Grit and oyster shell serve two completely different purposes. Grit is to help them digest their food and oyster shell is to replace the calcium that is lost when they make an egg shell.I do not have tums. The crumbles I mix in the with groovy mix has oyster shells in it. But not realizing the importance of it when I bought grit a month or so ago, it was just the little rocks, not the crushed shells. So although they are getting some, maybe not enough? I was going to use up the rocks, then buy the she'll grit. Perhaps I should go ahead and switch?