I buy ground eggshell from the feed store and offer them a dish of it periodically. My quail pretty much ignore it (and soil it) if I have it available all day, every day, but gobble it down when they only have access to it on a once weekly basis. DARK leafy greens served once a week or so, such as spinach or kale will also increase their calcium intake but should be offered as a treat, not a regular diet...(no more than 10% of their diet by weight, not volume). The issue can also be caused by a lack of vitamin D3, which aids in absorption and utilization of the calcium. Vitamin powder for poultry added to their fresh drinking water daily should offer sufficient amounts but I prefer a liquid vitamin supplement called "Chick Booster" that has a very long 'grocery list' of vitamins and minerals for my flock and includes Vitamin D3. I put 0.5ml per liter of water for adults, and double the dosage for chicks as they drink less than adult birds and not knowing the condition of the parent stock, to offset any nutritional deficiencies that may have been passed along to the chicks in their mother's egg.
Egg color, or lack thereof, can have health implications or be totally irrelevant. A quail hen can 'paint' her eggs to suit the environment. If she's laying in a location that is all the same color, such as sand... she may opt to not paint her eggs at all; to better camouflage them. The colorant is added to the egg as a final step before she lays her egg, and can easily be scraped off with a dull knife.
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