I know this is an old thread, but, my 8 ten week old sexlinks are roosting (in their tractor), and my 6 twelve week old black australorps are not roosting (in a tractor next to them). Instead they lay on the ground. I have seen one or two of the BAs on the roost bar, so, I know they can get up there. And there is easy access to the next boxes, which are not on the ground, but lower than the roost bar. I don't know why they sleep on the ground.