To which animals does "Oso ve'es B'no" apply?
Rashi: It applies to a mother and its son or daughter, or no matter which is slaughtered first. 1
Meshivas Nefesh: All forbid also the father and its child, just according to the opinion that is not concerned for the seed of the father, it is only mid'Rabanan.
Ramban: This is like the opinion that we are not concerned for the seed of the father.
What is the reason for the prohibition against slaughtering a mother and its baby on the same day?
Ramban #2 (Devarim, 22:6): Because slaughtering a mother and baby on the same day is akin to destroying the species.
Ramban #3 (citing Moreh Nevuchim): Because the feelings of a mother towards her children are the same in an animal as in humans, and it is therefore particularly cruel to kill a baby animal in front of its mother.
Why doesn't the Isur apply to Chayos and birds? We should learn from a Gezeirah Shavah "Shor-Shor" from Shabbos, like we do to obligate a pit for Chayos and birds that fall in!
Moshav Zekenim 1 : Toras Kohanim asked that we should learn from a Kal v'Chomer from Kisuy ha'Dam, which does not apply to Behemos, and it applies to Chayos and birds! It answers that Shor excludes Chayos, and Seh excludes birds.
Even though every Gezeirah Shavah needs a tradition, it was a proper question, since there was a tradition for"Shor-Shor"; Chachamim were authorized to decide to which occurrences of the words it applies, and what to learn from it. (PF)
Why is it permitted to slaughter a pregnant Behemah? We hold that the fetus dies first (and it is considered that the fetus was slaughtered)!
Moshav Zekenim citing R. Simchah: We learn from "Kol
Do not say that it is forbidden, just if one slaughtered it, the Torah permits it, The Torah would not teach a Heter that comes only via an Aveirah (Yevamos 78a. However, perhaps it teaches so for one who did not know that the animal is pregnant. This is like Ones! - PF. Why did the question mention that the fetus dies first? Since it is considered that he slaughtered both, in any case it should be forbidden! R. Meir Eliyahu - this is so one cannot answer that a fetus is a limb of its mother.)
QUESTIONS ON RASHI
Rashi writes that the Isur applies only to a mother and its child, If so, why does the Torah write "Oso v'Es Beno"?
Ramban, Moshav Zekenim: The previous Pasuk said that a Shor, Kesev or Ez must be with its mother for seven days before one may offer it. Now it teaches another law of these species - the parent and child may not be slaughtered on the same day. (It used the masculine, like the previous verse, even though this verse applies to mothers and not fathers.)