There is a discussion on the James Oliver site asking if a free-range bird can have hock burns. There are statements that a free-range chicken could develop hock burn due to exposure to mud.
A chicken tractor (sometimes called an ark) is a movable chicken coop lacking a floor. By moving it to fresh pasture daily the chickens would not be exposed to bare earth plus all the chicken tractors that I've seen include shelter from rain, wind and other weather. Is this likely to reduce the chance of hock burn?