So if we make oyster shell available for our hens to free-feed, the chances of them overdosing themselves on it is pretty much zero, is that right? It's completely safe to leave it there all the time, right?Thank you @Sueby for contributing some very valuable and interesting information to this thread. Few people are aware of the extent that calcium is crucial to a hen's health and body functions, not just egg laying.
My biggest concern for my hen is that if she isn't absorbing enough calcium to make a decent shell, the risk of sudden death syndrome increases. This happens when a hen has low calcium levels and her body pulls all the available calcium from her blood stream to make a shell and this robs her heart of calcium needed to keep it beating, causing a heart attack and death.
This is why I keep the bottle of citrate in my run so it's handy to give a hen exhibiting signs of having trouble passing an egg. My thinking is that giving her an easily absorbable cacium tablet on her way in to lay a difficult egg could help her avoid sudden death from her body draining all her available calcium. Egg laying can be risky business for a hen under some circumstances.