One of my hens has been laying thin-shelled eggs for a week or so. I am finding a fragile or broken egg in the nesting box every day now. They get 17% layer feed and free choice crushed oyster shell, but they are on pasture now so not eating as much of the feed. I'd like to help her out by giving her calcium + D3 tablets, but I don't know who it is! Is it safe to give it to all of them? What else can I do?