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One thing you might try- especially if your birds are free ranging- is cut the amount of protein you're feeding a little bit; nitrogen and potassium interfere with calcium metabolism. Adding a magnesium source (food quality dolomite lime) may also aid in calcification. I've gotten the same advice whether dealing with puppies, lizards, cattle or humans: the chemical pathways are the same.
Vitamin C and Vitamin D also help with the absorbtion of calcium. Someone on the forums posted several weeks ago that they got a vit D supplement at the feedstore and it greatly helped.
Here are some other sources of calcium
1. Spinach & Swiss Chard
2. Salmon & Sardines (canned with bones)
3. Mustard, Collard, Kale & Turnip greens
4. Shellfish
5. Blackstrap molasses (can cause runny stools)
6. Corn Tortillas
7. Yogurt
8. Mozzarella & Cheddar cheese
9. Milk, Buttermilk (goat's milk and cow's milk)
10. Basil, thyme, dill seed, cinnamon, and peppermint leaves
11. Romaine lettuce
12. Rhubarb
13. Almonds, Peanuts & Brazil Nuts
14. Black Beans, Dried Beans (Cooked)
15. Sesame seeds
16. Fennel
17. Cabbage, Bok Choy, & Chinese cabbage
18. Summer squash
19. Green beans
20. Garlic
21. Tofu & Soybeans
22. Brussel sprouts & Broccoli
23. Oranges (Some people do not feed citrus to chickens)
24. Asparagus & Okra
25. Crimini mushrooms
26. Foods Fortified with Calcium: Some Orange Juice, Breads, & Cereals
Rhubarb, however, means rhubarb with no green leaves as those contain oxalic acid that blocks calcium absorbsion.
As a matter of fact, one thing that might need watched out for in free ranging chickens is if they are eating sheep sorrel (Rumex acetelosa) or any Oxalis species, which all contain lots of oxalic acid. Another thing that can interfere with calcium absorbsion (although it has other more serious toxicities, including kidney and liver damage) is drinking water that has been contaminated by fallen oak leaves.
And, of course, DDT famously interferes with eggshell formation, but unless you're on an old farm where DDT was stored and is leaching into the food chain, that's unlikely to be a cause of thin eggshells.