Sefer HaChinuch ספר החינוך

To not take the mother upon the young: That we should not take a bird’s nest — the mother and the chicks or the eggs together — but rather that we should send away the mother. And about this is it stated (Deuteronomy 22:6), “do not take the mother upon the young.”

I have written about the roots of the commandment and a few of its laws and all of its matter in its positive commandment in this Order (Sefer HaChinukh 545) — see there. And there we also spoke about this negative commandment; that it is rectified by the positive commandment of “you shall surely send away the mother.” And they, may their memory be blessed, have already taught us in Tractate Makkot 15a in the chapter [entitled] Elu Hen HaLokin [about] all negative commandments that have a positive commandment (that reverses the result of transgressing the negative commandment) — that if he fulfilled its positive commandment, he is exempt [from punishment]; [but] if he did not fulfill the positive commandment and it is [now] impossible to fulfill it, he is liable for lashes. And as it is stated over there, “As Rabbi Yochanan said to the teacher, ‘It is taught, “He fulfilled it and he did not fulfill it.”’” And that is the correct textual variant (like the variant of Rif and Ramban). And from this opinion we learned that any time the mother dies or another man has sent it away — and even though he has now not negated the positive commandment with his [own] hands, as behold, he did not kill it, but rather it died on its own; and there is no need to say if he killed the mother with his [own] hands — that he is liable for lashes according to everyone. But any time that he sent it before it died — even though he did not send it when he took it [with] the nest — he has not violated the negative commandment nor the positive commandment, since the Torah rectified it with the positive commandment; and behold, he [now] fulfilled it. And [this is so] even though it is not fitting to do so — as maybe the mother will die or the sender [will die] before the sending and he will not be able to fix [it]; and also since “enthusiasts are prompt with commandments,” and “how good is a thing in its time.”