Commentary for Tahorot 1:3

נבלת עוף טמא – that its end is not to defile a stringent defilement/uncleanness which does not defile humans and garments.

The carrion of an unclean bird requires intention; and it must be rendered susceptible;
It conveys food uncleanness if its minimum bulk is that of an egg;
The consumption of a half of half a loaf's bulk of it renders one's person unfit to eat terumah;
There is no rule that an olive's worth defiles in the gullet;
He who eats of it need not wait for sunset;
No guilt is incurred on account of it for entering the sanctuary;
But on account of it terumah must be burnt.
He who eats a limb of it while it is alive is not subject to the penalty of forty stripes;
Slaughtering it does not render it fit.
The large feathers and the down contract uncleanness and convey uncleanness and combine with the flesh to constitute the prescribed minimum.
The beak and the claws contract uncleanness and convey uncleanness and combine [with the flesh to make up the prescribed minimum].
The mishnah now proceeds to talk about the purity rules governing the carrion of an unclean bird, one that is not "kosher." As we will see, some of these rules are the same as those we saw with regard to the carrion of a clean bird, but some are different.
Section one: The carrion of an unclean bird (a bird that one is not allowed to eat) is not unclean in a ritual purity sense, in and of itself. In order for it to become unclean, first one would have to have the intention to eat it and then it would have to be rendered susceptible to impurity through contact with a liquid. If these two things happen, and then it comes into contact with a source of impurity, it is impure. This differs from the carrion of a clean (edible) bird, which is a source of defilement in and of itself.
Section two: If it becomes impure, it now conveys impurity if there is the minimum size of an egg.
Section three: If a person eats an amount of carrion of unclean bird that is equivalent to the size of two eggs (half of the size of half of a loaf of bread) he cannot eat terumah until he immerses in a mikveh. This is the usual amount to cause a person to become impure through eating something impure.
Section four: Unlike the carrion of a clean bird, there is no special rule concerning one who swallows the amount of an olive. Such a person remains pure.
Section five: One who eats an amount the size of half of a half of a loaf of bread is impure, but he is pure immediately after he goes to the mikveh. He need not wait for the sun to set.
Section six: The defilement caused by eating this amount is only rabbinic. Therefore, if someone enters the sanctuary after having done so, he is not liable for transgressing a biblical commandment.
Section seven: Nevertheless, if this person touches terumah, the terumah must be burned. This is because terumah can be burned even if the level of the impurity is only rabbinic.
Section eight: The biblical prohibition of eating the limb of a living animal refers only to clean animals (and birds). Therefore, one who eats the limb of a living unclean bird is not liable for transgressing the biblical commandment. [Please understand this doesn't mean that one can do so, just that one who has done so is not liable for transgressing that particular negative commandment. He has transgressed another.]
Section nine: Obviously, slaughtering it does not render the unclean bird fit for consumption. This type of bird is unclean and forbidden for eating in all cases. The truth is that this clause is overly obvious. It is only here because it contrasts with the case of the clean bird.
Sections ten and eleven: When it comes to the unclean bird, since it is never permitted to be eaten, the edible and inedible parts combine to convey uncleanness.

צריכה מחשבה – that he will intend to eat/consume it.

והכשר (and preparation)– of water or the rest of the liquids [for susceptibility to defilement], and afterwards it will come in contact with a creeping animal.

לפסול את הגויה (to render the body unfit) – if it had been defiled, and he ate from half a loaf’s bulk, which is an egg and a half [of an egg], his (i.e., that of a Kohen) body is made unfit to eat heave-offering until he immerses [in a Mikveh].

ואין בה כזית בבית הבליעה – as it is written (Leviticus 22:8): “He shall not eat anything that died or was torn by beasts.” He whose prohibition is because of the food of a carrion defiles through eating in the esophagus, excluding this whose prohibition is not because of eating carrion but rather because of eating anything that is impure/

והאוכלה אין טעון הערב שמש – the person who consumes half of a loaf of it after it was defiled, for he invalidates the body to eat heave-offering, if he immersed [in a Mikveh] after he consumed it, he is permitted [to consume] heave-offering immediately, and does not require [the arrival of] a sunset.

ואין חייבין עליה על ביאת מקדש – if he ate one-half a loaf of carrion of an impure bird and entered into the Sanctuary, it is defilement of the Rabbis, and even though that burn heave-offering upon it.

והאוכל אבר מן החי ממנה סופג את הארבעים – and even though that eat an olive’s bulk whether it is flesh or sinews and bones, for now even according to the one who holds that [eating] the limb of a living animal requires an olive’s bulk for eating is written regarding it, here there is an olive’s bulk, nevertheless, he does not receive forty (really, thirty-nine) lashes. But because of something impure, he is not obligated, for there isn’t an olive’s bulk of flesh.

ואין שחיטתה מטהרתה (for slaughtering it does not render it clean) – for eating. For a son of Noah who is permitted [to consume] an impure bird but is prohibited from [consuming] the limb of a living animal, but if he slaughtered it and it is moves convulsively/kicks, his slaughtering of it does not purify it for eating until it dies. Even though that with a clean [animal] even it is convulses, the slaughter is permitted for a son of Noah, in consequence of that it would be fit for an Israelite (i.e., the legal status required for its legitimacy might have easily been obtained). And this is how our Mishnah is explained at the end of the chapter, “The Sciatic Nerve”/גיד הנשה (see Talmud Tractate Hullin 102a).