Pheasants pace because they want to move to where they see, and when they are behind wire, they naturally pace back and forth trying to get to the other side. A simple method that has worked for me is sight barriers. I have a 24-30 inch wall from the ground up the entire perimeter of the pen that prevents the pheasants from looking out. The less they see beyond the confines of the pen from ground level, the less they are prone to pacing. It also helps to put the barrier between separate pens so the roosters don't fight through the wire. I made my barriers from vinyl house siding. They can be cut to length and snap together to create as high a barrier as you want. They can be nailed to the pen supports or zip tied to the wire and give the pen a neat finished look. It will not cure all pheasants, but I have some of the most relaxed ringnecks you ever saw. And like others have said, plenty of natural cover helps too.