Cuttlefish bone is good for birds

Update: 4/20/2013

Cuttlefish bone is neither a bone nor of a fish, but it is good for birds, in any case.
Cuttlebone has too much phosphorus . . .
Mr. Robert G. Black, author of Nutrition of Finches and Other Cage Birds, defends (by referring contesters to the bibliography of his text) in forums, for example, and propagates this misinformation. Dr. David J. Kersting, D.V.M. has written just that under supervision and defense of President Gerald Oldberg of the American Cockatiel Society. I even have had to correct a Wikipedia entry over this.
It is wrong, simply. Cuttlefish bone is as suitable as eggshell as a calcium source. Maybe eggshell is supplied more inexpensively and conveniently in households where eggs are consumed liberally, but nutritionally, cuttlebone is not inferior.
I suspect that the confusion arose out of semantics. So firstly, Cuttlefish bone is not a bone. Bone is comprised of calcium phosphate, typically hydroxylapatite, and it does contain phosphorus.

And while on that topic, Cuttlefish are not fish. Cuttlefish belong to a sub-class of Cephalopoda, Coleoidea, characterized as being without shell or having internalized shell.
Both cuttlebone and eggshell are composed of calcium carbonate; neither contain more than trace amounts of phosphorous. The amount of nutritionally available calcium in 10 mg of either option is equivalent.
The difference between eggshell and cuttlebone is in crystal structure. Interestingly, while feeding of eggshell may seem more appropriate since the material is presumable more “native” to avians, the calcium carbonate in cuttlebone is slightly less stable.

Well, in 10 to 100 million years*, aragonite settles as calcite, so on the time-scale of metabolism in aviary birds, especially Passeriformes, this thermodynamic instability cannot be regarded as any kind of advantage.
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