Calcium deficiency can cause egg-binding in laying birds. A calcium deficient bird may produce eggs whose shells are softer or more fragile than normal. It is very difficult for a bird to lay a soft-shelled egg, since the muscles that push the egg out tend to deform the egg rather than moving it. The hen is unable to pass an egg that has formed. The egg may be stuck near the cloaca, or further inside.
An egg-bound bird can die of exhaustion, or from an infection following rupture of the egg inside the bird. Also it is possible an egg could break inside in the oviduct in response to a non-fatal blow (batted by cat, picked up roughly by a person), causing an infection that can be fatal within a week. This is probably a common reason apparently healthy birds are found dead in a box.
Captive birds and reptiles fed a diet consisting primarily of mealworms, especially when growing rapidly, can also develop severe bone disorders caused by an abnormal ratio of calcium to phosphorus in the diet. People who feed a lot of mealworms to their pets routinely dust the mealworms with calcium powder or feed a calcium and vitamin enriched diet to the mealworms (called gut loading) just before feeding to make them more nutritionally complete. Breeding birds are also offered calcium in the form of ground oyster shell or cuttlebone to supplement their diet. Eggshells are mostly composed of calcium. When parakeets are laying, they develop a craving for calcium and will devour a cuttlebone in a few days.
Many areas in the Northeast have also have soil which is severely calcium deficient because the calcium has been leached from the soil by acid rain.
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