Hmmm...I honestly don't know. They get sunlight from 7 am to 7 pm daily through the window that their cage is right in front of. Does vitamin D get absorbed through window exposure, or do you need to be in the actual sunlight outside? Any idea if chickens can metabolize vitamin D supplements?
I don't think they can absorb enough through glass. Especially modern glass. Egg yolk has vitamin D. You can feed them hard boiled yolks. Or you can try the reptile calcium supplement that had D in it. You can also get a full spectrum light.