My thoughts, I started my quail in a tractor and they loved it, I trained them to use steps to get in there coop to show them where the food was and water. Worked fine.
PROBLEM...
After about 3 weeks they started acting a little skittish. So I let it go for a little longer and came home from work at lunch one day to find my friend the Sharp-shinn sitting on the side of the cage with one of the roos in his talons... The roo survived and I remodeled the tractor and put some Half inch mesh over the 1" chicken wire. all was good for a while. Then 2 weeks ago someone decided to show up again. This time they couldn't get through the mesh but some how managed to get 1 foot under the 2x4 on the ground because of a tiny gap caused by my lack of care for how pretty my lawn looks... That hen didn't survive, though the hawk never got a meal out of it. He managed to scare the hen to death and she just died with no wounds. I then noticed another hen with a bloody head (caused by their spring ability) she hit her head on the mesh above and yeah... She survived and is growing feathers back now.
All in all I don't suggest it. Yes it's nice that they can be on the ground but Unless you can't fully keep predators out or even away from the Pen its just not worth it... I'm building a 25'x25' garden next year and I am completely enclosing it with nets and wire around it and I will have the quail in there (hoping to have at least 25-30 running free). I think quail are just to wild to keep confined out in the open available to predators...
I now have a 5'x2'x1' cage with double mesh floor along with a 1" netting on the roof so they can't hurt themselves anymore... I'll get some pictures tonight and post them. I think it should be pretty predator proof, only concern now is if the stupid buck tried to rub on it in the spring. He might have to meet my 12ga...