Mishnayos Kinnim Perek 2 Mishnah 4

קן סתומה – he did not explain which was [designated/specified] for a sin-offering and which was [designated/specified] for a burnt-offering.

וקן מפורשת – that it is known that this one was [designated/specified] for a sin-offering and that one was [designated] for a burnt-offering. But here, we are speaking when the two pigeons were combined together after they were designated/specified, and now it is not known which is for a sin-offering and which is for a burnt offering, and this is shown in the concluding clause [of the Mishnah] that teaches: “or [if] at first one which had been designated flew off to the first undesignated pair, all of them are left to die, but if the individual pigeons that had been designated/specified did not became mixed with each other, why should they be [all] left to die, but if the burnt-offering flew off, the remaining [bird] for the sin-offering should be offered up, and similarly, the unspecified/undesignated couple of sacrificial birds , should be offered up as one burnt-offering because of the burnt-offering that became mixed up with it. But we are speaking of when individual pigeons that were designated became mixed up one with another, but if one individual pigeon from the pair of sacrificial birds had not been designated/specified had been combined with that which was designated, he (i.e., the Kohen) should take one pair for second [bird] and designate one for a sin-offering and one for a burnt-offering, and the third that had been combined/mixed up together, all of them should be left to die. If one from the three that had been mixed up/combined returned and became combined with the individual [bird] remaining from the undesignated/specified group, or at the outset of flying off, combined one from the designated [pair] to that which was not designated/specified, which is the first as we stat, meaning to say, that at the outset of flying off, one went and combined with an undesignated pair, all of them should be left to die, for since the individual pigeons which had been designated had combined together, they are no longer offered up, and when one of them flee off in another place, that is, one from the ones left to die that flew off among those that were to be offered up, all of them should be left to die.