I let the sun in when I can. I would like to get them on wheels so I could roll them out. I might have to find a vitamin D supplement for them. I have thought about the need for sun. Which is why I set Cuddles in the sunshine today.
We have the same issue with breeding caged birds. My bird room is in my basement. I eventually went to fluorescent light fixtures in the ceiling with special UV producing tubes in them. But I also supplement with vitamin D and calcium in their water. What I use is called Cal-D-Solve. I've never checked to see if it would be economical to use for poultry. There may be something better on the market that would come in larger quantities.