A newborn domesticated animal may be slaughtered immediately if we know that it completed its full gestation – nine months for cattle and five months for sheep and goats. When this is not known, we wait until the eighth day after birth; by then, it is clear that the newborn is viable and not a nefel. Since in practice we generally do not know whether a newborn animal completed its full gestation, newborn animals are not slaughtered before the eighth day after birth (SA YD 15:2).
This is different from offerings, for which the eighth day is itself required by the Torah. As it is stated: “When an ox, sheep, or goat is born, it shall remain seven days under its mother; from the eighth day onward it shall be accepted as a fire-offering to the Lord” (Vayikra 22:27). For ordinary meat, however, the eighth-day wait serves only to establish that the newborn is viable.
A chick becomes fit for sheḥita only after it has fully broken through its shell and emerged into the world. As long as it remains inside the egg, it is classified as a sheretz ha-of. Therefore, if the shell was broken open and the chick was removed, it may not be slaughtered or eaten. Only after it fully develops like other birds does it become fit for sheḥita (SA YD 15:1; Simla Ḥadasha 1). According to many authorities, even a chick that broke through the shell on its own should not be slaughtered before large feathers have grown on its wings, since eating such an undeveloped chick raises a concern of bal teshaktzu (Shakh 15:2; Tevu’ot Shor 15:2; AHS 2–3; Taz permits it).
