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If you "throw" oyster shell to them and they really gobble it up, they could be calcium deficient. Of course they could also just think its a treat, lol. Some chickens will gobble up whatever hits the ground! Oyster shell should always be out for layers, they know how to control their own levels.
One common misconception about calcium is that you can judge a hens need for it by eggshell. This is not true, though thin eggshells MAY mean the bird is calcium deficient, by the time it shows in the egg shell she is very low. Egg shell formation is not the only thing birds need calcium for!
Another one is that layer mix is bad for Roosters do tue high calcium content, while it may not be the optimal dietary mix to actually impact the roosters health the calcium levels would have to be much higher over long periods of time. Studies showing the negative effects have been a bit taken out of proportion. There have also been studies showing that Roosters fed high calcium diets are more fertile....
One thing to remember about "studies", often the reports are based on "Optimal" nutrition and they fail to differentiate between "optimal", "adequate" and "harmful". Sometimes studies "conclude" with something to the effect of : "This research would appear to suggest that calcium levels above x.xx% could cause damage to the internal organs of male fowl"...or some other vague supposition. By the time it makes it to the general populous this has changed into "Layer feed kills roosters" .
As for when to start Layer feed...I start when I run out of chick starter, LOL