Chicks sleep in nesting box

Ok - I have 6 hens who are about 10 weeks old. This is my first time raising chicks so I have no idea what to expect. When I moved them from their brooder box to their outdoor coop about 5 1/2 weeks ago, they right away began sleeping in the inside portion of their coop, but within the first week or two they all climb into the two nesting boxes to sleep. I did not even put any bedding in there at first because I didn't want to encourage them to be there. They have roosting posts in the enclosed coop area but they don't use them. The coop was a kit and the roosting poles are squared at the top and seem a bit unstable (they wobble back and forth a little bit) so I am thinking about changing them out so they are more rounded and stable. Any ideas on why the chicks are sleeping in the nesting boxes and how to get them not to sleep in there before the eggs start coming and they squish them all?