A new mishnah: When the bird is killed improperly in the context of sacrifices, it does not necessarily constitute a carcass, in terms of conveying impurity. Specifically, melikah vs. slaughtering. Note the question of maturity - including in terms of appearance - of the birds. The Gemara dives into how the phenomenon of melika makes the difference as far as the carcass of the bird is considered. Including an attempted parallel to the red heifer.