The Gemarah Chullin (140a) has a drasha to exclude the birds of an עיר הנידחת to be used for the shechita bird of a metzorah. Rashi asks but the bird has to be burned as part of the עיר הנידחת so how could it be slaughtered for use of the metzorah? Rashi answers it could be burned after it is slaughtered. The Rashash points out that Rashi clearly assumes that the animals must be burned even though the possuk says to kill the animals לפי חרב? He answers that the possuk is referring to animals but birds Rashi would hold is in the same category as the שלל of the city. He cites the Rambam (Avodah Zarah 4:6) disagrees and holds all living creatures get killed.
The Achronim ask the Gemarah Snhedrin (112a) has a safek if the act of shechitah works at all to remove the tumah of נבילה, so on the צד that it does not how could the Gemarah assume without a possuk one could use the bird for the metzorah if the shechitah will not be valid? According to this Rashash, it is not a question for the Gemarah Sanhedrin is referring to an animal which has a law of killing, then the Gemarah has a doubt if the act of killing as shechitah is just viewed as fulfilling the law of עיר הנידחת, not an act of shechitah at all or it is still defined as an act of shechitah. But regarding birds where there is only a law of burning, the act of shechitah itself will not be defined as fulfilling the law of עיר הנידחת and hence the shechitah will be an act of shechitah.
The other way the Rashash says is there is no machlokes Rashi and Rambam but that both are done to the livestock, they are both killed and burned.
In Sanhedrin the Rashash wonders why the Gemarah has a question about the tumah of נבילה if other cases when an animal is put to death such as שור הנסקל the tumah does go off (he cites proofs to this)? It would seem that the Gemarah understands that when there is a law of לפי חרב then the act of shechitah is not defined as an act of shechitah but rather as an act of merely killing the animal and will not remove the tumah but in other places where an animal has a דין to be killed, the shechitah act will still be valid to remove the tumah of נבילה.
