Hmm ok, I typically throw the oyster shell crumbles out in the run don't keep it in the coop. I will try to figure out what works best. I feed a non-GMO layer pellet mixed with a small amount of scratch. I have 2 really picky hens tried to switch food once before and they stopped laying and got a bit thin.
Too much calcium will cause kidney failure in chicks. It might be best in your situation to either isolate the 2 picky hens and continue them on the regular diet or have the broody and chicks isolated with only chick starter feeds.
Don't take my word for it, please do research it to your own satisfaction so you can make the best choice for your flock, but here are a couple figures-- and please, please do read through the attached thread -- has good info and is a good starting point.
(Edited to add: Just like any other subject, it has as many opinions and varied experiences as there are people, as you will see!! That's why we bring up areas of concern so at least you're aware of a potential issue.)
Chick starter (purina) has min Calcium of .75%, max of 1.25%
Layer 'oyster strong' (purina) has min Calcium of 3.25%, max 4.25%
So, a chick eating layer is getting more than 2.6-5.7 TIMES the level of calcium it should which is a serious issue (edited to add) over a period of time until the calcium builds up.
Also edited to add: Figure arrived at by dividing the min Calcium of layer (3.25) by the max Calcium of starter (1.25) for 2.6, and 5.7 was dividing max Calcium of layer (4.25) by the min Calcium of starter (.75)
Here's a thread to start you off-- (edited to add) though it's discussing having fed game bird/turkey starter to chicks with max calcium of 2%, not layer with max calcium of 4.25%.
https://www.backyardchickens.com/t/1024086/i-fed-my-chick-too-much-calcium-now-what
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