Cockatiels have been eating mineral grit for hundreds of thousands of years because it is essential for normal avian digestion and neuromuscular physiology. Tiels do not hull all of their seeds. If a Cockatiel were to hull each seed in the wild it would be easy prey for predators. Therefore, Cockatiels (and other psittacines) will carry seeds in their crop until their proventriculus is ready to digest them. Hydrochloric acid is secreted in the proventriculus which begins the digestive process, however, the seeds are digested in the bird's ventriculus using pieces of insoluble mineral grit (quartz, granite, and other minerals).
Tiels use grit to digest their seeds and fibrous leafy food and as a source of electrolytes to prevent neuromuscular pathology. Without soluble and insoluble mineral grit Tiels can suffer from inflammatory neuromuscular pathology of the gastro-intestinal tract.
A bird's physiology is not the same as a dog or a cat. A bird uses insoluble grit to digest seeds in their ventriculus. The ventriculus is composed of keratinous plates that use the grit to grind and pulverize seeds and fiber. Birds deprived of grit can become impacted with ingesta. Impaction is not caused by grit. If this were true there would not be birds in the world today because Cockatiels and most birds have instinctively used insoluble pieces of mineral grit for hundreds of thousands of years as a natural aid to digest their diet of seeds and fibrous plant matter.
Cockatiels are indigenous to Australia. Here is an article by Mike Owen Queensland Australian Rep. World Parrot Trust copied from the internet.
"Almost all Australian parrots have access to grit. . . . President of the Australian chapter of AAV, has only ever seen two instances of impacted crops in 15 years of practice. . . . all wild parrots apparently have been found to have some grit in their gizzard, and I don't think they would swallow it if they didn't find it useful. . . . .Corellas, Galahs, Major Mitchells, budgies, and other species, deliberately land on sand banks in dry inland river beds and peck away and swallow sand grains. It is a deliberate action on their part. . . . .A Rosella for instance might have up to 50 grains of grit in the gizzard. . . .presumably reflecting the amount of wear that the grain has undergone. . . ."
"Actually when an autopsy is done on a seed-eating parrot, it is surprising just how much seed seems to be swallowed unhusked. Some birds might have 20% or more of the seed in their crop which is unhusked, particularly the smaller millets and pannicums."
"Parrots can certainly live their whole lives without grit. The question is whether having a significant amount of grit helps that bird to have a less-stressed (= more efficient) digestive system. I believe that it does. Anything that makes it easier, and more efficient for the bird to grind up the seed before the digestive system gets to work must be a help to the bird."
This idea that a sick bird will gorge on grit is a common one in the USA - all I can say is that I have never, ever, come across such a case. I have never seen an autopsied parrot with a gizzard full of grit and never seen grit at all in the proventriculus. I also find it incredible that a single vet in Florida sees hundreds of grit impaction cases a year, while Australian vets see none! Something is wrong here. Perhaps with USA vets not having exposure to wild parrot autopsies, they are not used to the large amount of grit that can occur in healthy wild birds. What they are diagnosing as gizzard impaction, to Australian vets might be a healthy and normal grit load for a bird." Quote: Mike Owen Queensland Australian Rep. World Parrot Trust http://www.holisticbird.org/diet/grit.htm