Crushed oyster shells need more crushing

[nuthatched]

How about just buying Avian (for small pet birds) grit? Avian grit it oyster shell.
https://www.tractorsupply.com/tsc/product/browns-avian-grit-plus-for-canaries-finches-20-oz

 

[CabritaChicks]

I bought a massive mortar and pestle, that I display (decorative). I can then fully crush the oyster shell- mixed with hard boiled egg shells (microwaved). If needed- but our beach is filled with all sorts of shells.​

Both are primarily calcium carbonate (CaCO₃), but eggshells contain a finer crystalline structure and trace organic material (like collagen-like proteins).

Particle size (coarser = slower release = better long-term absorption), finer is a faster absorption- in which I use the eggshells.

	Oyster Shell	Ground Eggshell
Calcium content	~38–40%	~33–35%
Absorption efficiency	50–70%	40–60%
Gizzard retention time	Longer (coarse)	Shorter (fine)
Ideal use	Free-choice	Mixed in wet feed
Cost/sustainability	Store-bought	Free, recycled