HaSh-m told Noach to take seven pairs of each species of bird. How do we know that that pertains specifically to the Tahor species?
Rashi: Because we learn what is not specified (the birds) from what is (the animals). 1
This is a principle through which the Torah is explained.
QUESTIONS ON RASHI
Rashi writes: "'From the birds, seven...' - this refers to the Kosher species" (although this is not specified in the Pasuk). Regarding the non-Kosher species (Bereishis 6:20), the Torah refers first to the birds and then to the animals. The author of the Terumas ha'Deshen asks, this Pasuk also could have been written, "From the Kosher birds, seven, as well as from the animals..." [and we would then derive the animals from the birds mentioned in this same verse]?
Terumas ha'Deshen: The majority of birds are Kosher, whereas the majority of animals are not. Had the verse stated ... as well as from the animals," without specifying, we would have assumed that it referred to the non-Kosher majority.
Gur Aryeh: When referring to the Kosher species that would be used as offerings, the verse mentions animals first, because they are used more extensively for offerings than are birds. In the context of preserving the species, however, the birds are mentioned first, for they were created before the animals. 1
Gur Aryeh: There are other verses regarding the Mabul in which animals are mentioned first; this is because their creation was closer to that of man, and to the fulfillment of creation as a whole.
