(19) The choice first fruits of your soil you shall bring to the house of the LORD your God. You shall not boil a kid in its mother’s milk.
"Three times, at Exodus 23:19, Exodus 34:26, and Deuteronomy 14:21, Scripture ordains that one may not eat the flesh of a kid that has been cooked in its own mother’s milk. The oral tradition connected with these verses vastly expands the concept to include three major areas of prohibition: dairy and meat products may be neither eaten nor cooked together, nor may one derive even ancillary benefit from such a forbidden mixture (MT Hilkhot Ma·akhalot Asurot 9:1). Although there is no specific source in the Bible or the Talmud that clearly states the reason for this supremely stringent policy of separation, many ideas have been suggested throughout the ages. Maimonides, for example, writing in the Guide for the Perplexed (III 98), took a historical approach, writing that he imagined these laws to be a kind of reaction to the pagan custom of boiling a kid in its mother’s milk, which practice he imagined tied to pagan fertility rites. More recently, however, Professor Edward L. Greenstein of Bar-Ilan University has taken a more anthropological approach, writing that the prohibition" "which is meant to sustain life, may not be turned into a means of preparing an animal for eating. A clear distinction must be made between life, which is godly, and death. The post-biblical Jewish tradition underscores the distinction by broadening it: not only milk, but all dairy products and the utensils used for serving them must be kept apart from meat products and utensils” (Etz Hayim, p. 1461). Whatever its rationale, however, the rabbis understood the law to apply to situations far beyond the literal meaning of the biblical verses. The flesh of mammals and birds are both considered meat. Furthermore, even foods made with meat additives or meat by-products are considered meat in this context, even if there are no actual pieces of meat present."
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:
Rabbi Akiva says, "Wild animals and birds are not [forbidden in mixtures with milk] from the Torah, as it says, 'You may not cook a kid in its mother's milk,' three times. It exempts the wild animal, the bird, and the prohibited domesticated animal." Rabbi Yose HaGelili says, "It is stated, 'You may not eat any nevelah [an improperly slaughtered animal of a permitted species],' (Deuteronomy 14:21) and it is stated, 'You may not cook a kid in its mother's milk,' (Ibid.). That which [may become] forbidden as nevelah is forbidden to cook with milk. A bird, which [may become] forbidden as a nevelah, one might think that it should be forbidden to cook with milk, [but] the Torah says, "In its mother's milk." This excludes a bird, which does not have mother's milk.
(ג) אֵינוֹ נוֹהֵג אֶלָּא בִּבְשַׂר בְּהֵמָה טְהוֹרָה בַּחֲלֵב בְּהֵמָה טְהוֹרָה, אֲבָל בְּשַׂר טְהוֹרָה בַּחֲלֵב טְמֵאָה, אוֹ בְּשַׂר טְמֵאָה בַּחֲלֵב טְהוֹרָה, מֻתָּרִים בְּבִשּׁוּל וּבַהֲנָאָה. וּבְשַׂר חַיָּה וָעוֹף, אֲפִלּוּ בַּחֲלֵב טְהוֹרָה, מֻתָּר בְּבִשּׁוּל, וּבַהֲנָאָה; וְאַף בַּאֲכִילָה אֵינוֹ אָסוּר, אֶלָּא מִדְּרַבָּנָן. אֲבָל דָּגִים וַחֲגָבִים, אֵין בָּהֶם אִסוּר, אֲפִלּוּ מִדְּרַבָּנָן. הַגָּה: וְנָהֲגוּ לַעֲשׂוֹת חָלָב מִשְּׁקֵדִים וּמַנִּיחִים בָּהּ בְּשַׂר עוֹף, הוֹאִיל וְאֵינוֹ רַק מִדְּרַבָּנָן. אֲבָל בְּשַׂר בְּהֵמָה, יֵשׁ לְהַנִּיחַ אֵצֶל הַחָלָב שְׁקֵדִים, מִשּׁוּם מַרְאִית הָעַיִן, כְּמוֹ שֶׁנִּתְבָּאֵר לְעֵיל סִימָן ס''ו לְעִנְיַן דָּם (ד''ע).
(3) The law only applies with meat from a pure [kosher] animal and milk from a pure animal, but meat of a pure animal in milk that is impure, or meat from an impure animal in pure milk, has the eating prohibition but not the restriction on cooking or deriving benefitting from. Meat of a wild animal and of fowl, even in milk which is pure, is permitted to cook and benefit from; and even eating is only prohibited rabbinically. Fish and grasshoppers are not prohibited [with milk] even rabbinically. We make milk from almonds and place bird meat in it, since [milk and bird meat] is only rabbinically [forbidden]. But with meat from a domesticated animal, place almonds next to the milk, so that people don't misunderstand. This is as we said above, in chapter 66.
“In the ancient world, meat was eaten with relative rarity, primarily for special occasions,” Kraemer told me. “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.”