If somebody slaughtered 100 animals in one place, they only need one act of covering the blood to take care of all of them. The same thing applies to birds. If there are 100 birds slaughtered in one location, you only need one act of covering the blood for all of them. A person shouldn’t think that he has to do a single act for a single animal; it’s about the blood being covered. So one full action of covering will take care of the obligation.
The same is true if there’s a chaya (an animal) and a bird in one place: you could do a single act of covering the blood for all of them. You may think that when it comes to a chaya or an of (bird)—birds and animals—they’re two separate mitzvot (commandments), and you need two different coverings. And that’s what the Mishnah here is teaching us: no, it’s all part of one mitzvah, and you only need one act when slaughtering a chaya and a bird together. One action is sufficient.
Rabbi Yehuda says that when you slaughter a chaya, an animal, you should cover the blood, and then you should slaughter the bird and cover its blood. According to Rabbi Yehuda, there’s a separation in the pasuk (verse), where it says “chaya or of”—a chaya or an of. According to Rabbi Yehuda, we learn from here that when there’s a chaya and a bird that they want to slaughter together, they still need two separate acts of kisui hadam (covering the blood), one for the chaya and one for the bird. According to Rabbi Yehuda, the slaughterer should cover the blood of the chaya before he slaughters the bird. He views the two coverings that he needs—one for the animal and one for the bird afterwards—as if they were two separate mitzvot: the mitzvah of slaughtering the chaya, then covering the blood, and then slaughtering the bird and covering its blood.
If somebody slaughtered a chaya or a bird and was violating the halakha (Jewish law) by not covering the blood, and another person saw that the blood wasn’t covered, the second person has to cover the blood. The pasuk tells us, “He should pour out its blood and cover it with earth.” We see from here that the obligation of kisui hadam falls on the person who poured out the blood, which is the shochet, the person who slaughtered. But if the shochet doesn’t fulfill his obligation, anyone who sees it has to cover the blood, because the next pasuk says that Hashem says to the children of Israel—which puts this obligation on everybody if the slaughterer doesn’t take care of it on his own.
If somebody covered the blood and then it became uncovered, you don’t have to cover it again. The mitzvah of kisui hadam only has to be done one time. If the wind blew dirt over the blood, and then the earth covered it, and then it became uncovered, he’s obligated to do the kisui hadam again, because in this situation, the kisui hadam was never really done; it happened on its own. So you didn’t even do the mitzvah one time on your own. Therefore, you’d have to cover it again in this situation.