We have had a mature hen taken by a buzzard, and eaten in front of our eyes! More of a problem with young birds seems to be Corvids here, but they don't take larger birds. I think that chrismahon's suggestion of stringing ropes across the area is good, although I would use the blue polyprop rope you can get from ships chandlers, and probably lots of other places too, at it shows up well and is cheap. Just make sure the ropes are a bit saggy so no birds can perch on them.