The necessity of providing grit to pet birds as an aid in digestion is controversial. Although there is a long history of feeding grit to pet birds, the practice has now come into question.
Types of grit
When discussing grit, it is important to realize there are actually two types: soluble and insoluble. Soluble forms of grit include cuttlebone, oyster shell, limestone, and gypsum. Soluble grit is dissolved by acids as it passes through the bird's digestive system, therefore there is little danger of it accumulating in the digestive system or causing an obstruction. Because it dissolves, it does little to aid in the digestion of whole seeds. It does, however, serve as a source of calcium and other minerals.
Insoluble grit is generally in the form of silica, and may range in size from sand to small pebbles. Insoluble grit remains in the gizzard and is thought to aid in the mechanical breakdown of food.
Grit and digestion
Birds have a muscular portion of the stomach called the gizzard, which grinds and crushes food. The smaller particles, then, are more easily broken down by digestive enzymes as they pass through the digestive tract. Some wild birds eat grit, which passes to the gizzard where it helps in this grinding process. It aids in removing the outer fibrous shell around some seeds (e.g.; sunflower seeds), if the shell was not removed with the beak prior to the seed being swallowed.
Diet and grit
The diet of wild birds is different than that of pet birds. Wild birds must eat whatever is available, and many of the seeds may have very tough shells or hulls, thus the need for grit. Doves and pigeons do not remove the hulls from seeds, so they also require grit. Pet birds, on the other hand, are generally fed pelleted diets or seeds which are easily hulled and digested without the need for grit. The use of grit will not make up for offering a poor diet. For proper food digestion, it is more important that a bird be fed an easily-digestible, nutritionally-balanced diet which also includes fruits and vegetables.
Most authorities agree that healthy psittacines (e.g.; parrots, budgies, cockatiels) fed a proper diet do not need grit. Some studies have shown that passerines (e.g., canaries and finches) have a behavioral or nutritional need for the ingestion of soluble grit. No evidence suggests that the provision of insoluble grit in the canary diet has any benefit and oversupplementation may lead to health problems. Soluble grit offered to passerines should only be in very small amounts - several grains every few weeks. Birds with pancreatic disease (the pancreas produces most of the digestive enzymes) or certain digestive problems may benefit from the addition of small amounts of grit to their diet. If your bird has a digestive problem, consult your veterinarian regarding your bird's need for grit.
Potential problems with grit
Some birds, especially psittacines (members of the parrot family) may eat too much grit, if it is available. This can irritate the digestive system or actually cause an impaction (blockage) of the crop, ventriculus, or proventriculus. Pet birds on a poor diet may also eat too much grit in an attempt to fill their nutritional requirements.
If offering grit...
Grit should not be placed on the cage floor where it can become contaminated with droppings. Nor should a dish of grit always be available, since it could easily lead to overconsumption. If a bird is offered grit, and seems to be eating too much of it, the bird should be examined by a veterinarian. Do not offer grit containing charcoal; charcoal can interfere with the absorption of certain vitamins such as A, B2, and K, resulting in deficiencies.
It appears that most pet psittacine birds, if fed properly, do not require grit. Offer a limited amount of soluble grit to passerines. If in doubt as to whether your particular bird should have grit, consult your veterinarian.