Okay then its official, U stormcrow said it! Be careful with the calcium it will damage your hens!Diesel does damage to a gasoline engine, too, but that doesn't mean it should be taken off the market.
The subject is well studied, and widely accepted in the poultry science community. I've linked many of the studies here on BYC, but short lunch, no time to do so again now.
here's some anecdotes
Merck Vet
From 1979
1981
hopefully that's enough to get you started on your own research
As you've noted, many don't recognize the signs. Initially, as a progressive pathology, they are evident clinically, internally. As it progresses, many of the external signs, such as gout, are erroneously attributed to the aging process. In particularly egregious cases, sudden mortality has been traced to it (as result of miss mix from the mill), and there are some studies finding evidence of bone deformity, particularly in the feet, from long periods of excess calcium intake.
Additionally, not all calcium is the same. Dicalcium phoshate (used much more frequently in the Mediterranean basin than here in the US) as primary source of calcium in feed seems to buffer the effects somewhat - its the extra phosphorus. That's studied, too, but I'd have to search for the link, and I've already devoted more time to this than I should have. Calcium Diphosphate is also used, with similar buffering effect. Oyster shell, popular here in the US, is calcium carbonate of course. Calcium Citrate (i.e, Citrical) is good for us humans, and sometimes offered to chickens needing a fast fix for medicinal reasons, but spikes their blood calcium due to how rapidly they digest it - not recommended for daily use.
@junior67 I do what you suggest, albeit with a different feed brand. I do NOT recommend it. Its educated risk taking on my part. I mitigate that risk, in part, by raising my birds to about 12 weeks on a standard (1%+/-) calcium feed while they are developing (and most susceptible to nutrition - related damage) and by culling my males early. Few make it more than a single season. Most don't get a season at all. Very little time for the excess calcium (my mix averages about 2.8% +/-) to start doing damage.
If you are seriously considering this method, I would suggest you do your own research, weigh the risks you are willing to take in view of your own goals and management style, decide for yourself if the minor cost savings is worth the trouble and the risk, rather than crowdsourcing this answer.
