Kinnim, Chapter 2, Mishnah 4. Until this point the chapter has dealt with mixtures made up entirely of birds that are setumot, undesignated: the owner separated a pair as his obligation, but never announced which of the two would be the chatat and which the olah, so either bird can still serve either purpose. That flexibility is exactly what allowed the earlier rulings to be as forgiving as they were. Now the mishnah introduces a new element into the picture: a pair that has already been designated, where the owner (or the kohen) has fixed one bird as the chatat and the other as the olah.
The mishnah begins: "Kein setumah v'kein meforeshet" - before us stand two keinim. One is setumah, its two birds still interchangeable, either one available for the chatat and either one for the olah. The second is meforeshet, its birds already sorted, this one for the chatat and that one for the olah. "Parach min hasetumah lameforeshet" - one bird abandons the setumah kein and settles among the birds of the meforeshet kein. Three birds are now bunched together, and identification is impossible: which is the chatat, which is the olah, and which is the intruder that carries no role at all?
Restoring the Pair That Was Left Behind
The mishnah rules: "Yikach zug lasheini" - the owner of the undesignated pair is not required to start over and purchase an entirely new pair. He brings a single bird as a mate for the one that remained with him. Why is that enough? Because the bird that stayed behind was never assigned any role. Nothing about it has been compromised by the disappearance of its partner. Its former partner's flight does not invalidate it; it simply needs a new companion so that the obligation of a pair can be fulfilled, and the newly purchased bird supplies that.
When the Confusion Travels Back
The mishnah goes on: "Chazar" - suppose one of those three birds now leaves that group and crosses over to the pair that was just reassembled with its freshly purchased mate. Or, alternatively, "o sheparach min hameforeshet larishon" - suppose the movement ran the other way to begin with, and it was one of the birds already carrying a role that flew across and mingled with the two interchangeable ones. Here the ruling turns out to be far harsher.
What we are looking at now is two birds without any assignment together with a third that already bears one, and we can neither pick out which bird it is nor say what it was assigned to be. The mishnah therefore rules: "harei kulan yamutu" - every one of these birds is set aside and left to die. None may be brought on the mizbeach, since whichever bird a kohen takes in hand, its status as chatat or as olah remains unknown.
This follows the principle established at the opening of the masechet. A chatat and an olah are offered by different procedures, and a bird brought under an unresolved doubt as to which it is cannot be sacrificed at all. Wherever a genuine safek arises regarding what must be brought, every bird caught up in that doubt is placed aside and left to die.