Oyster shell for chickens is a slow-release calcium source that laying hens use to build strong eggshells; in UK backyard flocks it’s the simplest, most reliable way to prevent thin shells, soft shells and calcium depletion, especially during peak lay.

Why calcium matters for chickens

Eggshells and bone health

An eggshell is almost pure calcium carbonate. Each time a hen lays, she must mobilise around 2.5 grams of calcium—more than many beginner keepers realise. If the diet doesn’t supply enough, her body will pull the mineral from bones. This can lead to brittle bones and conditions such as cage‑layer fatigue. Calcium also supports muscle contractions and nerve function, so deficiencies may show up as weakness or wobbly movement.

How feed provides calcium

Commercial layers’ pellets are formulated with about 4 % calcium. This is sufficient for most flocks when pellets make up the bulk of the diet. However, feeding growers’ pellets, corn or lots of kitchen scraps dilutes that balance. Hybrid layers and ex‑battery hens that lay prolifically may need extra calcium beyond feed. This is where soluble supplements such as oyster shell come in.

Wondering which chicken varieties lay the most eggs, check our our guide on the best chickens for beginners and how many eggs you can expect in our guide to the amount of eggs to expect.

What oyster shell does — and how it works

A soluble, slow‑release calcium source

Oyster shell is a form of soluble grit. Unlike flint or granite grit, it dissolves in the digestive tract. It’s usually made from crushed seashells such as oyster or cockle and is mostly calcium. The pieces dissolve slowly. They release calcium over several hours. This steady supply is ideal for a laying hen. Her body builds the night’s eggshell after dark. The large flakes also sit in the hen’s gizzard for longer than powdered limestone, providing a more consistent dose.

Why layers pellets aren’t always enough

Although complete layers feed contains calcium, mixing in greens, wheat or treats dilutes that ratio. Heavy egg‑layers and hens returning to lay after winter often consume more calcium than they receive. As a result, they may lay thin‑shelled or “soft” eggs. They might even stop laying altogether. Providing oyster shell ensures birds can replenish their own reserves. This method avoids forcing extra calcium on birds that don’t need it.

This year we had out first egg with no shell and immediately diagnosed calcium deficiency. The solution was making oyster shell freely available in the chicken run.

The key benefits of oyster shell for your flock

• Stronger eggshells – Soft or paper‑thin shells are usually the first sign of calcium deficiency. A free‑choice supply of oyster shell allows hens to build dense shells that are less likely to crack or break. In my own small flock, I noticed shells firmed up within a week of offering oyster shell.
• Healthier bones and muscles – When dietary calcium is plentiful, hens don’t have to leach it from their bones. This helps keep the keel bone and ribs strong and supports muscle contractions during laying.
• Regulated calcium balance during peak lay – Hens in peak lay (spring and early summer) often require more calcium. Soluble grit dissolves slowly so birds absorb calcium overnight when eggshell formation occurs. Older hens or hybrids that lay large eggs benefit from the steady supply.
• Reduced egg eating – Hens sometimes break and eat eggs to recoup calcium. Offering oyster shell satisfies that need and can reduce egg‑eating behaviour, especially in older or stressed hens.

When and how to feed oyster shell

Timing and age

Offer oyster shell from around 18–20 weeks of age or when you see the first egg. Pullets that haven’t started laying shouldn’t be fed supplemental calcium. Excess calcium can disrupt the balance of phosphorus and magnesium needed for growth.

Separate container and self‑regulation

Always provide oyster shell in its own container rather than mixing it into feed. This mirrors RSPCA advice on feeding pet chickens about providing a balanced diet with access to grit. Hens have an instinct for regulating their intake. If soluble grit is mixed into feed, birds often flick it out or waste it. A small hopper or bowl mounted at chest height keeps the shell clean and dry; top it up weekly. Encourage hens to help themselves – they will take only what they need.

Still wondering what to feed your chickens, here’s our guide on what to feed chickens at different stages of their lives

Integration with water and Vitamin D

Calcium absorption depends on vitamin D, which hens get from sunlight and fortified feed. Make sure birds have access to daylight and constant fresh water. Winter flocks benefit from a little cod liver oil or a vitamin supplement to aid absorption.

Affiliate disclosure: This section contains affiliate links. We may earn a commission if you click these links, at no extra cost to you. Our recommendations are based on UK‑available products that we use and trust.

Recommended Oyster Shell & Supplements (UK)

Oyster shell vs grit — what’s the difference?

Many new keepers confuse soluble oyster shell with insoluble grit. They serve different functions and both are important.

Insoluble grit sits in the gizzard and helps birds grind seeds, grains and fibrous food. It is essential for birds on wholegrain diets or free‑range flocks with access to grasses and weeds. Soluble grit dissolves to supply calcium and should always be available to laying hens.

How much oyster shell do chickens need?

There is no fixed ratio because hens regulate their own intake. As a rule of thumb, a hen uses about 2.5 g of calcium per egg. Layers’ pellets provide roughly 4 % calcium, so a bird eating 120 g of feed daily receives around 4.8 g of calcium—enough for an egg. However, many hens consume less pellet when foraging or receiving treats, so a supplemental source is wise. For a small flock, a bowl containing 25–30 g of oyster shell per hen per week is usually adequate. Check the dish and fill it up weekly. Older hens may require a little more, while cockerels and non‑laying pullets will ignore it.

If you notice eggs getting thinner or birds eating eggs, increase the amount available. Conversely, if the dish sits full, your birds probably don’t need extra.

Common mistakes to avoid

• Feeding calcium too early – Supplementing pullets under 18 weeks can upset the balance of minerals and slow growth. Wait until they begin laying.
• Relying on layers pellets alone – Feed contains calcium but treats and greens dilute the ratio. Provide extra oyster shell, especially for high‑production hybrids or older hens.
• Mixing oyster shell into feed – Hens tend to flick out the grit and waste both feed and shell. Offer it separately so birds self‑regulate.
• Using builders’ lime or sea grit – Only crushed oyster shell or limestone formulated for poultry dissolves correctly. Builders’ lime and sharp sand can cause impaction or toxicity.
• Ignoring water and vitamin D – Calcium absorption depends on vitamin D and hydration. Ensure birds have fresh water and access to daylight or a supplement.

FAQs

Do all hens need oyster shell? Only laying hens require extra calcium; pullets under 18 weeks and cockerels can manage on normal feed.

Can I mix oyster shell into feed? It’s better offered separately so hens self‑regulate. Mixing it into feed leads to waste and can result in some birds consuming too much.

How much oyster shell should I provide? A small dish topped up weekly is enough. About 25–30 g per hen per week is typical for backyard flocks. Hens will only take what they need.

Are crushed eggshells a good substitute? Crushed and baked eggshells are mainly calcium. They can be used. However, they break down faster and may encourage egg eating. Bake and crush them before offering.

What’s the difference between soluble and insoluble grit? Soluble grit (oyster shell) dissolves to provide calcium. Insoluble grit (flint or granite) stays in the gizzard to grind food. Hens need both when eating anything other than finely milled layer feed.