⠀ can chickens get any illness from not enough sunlight?

[healingbirds]

My chicks are still to young,to be outdoors,cause its too cold,the barn is kind of dark ,can i feed them some supplement to make sure they get the vit D from the sun,that they are missing. The are porcelian bantams and look really pale,i lost one for no apparent reason ,there was no sign of obvious reason for death. It made me start wondering? If they are getting enough sunlight.?

 

[Ol Grey Mare]

Hi and welcome. So sorry about your loss.
As long as you are providing them with an appropriate feed (ie chick feed), their nutritional needs are being met - many flocks live their entire lives without ever seeing sunlight (not just battery birds but the "cage free" layers as well - since all "cage free" means is they are still inside giant buildings, just able to roam about inside vs. being in those tiny battery cages). Commercial feeds are designed to be "complete" and provide sufficient/minimum required amounts of all nutrients to support the bird. You say the barn is dark (understandable, have spent much of my life in barns, lol), is there any lighting - windows, doors that are able to be left open or supplemental lighting from bulbs?
However, having concern about their general look and having lost a bird, it may be time to consider all other factors that may be playing into their condition. How old are the birds? How long have you had them? Is the bard dry and draft free? If the birds are not yet fully feathered, are you providing supplemental heat? When you say they "look pale" - are you referring to comb/wattle coloration or?