The Calcium Citrate is for quick uptake and given to a hen that is in crisis for 3-5 days to see if it makes a difference.
Oyster Shell, egg shells - those are great for hens and should be offered free choice so the hens can pick up pieces as they want. Giving the extra calcium is often...
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Do you have photos of her and her poop?
If she's acting like she's having a hard time passing an egg, then I'd give her extra Calcium asap.
You can find Calcium Citrate with D3 in the vitamin aisle of stores like Walmart, CVS, etc. Give 1 tablet daily, just pull down on her...