I wouldn't worry about excess dietary calcium in meat birds since they're not going to live long enough to worry about visceral gout, urolithiasis, kidney or other organ damage.
That said, when other poultry are affected by excess calcium, they just die and the cause is often not determined by a cursory necropsy.
http://nhjy.hzau.edu.cn/kech/synkx/dong/2bao/UrolithiasisChina.pdf
http://www.ncbi.nlm.nih.gov/pubmed/20434855 .
Mortality of male broiler breeders is nearly 4 times that of hens attributed to excess dietary calcium.
http://www.pjbs.org/ijps/fin1947.pdf
Commercial egg producers try to wait till seven to ten days prior to the oviposition of the first egg to start a prelay ration with increased levels of calcium.
In the past, if they increased too early, they would suffer losses to their pullet flocks.
After a lot of research, they found that adding an acidifier to the feed would alleviate the problem in case their lighting program didn't stimulate lay at the right time.
My assumption is that adding ACV to water may serve the same purpose.
It's also been found that some breeds and strains of chickens are more tolerant of excessive dietary calcium.
As for forage supplying higher levels of calcium. That depends on what is being consumed but studies don't show it to be true.
http://www.organicvaluerecovery.com/studies/studies_nutrient_content_of_insects.htm
"As expected, insects contain little calcium and high levels of phosphorus. While most wild-caught insects also appear to be low in calcium, in general, the values are somewhat higher than those reported for captive-raised insects. High calcium levels have been reported in only a few species of insects. Insects that have been shown to contain substantial quantities of calcium include stoneflies (1.15% dry matter basis), housefly pupa, Musca domestica (0.93% dry matter basis), from larvae raised in poultry manure containing 5.1% calcium, and the previously mentioned Musca autumnalis puparia."
Studies of insects raised for feeding insectivores, most were found to be deficient in calcium for the health of the reptiles and other zoo species.
http://www.timberlinefisheries.com/...content of four species of feeder insects.pdf
I remember when raising lizards and turtles, it was necessary to dust crickets with calcium powder otherwise they would have bone density issues.
As for green forage crops, they usually contain 0.5-1.5 % Ca and not all of that is bio-available.
http://www.extension.umn.edu/agriculture/dairy/feed-and-nutrition/minerals-in-forages.pdf
A 5 year study of forage indicated an average Ca content of under 1%.
http://mbfc.s3.amazonaws.com/2008_fact_sheets/p_forage_quality_final_june_26_08_p.pdf
I'd be interested to find what the forage source is that contains 10% calcium.
ETA
The best analogy I can give is the difference between occupational injury and occupational disease. If one falls off a ladder or scaffolding or get crushed in a machine, they will suffer broken bones or worse. The effect is immediate and they're sure what the cause was. That's occupational injury.
Occupational disease is something like mesothelioma from asbestos or cancer from chemical exposure - especially MEKs and PCBs. The injury is immediate but the pain and disease can come years later. People just get sick and don't equate that to the jobs they no longer work at, but the pain and disease is nonetheless from that exposure years ago.
The same goes for processed food. People don't know that aspartame may be killing them because the snack or diet soft drink tastes good and they don't immediately feel discomfort. They're still being poisoned, nonetheless.
A raccoon attack is immediate, Marek's and AI are pretty quick. Nutritional disorders are much slower, but they're there nonetheless. A poultry diet can be low in vitamin E, D or selenium and the chickens won't die immediately but they may do so prematurely.
People can feed layer to roosters for 4 years and they show no effects. Then they just die and they never know that they've slowly been overdosing calcium.
A lot of people don't worry about it. I never used to worry about it. But after reading the latest research and knowing the damage it does, I worry now and take measures so that I don't prematurely lose rare breeders to something I can prevent.
For those that have common breeds, their management would surely be different.
There's a big difference between anecdotal evidence and science.