Q. You have previously written that the separation of milk and meat derives from the verse in the Torah, “Do not seethe a kid in its mother’s milk”. If the reference is to a kid, why does the law extend to all milk and meat products? Chickens don’t have milk, so why include them?
The parameters of the law are addressed carefully in Rashi. He notes that g’di, a kid, is never used on its own except for these three cases. Usually it comes together with the word izzim, goats (e.g. Gen. 38:17, “I will send a g’di of the goats”). Where the word appears independently it does not mean a goat’s kid but simply a young animal.
Why the law mentions young animals and not animals of any age is presumably that the meat of the young is more tender and that is when the temptation to combine it with milk is greater.
On the question of chickens, Rashi quotes the Talmudic explanation that “in its mother’s milk” excludes species such as birds which have no milk. Including chickens in the milk-and-meat law is a rabbinical and not a Biblical rule, though some argue that it is Biblical. On the whole subject of kashrut, see I. Grunfeld, “The Jewish Dietary Laws”, 2 vols., 1972.