That is that He commanded us to check for the signs of birds, such that only some of them are permissible. And the signs of the birds are not stated in the Torah. They rather come from an investigation of our finding the birds that are written to be forbidden, and our [then finding] phenomena that they have in common - and these are the signs of an impure bird. And since also regarding birds are we involved with the issue of which is impure and which is pure, that [involvement] is a positive commandment. And the language of the Sifrei is, "'You may eat any clean bird' (Deuteronomy 14:11) - that is a positive commandment." Behold what we have hinted was made clear. And the regulations of this commandment have already been explained in Chullin. (See Mishneh Torah, Forbidden Foods 2.)