The beginning of the Torah describes the creation of the world. Hashem created the world in six days and completed it on the seventh day. The Torah out lines what was created on each day of creation.
On the first day Hashem created the heaven and the Earth. On the second day He created the Rakeiya. On the third day He created plant life and vegetation. On the fourth day He placed the luminaries in the sky. On the fifth day He created fish and water creatures, and birds. On the sixth day He created animals and people.
The Mishnah in Kailim (17:14) discusses different things that can become tamei. The Mishnah says that things created on the first day are able to be mekabel tumah. Things created on the second day cannot. Creations of the third day are susceptible to tumah. Things created on the fourth and fifth days are not able to contract tumah. Things created on the sixth day can become tamei.
There are two interpretations of this Mishnah. The Rav m’Bartenura and the Rash explain that the Mishnah is referring to vessels that are formed from things that were created on each of the days of creation. So vessels made from earth, which was created on the first day can become tamei. However, vessels cannot be made from the Rakeiya and thus utensils made from things created on the second day are not able to become tamei. On the third day of creation, Hashem created trees. Utensils created from wood are susceptible to tumah. On the fourth day Hashem placed the luminaries and utensils cannot be made from them. On the fifth day Hashem created fish and birds, and utensils made from them are not able to become tamei. On the sixth day Hashem created animals and people, and if one makes utensils from either their bones or skins they will be able to become tamei.
The Rambam however explains that the Mishnah is to be taken literally and things themselves that were created on different days will or will not be able to accept tumah. The Rambam asks a question on his own explanation. A kosher bird that dies without shechita is a nevaila. If one eats this nevaila it will make him tamei when he swallows it. This should contradict the Rambam’s explanation, for birds were created on the fifth day and the Mishnah said that utensils made from things created on the fifth day are not able susceptible to tumah. The Rambam answers that such tumah is a chiddush and thus is not included in the Mishnah’s list of tumah.
However there is an obvious question on the Rambam’s explanation. Birds, which were created on the fifth day, can accept tumah when they become food. This tumah is not a chiddush. How can the Rambam explain that the Mishnah is referring to the items themselves when birds can become food, thereby making them susceptible to accepting tumah?
The Kli Chemda quotes the Nemukei Hagrib who suggests the following solution. We know that Adam Harishon was only allowed to eat plantation fruits and vegetation. He was not allowed to eat animals. However, there is a dispute whether Adam Harishon was permitted to eat animals if they died on their own, and the only prohibition was to kill animals and eat them. Or if the prohibition was to eat any animal even if it died on its own.
The Rambam in Hilchos Melachim (beginning of the ninth perek) says that Adam Harishon was not permitted to eat meat at all, even if the animal died on its own. Therefore it is understandable that he says that anything created on the fifth day will not become tamei, for birds could not become tamei for Adam Harishon. It was not until Noach, when people were permitted to eat animals, that meat became a food, which can accept tumah.
However, Tosafos and the Ramban opine that Adam Harishon was only prohibited from killing an animal and eating it. Had an animal died on its own, Adam would have been permitted to eat it. Therefore, the Rash and the Rav m’Bartenura, who agree with them, suggest that a bird would be considered food and thus susceptible to tumah.
The chiddush of this pshat is that the Mishnah is using a time frame that is not applicable today. From the time of Noach birds have been considered food, and thus things created on the fifth day can become tamei.
The problem that I find with this pshat is that even if animals were not yet permitted to Adam Harishon, they should still be considered food, albeit forbidden food. The halacha is that even food that is forbidden is able to become tamei (see Rambam Hilchos Tumas Ochlin 1:25). Thus even if Adam Harishon could not eat birds, they should have nonetheless been able to be considered food that can become tamei.