The 150th mitzvah is that we are also commanded regarding the signs of birds; that only certain species are permissible. These signs are not listed in Scripture, but have been determined by investigation; that after examining each and every of the prohibited species, we have found characteristics common to all of them. These are the signs of a non-kosher bird.1 That we have been commanded to examine birds and declare one type non-kosher and another kosher is the positive commandment.

The Sifri says, "The statement,2 'You may eat every kosher bird,' is a positive commandment.' " This states clearly what we hinted to above.

The details of this mitzvah are explained in tractate Chullin.