Egg shell quality is primarily dependent upon the proper amounts of vitamins and minerals in the diet. Calcium and vitamin D3 are the crucial nutrients involved in egg shell formation. Calcium functions in the actual egg shell formation. Egg shells are comprised of 95% calcium carbonate. Vitamin D3 is critical for the absorption of calcium by the hen. Poultry can synthesize a portion of vitamin D3 through exposure to sunlight but additional vitamin D3 is necessary in the diet. An excess of any of these major nutrients involved in shell formation can result in weak or soft eggs shells and reduced egg production.