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The pvc pen worked great but I am going to do some modifications before I get another batch. I am going to add wheels to the back of the tractor. Maybe you can give me more details on how you did your wheels. Trying to catch the birds was difficult with only one door. I am going to add at least one more door on the top to make catching the birds easier.
I think I can help you with both those problems!  As for the wheels, I got two of sliding joints (the one that rides in the middle of the tarp "rafter."  I put a section of PVC through that, and drilled holes in both, then mounted a wheel at the bottom, so I can raise and lower it.  I think I posted a pic awhile back- let me know if I didn't or if you can't find it.  
As for catching the birds, I built a inner false wall, complete with chicken wire, attached by a zip tie to the back side.  I mounted it to PVC pieces that were just larger than the frame, so it slides, and tied a rope to it.  When I butcher, I'll just cut the zip tie and pull the rope, and the wall will move, cutting the size of the pen in 1/2.  Let me know if you want pics of that too!  Hope it helps.