You might be interested in knowing the different kinds of calcium there are. Oyster shell is exactly the same kind of calcium as egg shells - calcium carbonate. It's one of three kinds of calcium and is highest in calcium. It doesn't matter if the calcium carbonate is from an oyster shell washed up on the beach or in a bag of "manufactured" oyster shell or a bag of real oyster shells crushed.
Here's my tutorial:
There are three different sources of calcium, all different, all digested and absorbed at varying rates of effectiveness. The most common source is calcium carbonate. This is what egg shells, oyster shell, and calcite derived calcium supplements are. It's the highest in calcium, but it's the most difficult to digest and absorb. Some hens absorb it so slowly and inefficiently that it's not able to adequately supply their shell gland. So, they often produce shell-less eggs or very thin shell eggs.
The second kind of calcium is calcium gluconate. It comes from fruits and vegetables. It's not very high in calcium and still hard to digest and absorb.
The third kind of calcium is calcium citrate. It's the by-product of the manufacturing process of making citric acid. This form of calcium is very easy to digest and absorb. For this reason, it works much, much faster than the other two types of calcium. This is the form of calcium that's best to use when a hen is having reproductive issues from the relatively minor one of shell quality to the most serious and life threatening one of egg binding.
One calcium citrate tablet with vitamin D given right into the beak once a day until the issue is resolved is what I strongly recommend. Here's what to buy.
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Calcium citrate is not meant for daily use as a calcium source, only a temporary intense calcium boost in a reproductive crisis. Good quality oyster shell is still the very best source of calcium you can provide for laying hens. But be sure they're getting the large oyster shell particles and not the powdery residue left in the bottom of the container as it runs through a hens system much too fast to be properly absorbed. The larger particles remain much longer in the digestive tract allowing for much greater absorption. This will help to assure your hen is keeping her calcium stores topped off and will have less tendency to have egg issues.