Sefer HaChinuch 470 ספר החינוך ת״ע

To check the signs of a bird: To check the signs of a bird [that distinguish them as being permissible to eat]. And about this is it stated (Deuteronomy 14:11), “You may eat from any pure bird.” And so did they say in Sifrei Devarim 103 (see Maggid Mishneh on Mishneh Torah, Laws of Forbidden Foods 2:4), “‘You may eat from any pure bird’ — that is a positive commandment.” I have written all of the content of this commandment, its root, its laws and in which place it is practiced and at what time in the Order of Vayehi Bayom Hashmini (Sefer HaChinukh 153) concerning the examination of the signs of a [domesticated] beast, a [wild] animal, fish, and locusts — as the law of all of them is the same. And over there, I also wrote that Ramban, may his memory be blessed, differs with Rambam, may his memory be blessed, in [the latter’s] counting of the checking with beasts and other species to be a positive commandment. And he holds that the [verse] only comes to give a positive commandment and a negative commandment to the one that eats from the impure [species]. And there in that same Order, I wrote (Sefer HaChinukh 157) a little of what I heard from my teachers, God protect them, about the signs of birds — and take it from there.