My normal thing is to drive around a little bit after midnite, and spot light a rabbit or 2, find a good hide, then go out 50 or more yards and split the rabbits and sling the blood around, try to get it 3 or 4 feet up a tree or bush, to put out the scent and then lay the rabbits out in plain sight. Go back to the hide and run the dying rabbit call for a couple of minutes. Go silent for 2 or 3 minutes, and spot them with the red lens covered spotlight. Depending on the terrain, after the first call or 2, you can wait for 5 minute intervals before shining again. I don't have a howler call, but sure want to try one.
Rule #1, never call from a tree, predator calls will work for more than coyotes, An owl coming at you in a tree can kill you!! I have called in owls, bobcats, coyotes, possums, raccoons, and funny enough, rabbits with a dying rabbit call. In the daytime I've had hawks come to a squirrel call, and man were they pissed. No I don't shoot hawks or owls.
Good luck with your sheep protecting.