Kinim, Chapter Two, Mishnah Four
1) An unassigned pair and an assigned pair: if one bird from the unassigned [pair] flew to the assigned [pair], then a pair must be taken for the second [bird].
2) If one bird flew back, or if in the first place a bird from the assigned pair flew [to the other pair], then all must be left to die.
Section one: There are two pairs in the situations described in this mishnah. In one pair the two birds have been assignedwhich one will be a hatat and which one will be an olah. The birds of the other pair have not been assigned.
One bird from the unassigned pair flies over to the assigned pair. The three birds are now mixed up such that we can’t tell which one of the assigned pair was a hatat and which was an olah. These three birds cannot be sacrificed and must be left to die. The owner can then take a second bird for the one bird left in the unassigned pair. This is the case of one bird from an unassigned pair that flies into a group of birds that must be left to die (see mishnah one). As we learned, in such a case he may take another bird for the bird left in the original pair.
Section two: If one of the three birds left to die flew back to the other pair, then all of the birds must be left to die, for in neither of them do we know which is the hatat and which is the olah. The same is true if originally one of the assigned pair flew to the unassigned pair. Since we don’t know which bird flew (the hatat or the olah) and which bird remained, none of the three birds in the first pair, or the remaining one bird, can be offered. All must be left to die.