We are up to Zevachim chapter 6, mishnah 4, which deals with the order of the service of a bird chatas - how it is actually performed. As we mentioned, it consists of three stages: melikah - killing the bird; haza'ah - sprinkling the blood; and mitzuy - squeezing out the blood. In this mishnah, we will briefly review all of them.
Melikah - how to hold the bird:
The entire bird is held in the kohen's left hand, and this grip is difficult:
The two legs of the bird - between the ring finger and the pinky, the two small fingers.
The two wings - between the middle finger and the index finger.
The head of the bird - wedged between the thumb and the index finger.
On his right hand, the kohen has a sharpened thumbnail, and with it he performs the melikah: he pierces and severs at the back of the neck, behind the nape of the bird. He cuts one of the pipes:
The veshet - the food pipe (esophagus).
The kaneh - the windpipe (trachea).
However, the Torah forbade him to completely sever it, meaning to actually separate the head from the body, and one who does so invalidates the entire korban. Thus, he kills the bird and cuts one of the pipes, but he does not detach the head from the body.
The text of the mishnah:
"Hayah molek es roshah mimmul orpah, ve'eino mavdil" - the kohen pierces with his thumbnail opposite the back of the neck. 'Oref' means the back part of the neck, opposite the throat; and he does not sever the head from the body, as we have learned.
Haza'ah:
"Umazeh middamah al kir hamizbe'ach" - at this point, the bird chatas differs from an animal sacrifice. With an animal sacrifice, the blood must be received directly from the neck of the slaughtered animal into a service vessel (and there is a discussion about a case where the blood fell to the floor first). With a bird, there is not enough blood for a service vessel, and therefore the blood goes directly from the neck to the mizbe'ach.
How is this done? The kohen holds the dead bird, and with a vigorous downward motion, he brings it down, and the blood splatters onto the wall. Ideally, this is done on the southwestern wall, and in any case, above the yesod and below the chut hasikra (red line).
The Squeezing:
"U'shiyarei hadam hayah mismatzeh al hayesod" - and the remainder of the blood would be squeezed out onto the base. This is the blood that remains. With a bird, there is no blood collected in a vessel like an animal; rather, there is a small amount of blood left. The wording of the Mishnah makes it sound as if he squeezes the bird's neck on the base itself, but this is not the case: the kohen presses the bird's neck against the wall and squeezes the blood directly from the neck. According to Rashi, he must squeeze the blood onto the wall above the base, and the blood drips down toward the base.
"Ein lammizbe'ach ella damah vechulah lakohanim" - the altar has nothing but its blood, and all of it goes to the kohanim:
The only part of the bird sin-offering that goes up on the altar is the blood alone - the sprinkling and the squeezing. It has no sacrificial parts, no eimurim (fats offered on the altar), or anything similar. Instead, all of it goes to the kohanim: they receive the entire bird to eat, and upon completing their service, they eat these birds; from that moment, it is theirs, regular food in every respect.
Once the squeezing is complete, they can remove the head, pluck the feathers, and cook it, all without any issue. The bird is eaten entirely by the kohanim, and no part of it is burned on the altar other than the blood.
In summary: In this Mishnah, we learned the three stages of the bird sin-offering - the melikah (nipping) with the fingernail at the back of the neck without severing the head from the body, the sprinkling of the blood on the wall of the altar above the base and below the red line, and the squeezing of the remaining blood against the wall toward the base - as well as the rule that the altar receives nothing but its blood, and all of its meat is eaten by the kohanim.