Yeah, we should all be offering oyster shell since free range birds might not get as much as "mimimum requirements" per the bag. Just have to make sure they are getting grit from some source wither it be us or the ground.
If your birds are on green grass or sifted soil in the city, you'll need to get grit since they can't find it on their own. I've never bought grit for my birds since they are on the ground early in life and there are literally 4 commercial truck loads of gravel on top of 4 previous loads of...