I don't know how to post a link to another thread, but "Vitamin D supplement?" from April has one really wonderful post about vitamins and minerals. So far what I'm reading, hens can synthesize vit D from ultraviolet light "through" their feathers. Don't know what exactly "through" implies though. Several other threads I came up with when searching the subject also wondered about the connection between low light levels and shell-less eggs. So, it seems to be one factor.
the fact that I thought about consulting books before the internet, or even BYC, shows my age.
ha ha me too !!!!!!!!!!!!!!

My books are saying feed & especially greens high in D levels.
I also have lights in my coops, as I said before.
The coops have 1 window each but are under a huge fir tree and 1 coop gets little to no light from the sun.
So...I have clamp on type lights with equivialnt 60 watt bulbs in each coop on at 5AM and off at 5 PM, on a timer.
In all my coops.
As spring progresses I have to up the timer to match the days length.
In fall I reduce but never below 12 hrs of light a day.