“ The Secretary of State is hereby designated and empowered to perform, without the approval, ratification, or other action of the President, the function vested in the President by Article IV of the Convention between the United States of America and Mexico for the Protection of Migratory Birds and Game Mammals to determine, by common agreement with the President of Mexico, other species of birds which may be added to the list of migratory birds specified in and subject to the protection of the Convention. ”