Mishna:
Beis Shammai: One may not take a bird on Yom Tov [to slaughter for the meal] unless he had hefted it from before Yom Tov [out of concern that when he picks it up he’ll find it too lean and will want to choose another one for his meal, he will then be guilty of having picked up a muktzeh bird without the excuse of needing it for simchas Yom Tov]. Beis Hillel: He can designate them before Yom Tov.
Rav Chanan bar Ami: Their debate is only about the first set of birds that are hatched in the season. Beis Shammai is worried he might change his mind [because people are reluctant to slaughter of the first group] and Beis Hillel do not make such a gezeirah. But in the second batch, everyone agrees it is enough to designate the birds before Yom Tov. [We rule like Beis Hillel so actually it doesn’t matter].
Question: Why must the birds be designated according to Beis Hillel? It should be enough just to declare one’s intention to take birds but it shouldn’t be necessary to designate specific birds.
Suggestion: You may want to answer that Beis Hillel do not hold of Bereirah [retroactive clarification].
Introduction to next subject: We’re discussing a case of Ohel Hameis, there is a room with multiple doorways and windows all of which are closed. Since we know that the meis will be carried out of one of them, we consider them to be part of the ohel and any keilim in the doorway, or on the windowsill is tamei [even though it is on the other side of the door/window], because we consider the door/window to be open. If one door is open, it is considered the exit path for the meis and all other doors and windows are considered closed and keilim there remain tahor.