I completely disagree about the camo necessity. Camo is for hunters, not game. Birds are the only game animals I'm aware of that can even see in color. Wear neutral colors (or camo if you want) but stay still.
As far as calls, a variety of things work. I'm not sure where you are located (and it probably doesn't really matter too much) but it is hard to beat wounded rabbits, cottontail or jack, or other squealing rodents.
It also is good to mix it up and throw something different at them from time to time. This is where having a good quality electronic caller comes in handy. You can have dozens of potential calls to use at any given time.
Howlers have never given me much other than locations. So that's what I use them for. Barks haven't provide me much success either. I'm not sure if it intimidates the coyotes or something like that.