As I was reminded ants and birds will eat your fly predators if your not careful. Fly predators are noturnal (come our at night) so should be turned loose at dusk. That can help protect them from ants and birds, too.
Now don't get me wrong, ants and birds are good fly control, too. Ants that find fly in the pupa stage will move them to their nest to be eaten by the young ants. Birds will dig through the horse manure and eat the fly pupa. So will different kinds of beatles.
but the flies can usually out breed and out populate all the insects and birds that want to eat them. We have to continue to protect the good insects so they can help destroy the bad.