Hullin, Chapter One, Mishnah Five
Yesterdays mishnah concluded by noting that the place on the neck where slaughtering is performed is exactly where nipping cannot be performed and vice versa.
The remainder of our chapter deals with other opposite types of situations. At first these deal with sacrificial issues, but then they move on to other issues as well.
1) [The age] which qualifies turtle doves [for sacrifice] disqualifies pigeons, and [the age] which qualifies pigeons [for sacrifice] disqualifies turtle doves.
2) At the period when the neck feathers begin to turn yellow in either kind they are disqualified.
Section one: Turtle doves are valid as sacrifices when they are older and pigeons when they are younger. There is no overlapping period when both are valid.
Section two: However, both are invalid when the neck feathers begin to turn yellow. When the young birds hit three months their feathers begin to fall off, first from the body and then from the neck and head. Once all of their baby feathers have fallen off, they get new adult feathers. The new feathers on the head are at first yellow in appearance. Once this yellowish color has appeared, the birds are no longer considered to be young, but they are not yet considered old. Therefore, neither turtle doves nor pigeons can be used at this in-between age.