yose, "This is [one] of the lenient rulings of Beit Shammai and stringent rulings of Beit Hillel." About which table were they speaking? About a table on which one eats. But for a table where one sets dishes on it, one puts this next to this and does not worry.