Chulin 1:4-5

Chulin 1:4

If one cut at the side of an animal’s neck, the slaughter is valid; if one nipped the head of a bird offering from the side, it is invalid. If one slaughtered an animal by cutting from the back of the neck, the slaughter is invalid; if one nips the head of a bird offering from the back, it is valid. (This is actually the proper place to perform this process, which is called melikah.) If one slaughters an animal at the front of the neck (as one should), the slaughter is valid; if one performs melikah at the front, it is invalid. The whole back of the neck is valid for melikah and the whole front of the neck is valid for slaughter; what is valid for slaughter is invalid for melikah, and vice versa.

Chulin 1:5

The age that renders the pigeons valid, renders doves invalid, and vice versa. (Pigeons must be grown and doves must be young.) Both species are invalid when the feathers on their necks start to glisten (an intermediate age).