I live in Alaska and I'm just starting out with chickens. My birds were all doing fine until recently when the weather turned sour. They've been holing themselves up in the coop. I started getting a lot of soft eggs (1-2 a day from 11 birds.). I moved their free choice oyster shell into the food in case they weren't consuming enough. They are eating a lot but I'm still getting soft shelled and shells so thin they crack upon laying. I do have a supplemental light on in the coop. Vitamin D deficiency in humans is common here and I know it can affect Calcium absorption. Can I supplement with Vit D and if so how much? I currently have 13 laying hens.