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David,
If you are feeding a complete feed for layers/ breeders feed and not offering a bunch of treats you should not need to supplement with Oyster Shells, but if you are adding treats and or feeding a feed that doesn't offer added Calcium then you should.
Laying hens need Calcium to form strong, sturdy eggshells. Hens without proper amounts of calcium lay thin-shelled eggs, or eggs with no shell at all! The amount of calcium a hen needs will vary with her age, diet, health, breed, and size. Older hens need more calcium than younger pullets, and hens in hot weather need extra calcium concentrations in their feed since they are eating less due to the heat and might not be getting all the calcium they need. Since eggshells are 94 percent calcium carbonate, oyster shells and ground up limestone are good sources of supplementary calcium.
Make sure you are not purchasing dolomite limestone, since this is harmful to egg production!
Now if you are supplementing with Oyster Shells you may also have to offer a good sores of Vitamin D.
The main function of Vitamin D is to maintain normal blood levels of calcium, and phosphorus. The lack of vitamin D can cause soft egg shells, and brittle, or thin bones in fowl. Vitamin D also helps keep your birds immune system strong, and can affect their over all growth, and development. Extreme cases of vitamin D deficiency can even lead to diseases like Rickets.
Good sources of Vitamin D would be Water Soluble Vitamin Supplements, Liver, and Fish.
Chris