Since the Midrash lumps together kan tzipor, milah, and the issur of offering as a korban an animal less than 8 days old all as expressions of G-d’s mercy, why does the Mishna single out the mention kan tzipor as objectionable – why not give all three examples?
Sidduro Shel Shabbos answers: according to the opinion that G-d’s commandments are gezeiros and not based on mercy, there clearly is a problem with mentioning milah/korban as merciful because it might lead to the incorrect conclusion that milah before day 8 is valid (see yesterday’s post!). The Mishna highlights the even bigger chiddush, that even mentioning mercy in reference to kan tzipor, where there is no danger of drawing an incorrect halachic conclusion, is problematic.
According to the opinion that the problem with kan tzipor is that it singles out birds as beneficiaries of G-d’s mercy to the exclusion of other creations, the reason the Mishna does not mention all three cases the Midrash gives - birds, animals, man – is because incorporating all three in one prayer would be permitted, as no single creation is singled out.
One other note: the S.S.S. questions whether can one infer from the fact that the Midrash/Mishna draws on a case of kan tzipor to illustrate G-d’s mercy on birds and does not use the din of not bringing a korban before day 8 that a bird (torim, bnei yonah) would be acceptable as a korban from birth without waiting 8 days.