Well, I would think that first of all, we'd need to know more about your conditions. The space sounds wonderful, but where are you located?
Some birds do better in the northern climates than others. Are you looking for cold hardy breeds? Or breeds that can handle the heat and humidity of the southern states?
You mention that the 30 acres is fenced, but not what kind of fencing. 4 strand barbed wire or electric will keep the horses in, but do nothing for keeping predators out or your birds in. With that kind of fence, either can go right under.
Hawks of course are always a concern. Free ranging in a wide open 30 acres with no source of cover is a sure recipe for hawk predation on any breed or type of bird you could keep.
Foxes, opossums, raccoons, and, if you have those, you likely have coyote's as well, are consummate predators of both birds and eggs. Those predators are, for the most part, nocturnal hunters. Unless you round up and lock your birds in for the night, you can probably expect a lot of predation on your flock from that quarter as well.
I'd love to be able to recommend birds that would fit your needs, but, I think we all need a bit more information before we can even try.