I have a dedicated brooder coop, which has a flat 2"x4" "high roost". I also built a "little girls" roost out of 1"x 2" and wood glue. When I start a new brood, I put in the "little girls" roost, and they inevitably start using it within a few days. One starts, and they all follow along. After a few weeks one will start using the "high roost", and they all follow her. Or him, which is often the case on the "high roost". If one turns out to be a roo, or more than one, they seem to tend to be the first ones to use it. By Week 6, they tend to all sleep on the "high roost". At that point I take out the "little girls" roost.