What ever you heard is is inaccurate. I offer oyster shells free choice and some times I will throw some into feed. Most of the oyster shell remain on the bottom after the feed is consumed. Chickens do pick out what they want to eat. I do the addition to their feed to encourage them to get extra calcium. I feed Alflock (2% calcium) because I have layers and non layers. Purina Layena pellets I believe is a layer feed that has about 4% calcium. You do not need (theoretically) to give any additional calcium. By providing Ouster shells free choice, you allow chickens their own decision if they want/need additional calcium.
They will not OD on free choice oyster shells. There will not be any bad affect if there is some also among their feed pellets. They will just ignore and leave behind. I am talking from long time observation and experience.
Processed calcium that is in feed, is a different story. Giving layer feed to non layer is not a good IDEA. They will NOT DIE INSTANTLY. Longer term it may harm chicken's liver and kidneys if they are non layers. Same goes for roosters. That is why My Choice is Alflock at 2% calcium. My layers supplement their calcium need themselves with green vegetation free ranging, and oyster shells. A bag of oysters shells lasts me a loooong time since I have a small flock.
Also did want to add that my chickens live long lives with the way I treat/feed them. Oldest was 13. Last on to go to other side went 11. I only keep chickens as pets.
WISHING YOU BEST,,,,,,,, and
