Why can't we eat poultry with dairy?
Popular misconception: The rabbis were afraid that a piece of chicken has a similar appearance as a piece of meat and that people who observe chicken being consumed with milk may think that the people are eating meat.
(יט)...לֹֽא־תְבַשֵּׁ֥ל גְּדִ֖י בַּחֲלֵ֥ב אִמּֽוֹ׃ {פ}
(19) ...You shall not boil a kid in its mother’s milk.
(11) You may eat any pure bird. (12) The following you may not eat: the eagle, the vulture, and the black vulture; (13) the kite, the falcon, and the buzzard of any variety; (14) every variety of raven; (15) the ostrich, the nighthawk, the sea gull, and the hawk of any variety; (16) the little owl, the great owl, and the white owl; (17) the pelican, the bustard, and the cormorant; (18) the stork, any variety of heron, the hoopoe, and the bat.
(א) משֶׁה קִבֵּל תּוֹרָה מִסִּינַי, וּמְסָרָהּ לִיהוֹשֻׁעַ, וִיהוֹשֻׁעַ לִזְקֵנִים, וּזְקֵנִים לִנְבִיאִים, וּנְבִיאִים מְסָרוּהָ לְאַנְשֵׁי כְנֶסֶת הַגְּדוֹלָה. הֵם אָמְרוּ שְׁלשָׁה דְבָרִים, הֱווּ מְתוּנִים בַּדִּין, וְהַעֲמִידוּ תַלְמִידִים הַרְבֵּה, וַעֲשׂוּ סְיָג לַתּוֹרָה:
(1) Moses received the Torah at Sinai and transmitted it to Joshua, Joshua to the elders, and the elders to the prophets, and the prophets to the Men of the Great Assembly. They said three things: Be patient in [the administration of] justice, raise many disciples and make a fence round the Torah.
(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) It is prohibited to cook any meat of domesticated and undomesticated animals and birds in milk, except for the meat of fish and grasshoppers, whose halakhic status is not that of meat. And likewise, the Sages issued a decree that it is prohibited to place any meat together with milk products, e.g., cheese, on one table. The reason for this prohibition is that one might come to eat them after they absorb substances from each other. This prohibition applies to all types of meat, except for the meat of fish and grasshoppers. And one who takes a vow that meat is prohibited to him is permitted to eat the meat of fish and grasshoppers. The meat of birds may be placed with cheese on one table but may not be eaten together with it; this is the statement of Beit Shammai. And Beit Hillel say: It may neither be placed on one table nor be eaten with cheese. Rabbi Yosei said: This is one of the disputes involving leniencies of Beit Shammai and stringencies of Beit Hillel. The mishna elaborates: With regard to which table are these halakhot stated? It is with regard to a table upon which one eats. But on a table upon which one prepares the cooked food, one may place this meat alongside that cheese or vice versa, and need not be concerned that perhaps they will be mixed and one will come to eat them together.
(ד) בְּשַׂר בְּהֵמָה טְהוֹרָה בַּחֲלֵב בְּהֵמָה טְהוֹרָה, אָסוּר לְבַשֵּׁל וְאָסוּר בַּהֲנָאָה. בְּשַׂר בְּהֵמָה טְהוֹרָה בַּחֲלֵב בְּהֵמָה טְמֵאָה, בְּשַׂר בְּהֵמָה טְמֵאָה בַּחֲלֵב בְּהֵמָה טְהוֹרָה, מֻתָּר לְבַשֵּׁל וּמֻתָּר בַּהֲנָאָה. רַבִּי עֲקִיבָא אוֹמֵר, חַיָּה וָעוֹף אֵינָם מִן הַתּוֹרָה, שֶׁנֶּאֱמַר, לֹא תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ, שָׁלשׁ פְּעָמִים, פְּרָט לְחַיָּה וּלְעוֹף וְלִבְהֵמָה טְמֵאָה. רַבִּי יוֹסֵי הַגְּלִילִי אוֹמֵר, נֶאֱמַר (דברים יד), לֹא תֹאכְלוּ כָל נְבֵלָה, וְנֶאֱמַר (שם), לֹא תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ. אֶת שֶׁאָסוּר מִשּׁוּם נְבֵלָה, אָסוּר לְבַשֵּׁל בְּחָלָב. עוֹף, שֶׁאָסוּר מִשּׁוּם נְבֵלָה, יָכוֹל יְהֵא אָסוּר לְבַשֵּׁל בְּחָלָב, תַּלְמוּד לוֹמַר בַּחֲלֵב אִמּוֹ, יָצָא עוֹף, שֶׁאֵין לוֹ חֲלֵב אֵם:
(4) It is prohibited to cook the meat of a kosher animal in the milk of any kosher animal, not merely the milk of its mother, and deriving benefit from that mixture is prohibited. It is permitted to cook the meat of a kosher animal in the milk of a non-kosher animal, or the meat of a non-kosher animal in the milk of a kosher animal, and deriving benefit from that mixture is permitted. Rabbi Akiva says: Cooking the meat of an undomesticated animal or bird in milk is not prohibited by Torah law, as it is stated: “You shall not cook a kid in its mother’s milk” (Exodus 23:19, 34:26; Deuteronomy 14:21) three times. The repetition of the word “kid” three times excludes an undomesticated animal, a bird, and a non-kosher animal. Rabbi Yosei HaGelili says that it is stated: “You shall not eat of any animal carcass” (Deuteronomy 14:21), and in the same verse it is stated: “You shall not cook a kid in its mother’s milk.” This indicates that meat of an animal that is subject to be prohibited due to the prohibition of eating an unslaughtered carcass is prohibited for one to cook in milk. Consequently, with regard to meat of birds, which is subject to be prohibited due to the prohibition of eating an unslaughtered carcass, one might have thought that it would be prohibited to cook it in milk. Therefore, the verse states: “In its mother’s milk,” excluding a bird, which has no mother’s milk.
(יג) לְפִי שֶׁהוּא אוֹמֵר: "רֵאשִׁית בִּכּוּרֵי אַדְמָתְךָ תָּבִיא בֵּית יי אֱלֹהֶיךָ, לֹא תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ", לֹא שָׁמַעְנוּ אֶלָּא בִזְמַן שֶׁהַבִּכּוּרִין נוֹהֲגִין, וּבִמְקוֹם שֶׁהַבִּכּוּרִין נוֹהֲגִין, שָׁם בָּשָׂר בֶּחָלָב נוֹהֵג. בִּזְמַן שֶׁאֵין הַבִּכּוּרִין נוֹהֲגִין שָׁם, מְקוֹם שֶׁאֵין הַבִּכּוּרִין נוֹהֲגִין שָׁם, לֹא שָׁמַעְנוּ. תִּלְמֹד לוֹמַר: (דברים יד,כא) "לֹא תֹאכְלוּ כָל נְבֵלָה... לֹא תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ." מַה נְּבֵלָה נוֹהֶגֶת בָּאָרֶץ וּבְחוּצָה לָאָרֶץ, בִּפְנֵי הַבַּיִת וְשֶׁלֹּא בִּפְנֵי הַבַּיִת, אַף בָּשָׂר בֶּחָלָב יְהֵי נוֹהֵג בָּאָרֶץ וּבְחוּצָה לָאָרֶץ, בִּפְנֵי הַבַּיִת וְשֶׁלֹּא בִּפְנֵי הַבַּיִת.
(יד) רַבִּי עֲקִיבָה אוֹמֵר: מִפְּנֵי מָה נֶאֱמַר בִּשְׁלֹשָׁה מְקוֹמוֹת? פְּרָט לִבְהֵמָה טְמֵאָה, פְּרָט לַחַיָּה, פְּרָט לָעוֹפוֹת.
(טו) רַבִּי יוֹסֵה הַגָּלִילִי אוֹמֵר: לָמָּה נֶאֱמַר (דברים יד,כא) "לֹא תֹאכְלוּ כָל נְבֵלָה", וְנֶאֱמַר (דברים יד,כא) "לֹא תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ." אֶת שֶׁהוּא אָסוּר מִשֵּׁם נְבֵלָה, אָסוּר לְבַשֵּׁל בֶּחָלָב, עוֹף שֶׁהוּא אָסוּר מִשֵּׁם נְבֵלָה, יָכֹל יְהֵי אָסוּר לְבַשֵּׁל בֶּחָלָב? תִּלְמֹד לוֹמַר "בַּחֲלֵב אִמּוֹ." יָצָא עוֹף, שֶׁאֵין לוֹ חֲלֵב אֵם,
(13) Variantly: whether non-consecrated or consecrated (animals). Rebbi says: Because it is written (in the same context as meat and milk) "the first of the fruits of your land," I might think that only at the time that bikkurim obtain and only in the place that bikkurim obtain, only there does (the prohibition of) meat and milk obtain, but not otherwise; it is, therefore, written (Devarim 14:21) "You shall not eat all carrion (neveilah) … You shall not cook a kid in its mother's milk." Just as (the prohibition of) carrion obtains both in the land and outside the land, both before the Temple and not before the Temple, so (the prohibition of) meat and milk.
(14) R. Akiva says: Why is it written in three places? To exclude an animal, to exclude an unclean beast, to exclude a bird.
(15) R. Yossi Haglili says: It is written "You shall not eat all carrion … You shall not cook a kid in its mother's milk." I might think that what is forbidden as carrion may not be cooked in milk, so that a bird which is forbidden as carrion, may not be cooked in milk. It is, therefore, written "in its mother's milk" — to exclude a bird, where "mother's milk" does not obtain.
Professor David Kraemer, Jewish Theological Seminary
“In the ancient world, meat was eaten with relative rarity, primarily for special occasions. For more common special occasions, such as the Sabbath, ‘smaller’ meat would have been most common, and that was typically fowl. So people simply thought of and spoke of fowl as meat. Since this is the way people thought of it, this is the way the rabbis categorized it.”