Hens go through a pretty amazing process building those eggs, and Oyster shell helps out at a very specific point. The shells are built last, usually overnight - and the process can drain your chicken of Calcium. The calcium in Oyster shell is digested at a different rate than that found in their feed, so they have some extra during those night hours when they are creating shell. Help them out by providing Oyster shell At all times, and they will self-regulate, eating it when they need it. Flaked Oyster & Seashell Mix.