If he prepared two birds and found three; they’re all forbidden.
Explanantion: What do you want? [It’s ossur any way you look at it]. If these are new birds, they were never designated. If two of them are the original birds, but there is another mixed in with them*!
If he prepared three birds and found two; they’re permitted.
Question: Why is this so?
Response: These are the same birds only that one flew away.
Suggestion: It would seem that our Mishna is taking the side of Rebbi against the Chachomim [this is a difficulty since the Mishnayos should always be the majority opinion unless explicitly stated otherwise], as discussed in this Beraisa:
If one prepared a hundred sus [of maaser sheini money] and then found two hundred: Rebbi: It is maaser and chullin mixed together. Chachomim: It is all chullin [people wouldn’t place chullin money with maaser money]. If one prepared two hundred and found one hundred: Rebbi: One hundred was removed and the second hundred remained in place [this is similar to our case in the Mishnah where we say one bird flew away and two remained in place]. Chachomim: It is all chullin.
Response: Birds are not the same as money. We learned about our Mishnah: “Rabbi Yochanan and Rabbi Elazar both say: Birds are different because they hop away” [In the case of money, since it is stationary, we assume that if part of it was removed, the entire sum would have been moved, but since birds can hop away, even the Chachomim would agree that one hopped away].
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* Tosfos: It does not become battel because: A. Living things never become battel b’rov. B. This is a case of Davar shyesh lo mattirin (009).