My chickens continue to sleep on top of their nesting box, and I want them to stop. I know they like to sleep up high but I made a roost ladder inside the coop, and still they prefer to sleep outside on top of the nesting box. I went through great effort to increase the slant of the box, and that worked for a while, but no longer. Is there anything slippery that I could apply that would help? Any other ideas?