I think a lot of it is simply misunderstanding
A lot of people don't know that there are different types of calcium and that they are absorbed differently. It is also possible that birds do make use of calcium carbonate more than we know! In mammals it absolutely is the worst version of calcium.
This!
Birds are so very different than mammals and there are so many species of birds that generalisations are always hard to make. But the internet loves generalisations and quick factoids, which can mislead well meaning people. And a wide range of research on calcium absorption in from mammal based studies i.e. humans/mice/guinea pigs/rats/etc
Lets use healthy chickens as an example. They consume small pebbles, dirt, sand and other debris that most mammals would not by choice. Chickens also have strong digestive acids- not dissimilar from our own- a tonic of hydrochloric acid and pepsinogen that their bodies regulate. However, unlike us, this acid is produced in the proventriculus and starts digestion before the material reaches the gizzard. Within the chicken's powerful gizzard this blend is then crushed and ground. It is a system of digestion that is chemical as well as mechanical. This isn't surprising when chickens swallow their food whole. They don't chew it or cook it like we do, and they don't shell seeds like a parrot. Yet chickens have this amazing digestive system, then can eat fallen fruit, insects, small lizards, veggies, seeds, roots, the list goes on. It is simply incredible. They def get much value from calcium carbonate via egg shells and oyster shells. Feeding egg shells back to hens is common practice, since it is one of the most effective ways for them to reabsorb their lost calcium. If you are worried a chicken may become an "egg eater" you can simply crush the shells up into a mash first. However I haven't had this issue, I find the hens are more than aware of a spent shell vs a good egg. They would normally eat the shells in the nest once their babies hatched in a wild setting.
Very, very interesting!
