Flatter, wider roosts are better for chickens' feet than perfectly round poles or dowels. You can use natural tree branches, but make sure there are no sharp spots that could injure feet, and pick branches that are a little wider than your chickens' foot. I sand down any rough spots on the branches I use, and also spray the branch thoroughly with Adams Flea and Tick spray, then leave it out in the sun to dry and air out.
I also use pine or aspen boards, 2" wide for my tiny bantams, with the edges sanded down.
We have a lot of bamboo growing on our property, but I've read that bamboo is too slippery for use as a perch even for perching birds (parrots, etc.), so I don't use it for that purpose.