We learned in the Mishna (040) that according to Beis Hillel, one must designate birds he would like to slaughter on Yom Tov, before Yom Tov. It was suggested that this is because Beis Hillel does not accept the principle of Bereirah - retroactive clarification - and so the exact birds must be selected before Yom Tov.
Challenge: Mishna: If a meis is in a room with a number of entrances, all are tamei. If one was opened, all are now tahor except for the one which was opened. If he intended to take the meis out from one of the doors, it rescues all other openings from becoming tamei.
Beis Shammai: This is only if that intention took place before the meis died. Beis Hillel: Even after he died. [So it is clear that Beis Hillel holds of Bereirah!]
Resolution: This Mishna was already explained by Rabbah: “To purify the doorways from now on.”
So too said Rabbi Oshaya: To purify the doorways from now on. Only going forward but not retroactively.
Rava: The openings are permitted retroactively. The reason Beis Hillel requires one to designate birds before Yom Tov and it is not clarified retroactively on Yom Tov is because we’re worried he might pick up [a bird] and reject it, pick up another and reject it, and he will have moved something he didn’t intend to use.
Challenge: But according to Beis Hillel it’s enough to designate a bird? [They are not worried he will come to reject it on Yom Tov and they do not require him to actually pick it up before Yom Tov]?
Resolution: This is true about Erev Yom Tov [because we required his to select a bird to avoid the possibility of his rejecting it on Yom Tov], but [if we don’t require anything before] we are worried he may reject the bird on Yom Tov because what seemed fat may turn out to be lean and he will have moved something that is not fit for him on Yom Tov. Or perhaps all of the birds will turn out to be too lean and he’ll leave over that nest and refrain from Simchas Yom Tov [but if a bird is designated before Yom Tov, he will slaughter that one even if it turns out to be lean].
Mishna:
If he prepared black birds and found white ones, or he prepared white birds and found black ones, or if he prepared two birds and found three; they’re all forbidden. If he prepared three birds and found two; they’re permitted. If he prepared them inside the nest and found them in front of the nest [on the little porch], they’re forbidden. If no other nests are nearby, they’re permitted.