Killing cones are the best way..As I see it,,A birds brain /head is very small..By placing the bird upside down..all the blood rushes to that part of the body..We all know that too much blood will make you black out or loose consciousness. So I look @ it as the bird is already there when i swing the blade..I also have seen that if the spinal cord is not severed then the birds reaction are more calm and subdued....