The science behind the grit

1. “Grit” and “calcium” are two different jobs

The word grit gets used for two completely different things, and a lot of confusion starts there.

Oyster shell is the second kind. A laying hen on a fibrous or free-range diet may need both: something hard to grind with, and calcium to build shells with. Agriculture Victoria's backyard poultry guidance puts the grinding job plainly:

Chickens do not have teeth so rely on grinding their food with their stomach. It is a good idea to supplement the birds' diet with grit such as crushed oyster shells to grind food. — Agriculture Victoria, Feed and nutrition for back yard poultry

2. A hen builds her shell overnight — while she isn't eating

Shell calcification happens mostly in the dark hours, when the bird is roosting and eating little or nothing. That creates an awkward gap: the shell gland needs a steady calcium supply at exactly the time the gut is emptiest.

This is where particle size does the work. Fine calcium dissolves and passes through quickly. Coarse particles are held in the gizzard and dissolve slowly, releasing calcium over a longer window. A 2025 Poultry Science study of 90-week-old hens puts it directly:

Coarse limestone shows a positive effect on eggshell quality due to its longer retention in the gizzard and subsequent increased availability of Ca for a greater time compared to fine limestone particles. — Star et al., Poultry Science, 2025

Oyster shell is naturally coarse and relatively slow to dissolve, which is why it has been fed free-choice to laying flocks for well over a century.

3. What the evidence does — and doesn't — show

We would rather be accurate than flattering, so here is the honest version.

Particle size matters more than which shell it came from. When researchers compared limestone, cockle shell, oyster shell and crushed eggshell in older hens, eggshell breaking strength came out much the same across the coarse sources. The clear difference was between coarse and fine particles — not between one shell and another.

Where coarse oyster shell did stand out was the hen's skeleton. In that same study, hens fed coarse oyster shell or coarse eggshell had significantly higher tibia bone mineral density than the other groups:

ESC and OS fed hens showed higher tibia bone mineral density (BMD) than the other groups (P < 0.001). — Lee et al., Scientific Reports, 2021  (ESC = coarse eggshell, OS = oyster shell)

That matters, because a hen short of dietary calcium will draw it out of her own bones to finish the egg. Over a long laying life, a slow-release source that keeps up with demand is protecting more than the egg.

So the fair claim for oyster shell isn't “it makes stronger eggs than limestone”. It's that oyster shell is a naturally coarse, slow-release calcium source well suited to free-choice feeding — and the research associates that kind of calcium with better bone mineralisation.

4. Why we grade ours the way we do

Because particle size is the active ingredient, we screen our shell into two grades rather than selling one mixed bag of everything:

• Coarse, approx. 1.6–5 mm — for full-size laying hens and larger poultry. Big enough to sit in the gizzard and release calcium slowly.
• Fine, under 1.6 mm — an easier size for bantams, ducks, quail and smaller laying breeds.

Match the grade to the bird. A bantam struggling with 5 mm shell will simply eat less of it, and a supplement the bird won't eat helps nobody.

5. How to feed it — and which birds shouldn't have it

Offer it free-choice in its own dish, never mixed through the feed. Mixing calcium into the main ration takes the choice away from the bird and forces the same dose on every animal that eats it. Laying hens that need extra calcium will help themselves from a separate dish.

6. What to look for in any shell grit

Whoever you buy from, these are worth checking:

• Is it graded, or just crushed? A bag that is half dust delivers fast-dissolving fines — not the slow release the research is about.
• What is actually in it? “Shell grit” can be a mix of whatever shell was going. Oyster shell is a single, known material.
• Is it clean? Shell collected off a beach brings sand and salt with it.
• Does the size suit your birds? One grade can't be right for both a Brahma and a quail.

References

1. Star, L. et al. (2025). Effects of dietary calcium source and quantity on the laying rate, eggshell quality, reproductive tract, liver fat level, and duodenum morphology in Dekalb white laying hens of 90 weeks of age. Poultry Science. pmc.ncbi.nlm.nih.gov/articles/PMC12221496
2. Lee, S.-H. et al. (2021). Superiority of coarse eggshell as a calcium source over limestone, cockle shell, oyster shell, and fine eggshell in old laying hens. Scientific Reports 11. nature.com/articles/s41598-021-92589-y
3. Roland, D.A. Sr. (1986). Eggshell Quality IV: Oystershell versus limestone and the importance of particle size or solubility of calcium source. World's Poultry Science Journal 42(2), 166–171. cambridge.org
4. Agriculture Victoria. Feed and nutrition for back yard poultry. agriculture.vic.gov.au
5. MSD/Merck Veterinary Manual. Management of Backyard Poultry. merckvetmanual.com