Netting the Pens
For the first time in the four years we have 
been here 
hawks have become a threat big enough 
to our birds that we decided 
it was time to protect them.
We started with different sized pens already enclosed with metal wire Panels.  Hog panels work best.
We used 3/4" PVC Electrical Conduit for the frame.  We bent the Conduit into an arch shape by securing the ends of the Conduit to the panels with Plastic Cable Ties.  Each "Rafter" was spaced aproximately 4' on center.
We connected the Rafters with a Conduit "Ridge" fastened with self drilling screws..
Bird Netting (25' wide) was stretched over the frame and fastened to the 
panels with Plastic Cable Ties.  We started working on a 12' wide 
pen.  We used 2 1/2 lengths of Conduit per Rafter (25') and glued the joint.  
It bent easily, but felt strong enough to support some weight.
It's worked well and should support some snow load--as well as the odd turkey looking for a place to perch.  We tried wider pens (13'-15') but didn't like the results.  We changed those to approximately 12' wide.
We purchased the Bird Netting from

It's been five years since we installed the PVC netting frames.  A couple of harsh New England winters did some damage, snapping  or bending them  from heavy wet snow load, so we upgraded to
 Clear Span Round Cold frames from FarmTek

This is a more expensive product but it holds up to any New England winter.
3/4" conduit can be purchased at Home Depot