My girls are about 1 yr old and I have had two thin shelled eggs within the past 3 weeks. No biggee, but I don't want to make it a habit. I feed them organic soy-free feed, oyster shells on the side, they get "treats" (fruits and veggies, freeze-dried meal worms, crickets) occasionally, but I am going to cut out treats for a month to see what happens. Let them find their own bugs, I am almost out of "egg shell powder" that I had ground up, so I am going to incorporate that again, since I had stopped when I started the oyster shell. So here is some info I found on-line:
Most people immediately think that Oyster shell grit is what a hen needs if she is laying soft shelled eggs but a dietary deficiency can also be the cause of a ‘soft' shelled egg. Whilst chickens need grit to grind their food down in their gizzard and calcium to form their egg shells, they also need a correct level of protein in their diet as well as minerals and various other vitamins. Vitamin D3 (Cholecalciferol) is used for the metabolism of calcium and phosphorus to be able to form egg shells as well as strong bones. Vitamin D is found in Cod Liver Oil but they shouldn't normally need this if they are fed the correct formulated layers feed and have free range and sunshine on their backs.
Corn treats and kitchen scraps
Wheat found in mixed corn typically contains about 10% protein. This isn't a sufficient amount for a hen that is producing eggs. Keep corn as a treat only. Kitchen scraps are a bit of a mixed bag of what a hen needs so whilst this is an efficient way to recycle them and save on feed costs, scraps should not really exceed 25% of a hen's diet. Scraps should be mixed with layers mash to make a crumbly mixture. The best way to ensure a hen is getting the correct diet is to use a balanced layers feed and then supplement this with greens and a vitamin and mineral drink that can be added to their water during the peak months of production.
Free Range Hens
If hens can be allowed to free range on grass or rough ground then this is much better than having to add top up vitamin and mineral drinks. They will be able to pick up a lot of the extra grit, vitamins and minerals they need from and be a lot less prone to health problems.
Some strains of birds can lay more soft egg shells as they age. This is particularly true of hybrid breeds that have been optimised to give as many eggs as possible during their first year such as the Bovan Goldline often found in ‘battery' farming. Once these birds reach 4 or 5 years old, you may find they start to lay eggs with soft shells.
Causes for thin egg shells
Calcium is the primary mineral that makes up eggshells and when not supplied in the diet, the hen does not have the basic materials needed to make the shell. The problem is produced when whole grains or feeds deficient in minerals and vitamins make up the bulk of the laying hen diet. Thin egg shells are observed when calcium, phosphorus and vitamin D3 are not provided in diets at adequate levels. It is more often observed during periods of hot weather because calcium is conserved and retained within the hen's body less efficiently.
The quality of the shells is improved by feeding a complete laying ration as the only diet. This diet supplies all nutrients in the proper proportions so the hen can produce good shells. If thin egg shells becomes a problem, it is advisable to add 2 pounds of oyster shells (as an oyster shell flour or hen-sized oyster shells) to every 100 pounds of complete layer ration.
This will provide a quick remedy to the problem and should restore egg shell quality within a short period of time. After the egg shell quality is restored, the addition of oyster shell can be eliminated and the complete layer diet can then maintain good egg shell formation. It is also advisable to also add a vitamin supplement to the drinking water while the oyster shell is being added to the feed. This will help ensure that calcium and phosphorus in the diet is being properly absorbed through the digestive system and will be available for deposition as shell on the egg.
Feeding Schedules for Chickens
Poultry feeds are referred to as "complete" feeds, because they are designed to contain all the protein, energy, vitamins, minerals, and other nutrients necessary for proper growth, egg production, and health of the birds. Feeding any other ingredients, mixed with the feed or fed separately, upsets the balance of nutrients in the "complete" feed. Feeding additional grain or supplement with the complete poultry feed is not recommended.
Feed chicks a "starter" diet soon after they hatch. Continue feeding the starter feed until they reach 6 or 8 weeks of age. The starter diet has the highest level of protein a chicken receives during its lifetime. As the chick matures, it requires a lower percentage of dietary protein and a higher level of energy.
After the chicks reach 6 or 8 weeks of age, feed them a "finisher" diet (to broilers) or a "developer" diet (to pullets or cockerels saved for breeding purposes). Feed broilers a finisher diet until they reach slaughter size. Feed the pullets and cockerels a developer until they are at least 20 weeks of age. When egg production starts, feed them a "layer" ration until egg production ends.
The minimum requirements for protein, calcium, and phosphorus in poultry feeds are shown. Remember, chickens saved for egg production are fed pullet-type diets, not broiler diets, regardless of being from broiler or egg-type stock.
Minimum
requirements Protein
%
Calcium
%
Phosphorus
%
Pullets
Starter (0-8 weeks)
20
0.9
0.5
Developer (8 to 20 weeks)
14
0.8
0.5
Laying Hens
Layer
16
3.0
0.5