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 Breeding & Genetics

Health & Nutrition

The Grit Issue

 

Article sections:
1.  Introduction
2.  What is grit?
3.  The history of the anti-grit position
4.  How sound is the anti-grit position?
5.  Where did the complete rejection of grit originate?
6.  The pellet company connection?
7.  Observed behavior in the wild
8.  Observed behavior in captivity
9.  Pro-grit experts
10. Conclusion
11. Footnotes
   a. Quotes from Avian Medicine
   b. Other book quotes
   c. Internet sources
      i.   Pro-grit experts
      ii.  Impaction and crop function
      iii. Wild behavior
      iv. Medical case studies
      v.  Other studies

1. Introduction

For many years, the standard advice on grit was to offer plenty of it to captive birds.  But now the overwhelming advice on the internet is that you should never give grit to a parrot because they don't require it, and it's a dangerous material that causes crop impaction (a blockage that prevents food from passing through the digestive tract). 

But does the modern advice to "just say no" make sense?  There is good reason to think that the dangers of grit have been greatly exaggerated and the potential benefits are being ignored. It's difficult to study the feeding habits of wild parrots because they move around so much, but there are quite a few species that are known to eat grit in the wild. Mother Nature must have equipped these species to deal with it.

The dire warnings on the internet seem to be the result of people  repeating what they've heard without doing a reality check on the information.  This article does not take a stand one way or the other. Instead it presents evidence-based information and expert opinions to help bird owners assess the grit issue based on reasonable evidence instead of what "everybody" says aka hearsay. If people read this discussion and don't want to offer grit because it makes them nervous, that's a perfectly good reason to not offer it. It is enough that they took the time to educate themselves about the issue.

 I think it's likely that someday the opinion pendulum will swing back and there will be a more  balanced view toward grit. I also suspect that there are currently a lot of people in the grit closet, who give their birds grit but don't talk about it because they don't want to be flamed on pet bird forums. My own experience: I've been giving grit to birds since 1961, and about the first 40 years of it were done in the way that's supposed to be the most dangerous: unlimited quantities of insoluble grit offered to birds on an all-seed diet, and they ate it with enthusiasm.  The number of grit-related problems encountered so far: zero.  The all-seed diets were a different matter however, and my birds started living longer after I learned more about bird nutrition.

It's difficult to find hard evidence on this subject one way or the other, and the information presented here is the result of several years of searching for information.  The article will attempt to provide an easy to read commentary with footnotes containing extensive additional information and references for people who want to know more. Links to footnotes will be indicated in a small font like this: (see Footnotes). Anyone who has more information on the subject is welcome to write to me about it.

2. What is grit?

The answer to that question is more controversial than you might expect, with some people using a narrow definition limited to insoluble grit only and ignoring the fact that soluble grit exists too. This narrow definition makes it easier to condemn grit IMO, but I don't think it's a realistic definition because it doesn't tell the full story about the way birds use grit. In general, grit is defined as small bits of minerals or rock.  It's the size of the chunk that identifies it as grit, not its chemical properties.  

Soluble grit is grit that dissolves in the digestive tract, adding useful minerals to the diet in the process. It may or may not help with food grinding, depending on how quickly it dissolves. The soluble stuff isn't necessarily water-soluble, but if water won't do the job then the digestive acids certainly will. Soluble grit includes material like oyster shell and limestone. Insoluble grit does NOT dissolve in the digestive tract, so it doesn't provide any nutrients and is apparently consumed solely as a food-grinding aid. Insoluble grit includes material like quartz and granite.

Soluble grit can help compensate for anti-nutrients in food.  Seeds, grains, and nuts are high in oxalates and phytic acid, which bind minerals in food, particularly calcium.  These anti-nutrients are present in vegetables too, sometimes in high amounts.  The consumption of additional calcium and minerals insures that the body has enough minerals in spite of the anti-nutrients.

Information sources don't always indicate whether they're using the narrow definition of grit (insoluble only) or the broader definition that includes both types.

There's a misconception that needs to be cleared up about crop impaction.  The crop is basically a temporary holding pouch for food, and there's not much digestive activity in it - almost all digestion takes place further down the line. Grit does not normally remain in the crop. It passes into the gizzard (aka ventriculus) where it is actually useful. Some sources say that crop impaction can be a misnomer that would be described better as gizzard impaction.

The use of grit aka gastroliths ("stomach stones") is ancient, dating back to the days of the dinosaurs at least. A recent scientific study looked at gastrolith use in ostriches to gain insight into gastrolith use by dinosaurs. The ostriches ate a diet of grass, straw, oats and vegetable granulate (mostly lucerne). They were free to consume as much grit as they wanted from the soil, and preferred quartz pebbles.  On average, stones were about 1/3 of the total contents of the "complete stomach" (proventriculus and gizzard), with a range of 10% to 50% of the total contents. This study is an indicator of the long evolutionary history of grit use, and also demonstrates that eating a large amount of grit may be perfectly normal.

3. The history of the anti-grit position

I've looked at dozens of bird-care books published before the year 2000, and not one of them was opposed to feeding grit to parrots. There were some that didn't mention grit at all, but the majority recommended it (usually in large quantities) and some were quite vehement about the need for grit. Experiences with my Cockatiels by Mrs. E.L. Moon (first published in the 1940s) includes a recipe for more than five pounds of grit, which she said was enough to last one pair of cockatiels for one year! That's a lot of grit for a couple of 3-ounce birds and their babies, and quite a contrast to the modern advice you sometimes hear saying that if you insist on giving grit to a parrot, you should limit it to one or two grains every six months.

This "let them eat grit" attitude seemed to change abruptly sometime around the year 2000 (except in Australia, where many birdkeepers are still unconvinced that there's anything wrong with grit).  The number of bird-care books being published dropped dramatically around the same time, but most books published since then are vehemently anti-grit, as is most grit-related advice on the internet.  The arguments are that parrots don't require grit because they shell their seeds before eating them, and that grit is dangerous. 

4. How sound is the anti-grit position?

The anti-grit mantra in the parrot world is that birds that hull their seeds (like parrots) don't require grit and birds that don't hull their seeds do require grit; but scientific studies indicate that birds in general don't actually require insoluble grit and will manage to digest food without it. Oat hulls can function as a type of grit, and presumably other types of tough seed hulls could serve the same purpose. Dry seeds themselves are a hard material and can function temporarily as a sort of grit even without the shell.  But stone is harder and more durable, and many birds (including parrots) evolved using stone grit. Their digestive tracts are designed to work most efficiently with this material.

In many cases, the hull is softer than the hard dry seed it contains. It's hard to argue that a bird that can grind up a hard seed is unable to deal with a flimsy hull. So why do parrots hull their seeds?  Because they can. Many seed-eating birds don't have the right beak design or foot dexterity to remove seed hulls, but parrots do and they take advantage of it.  Removing the hull is better for their digestion. There is no food value in a seed hull, it's just indigestible fiber. Some fiber in the diet is good, but too much of it interferes with digestion by physically getting in the way of nutrient absorption. If the hull isn't swallowed, the  bird doesn't have to process a lot of indigestible material through the system and carry around extra useless weight while it's flying. So parrots make a smart move by getting rid of it.

It's obviously true that parrots don't require grit, because millions of pet parrots are currently surviving without it. But this doesn't mean that they're surviving optimally.  When grit is part of a bird's wild diet they're consuming it for a reason. Poultry studies have established that chickens can survive without grit too, but their digestive efficiency improves significantly when grit is added to the diet, and they choose to eat grit when given the opportunity. How are we affecting our birds' health if we impair their digestive efficiency over a long period?  We don't know the answer.

What about the dangers of grit?  The general consensus seems to be that a parrot who is sick or mineral-deprived may overeat grit and suffer impaction of the crop/gizzard as a result.  It's doubtful whether this actually happens, unless a bird's ability to regurgitate or pass material normally through the digestive tract has been affected. If it does happen, this would be a good reason to use caution and limit access to grit to prevent overconsumption. But it's not a good justification for completely depriving all parrots of grit. There are dangers to not providing grit too. Many parrots will crave grit whether we let them have it or not, and a parrot who is grit-deprived is likely to gorge on any gritty material it can get hold of, with possible impaction or toxicity issues as a result.  A bird who is not grit-deprived is less likely to pig out on something inappropriate.

Most of the internet information on crop impaction deals with poultry. When it comes to blockage, the difficulties are generally caused by soft materials or foreign objects. Grit is considered to be the solution to crop impaction not the cause of it, because it helps break up tangled soft masses and large hard objects. There have been cases where parrots had an impaction caused by soft material, such as fibers from rope perches.

In general, it appears that much of the grit that birds eat consists of small particles capable of passing through the digestive tract, not large particles that cannot pass until they have been worn down. Grit-eating birds have physical mechanisms to retain useful grit and also to get rid of it easily when it is no longer useful. Grit is trapped in the folds of the gizzard where it helps with food grinding.  Birds apparently have a lot of control over grit retention. Pieces that have become too smooth to be useful may be passed or regurgitated, and when grit is plentiful birds tend to replace it frequently, eating and excreting large amounts of grit. Grit that is not retained usually completes the journey through the digestive tract and is passed out along with the feces.

This discussion brings up a point that seems to be generally overlooked: birds can regurgitate their crop/gizzard contents whenever they want to.  It is the technique they use to feed their mate and/or babies during breeding season, and it is easy and natural for them.  It's possible for soft, stringy material in the crop to tangle into a mass that's too large to regurgitate, but it's hard to see how this could happen with loose bits of hard material, assuming that the bird hasn't made them stick together by eating fresh glue or concrete powder along with the grit. A piece of stone small enough to be swallowed in the first place will generally be able to make the trip in the reverse direction. So even if a bird did eat a piece of grit that was too large to pass out of the gizzard in the normal direction, it has an alternate way to get rid of it.

(see Impaction and crop function)

5. Where did the complete rejection of grit originate?

There are certain events that stand out as possible causes for the sudden rejection of grit by the bird community at large. The medical text Avian Medicine: Principles and Applications  by Ritchie, Harrison and Harrison was published in 1994, and made statements that grit was unnecessary and potentially dangerous for parrots. The book is not entirely anti-grit, but could easily be taken that way if only the negative passages are seen (see Quotes from Avian Medicine). A previous medical text published in 1986 had a more neutral tone; it suggested limiting grit but did not speak out against it. A subsequent book published in 2005 also had a more subdued tone.

At the same time, the internet was rising in popularity, and the practice of avian medicine was also on the rise. So besides having an influence on new vets, the negativity expressed toward grit in the text was in an excellent position to spread rapidly through the bird-owning population as soon as a few well-meaning people saw the anti-grit commentary, took for granted that it was right, and started talking about it on the internet. A recently published medical book would be seen as the most up to date thinking on the issue, so offering grit got tossed on the trash heap along with old practices that really were bad, like the all-seed diet.

But where did this viewpoint originate? Avian Medicine mentions a prevailing anti-grit sentiment in the US, but there's nothing like this in the mass-audience bird-care books of the time, and the medical book doesn't provide  references to back up this statement. There were ways for rumor and anecdote to spread among the "upper crust" of the bird community even in the pre-internet era; veterinarians could talk to each other at professional conferences, and show breeders and other hard-core bird enthusiasts could talk to each other at conventions, shows and meetings. These events are probably where the bandwagon first began to roll. But the vast majority of bird owners didn't participate in these activities, and were limited to what they read in mass audience books, which seem to have been universally pro-grit. Magazines for bird fanciers were a potential way to reach a somewhat wider audience, but so far I haven't seen any anti-grit comments in my growing collection of old magazines.

It is evident that prior to the publication of Avian Medicine, American veterinarians observed problems that they believed were caused by excessive grit consumption. But crop impaction with grit is not observed in Australia, and an Australian source thinks that crop impaction cases in the US might be a misdiagnosis by American vets who don't know what a normal grit load for a wild parrot looks like (see Holistic Bird grit article). There was a shortage of avian vets in those days, so it's likely that their practice was focused on urgent cases not on well-bird exams. If that is true, then obviously they would see more sick birds with grit in the crop than healthy birds who consumed grit, potentially leading to incorrect assumptions about the cause and effect of the illness. 

There are other indications that grit was being questioned prior to the publication of Avian Medicine in 1994, but I didn't find any sources from this time period that actually advise against it.  Cockatiels! Pets-Breeding-Showing by Nancy A. Reed has some some strangely ambivalent comments about grit. The book was published in 1990. The author says grit has become controversial but it seems to mostly be a budgie problem. She uses grit with no problems and her cockatiels enjoy it and devour it when breeding.  Nonetheless, she will probably stop using it when her current supply runs out.  This is an interesting example of the bandwagon effect, where someone gives up something that is working well for them because of rumors that it might be bad.

The recommendation to avoid grit apparently isn't based on scientific findings.  In fact there don't seem to be any published scientific studies that focus on the subject of parrots and grit. I found only three published medical case studies that mentioned grit in the crop (all published in the 21st century). The only case where the grit was considered to be a problem involved baby birds thought to have been overfed grit by their parents (see Medical Case Studies).  I couldn't find any case studies where an adult parrot needed treatment or died because it chose to overeat grit.

Apparently it has been recognized for decades that excessive grit can cause problems for baby birds, who eat what their feeder gives them without having a choice in the matter. A tiny chick can't handle a particle size that would be fine for an adult, although an older baby would have more capacity. In addition, weaning babies don't always know the difference between food and non-food, and are notorious for eating inappropriate objects. They may not have developed the skill of voluntary regurgitation yet and might not be able to get rid of excess grit. So it would be wise to use extra caution when there are babies in the nest or when offering grit to very young birds.

The available information indicates that the publication of Avian Medicine was the first time that a major authoritative source expressed a negative attitude toward grit in writing, and they did it at a time when this information could begin to spread rapidly through the bird community via the internet.

6. The pellet company connection?

There are some interesting circumstances surrounding the general negativity about grit in Avian Medicine. The book doesn't mention that one of the primary authors is the owner of a pellet company (Harrisons) or that the nutrition chapter of the book was written by an employee of another pellet company (Dr. Randal Brue of Kaytee). I don't question the sincerity of these authors but it seems reasonable to question their objectivity, and it seems like a major conflict of interest to have a pellet company employee write the nutrition chapter, regardless of his qualifications.  The text never states a recommended pellet percentage, but pellet companies generally prefer for birds to eat a diet of at least 80% pellets if not more. It's true that a bird on a diet like that doesn't need much help with food grinding or supplemental minerals, but many people don't feed their birds that way and think that a more natural and varied diet is preferable. The text doesn't mention that a bird who eats a lot of natural foods might have different needs than a bird who mostly eats high-nutrition mush (pellets get soft when they're wet). Studies show that the type of diet makes a difference in how much grit wild birds use. Granivorous (seed eating) birds eat more grit than insectivores, omnivores, and frugivores. Researchers generally consider dry seed to be a hard, coarse food. A study on captive Willow Ptarmigan found that birds eating coarse food consumed and excreted 2-4 times as much grit as birds eating pellets.

The references at the end of the nutrition chapter show a single paper related to grit, called The use of grit in psittacine and passerine diets, published in 1987. This paper is apparently the basis for the grit-related statements in this chapter and possibly elsewhere in the book. It was sponsored by Kaytee and authored by the same Kaytee employee who wrote the nutrition chapter.  It is apparently an internal Kaytee document that has never been published in any place where it could be evaluated by peers in the avian medicine and nutrition community, or by anyone else for that matter. This is a red flag for anyone who knows how the scientific community works, and the importance of peer review and publication in venues that set standards for quality. There are a lot of non-Kaytee references at the end of the chapter, and unlike the Kaytee documents it looks like they were published in peer-reviewed journals and at professional conferences.

Because the grit study is a private document, we don't know what they did or what results they observed.  I requested a copy of the paper from the Kaytee company and did not receive a reply.

Not all veterinarians with pellet company connections agreed with the anti-grit stance.  A 1977 book by T.J. Lafeber DVM (owner of the Lafeber birdfood company) agrees that grit is not required for digestion and is potentially dangerous in some circumstances. But he goes on to say that seed-eating birds seem to have a physiological need for grit, and gives instructions on how to provide it. This is a very early mention of the anti-grit position, and it is rejected by the veterinarian who had the biggest pet bird hospital in the US in the 1960's and 70's, and started developing the first pellets in the early 70's.

A 1989 book by Dr. Lafeber continued to express the opinion that "nature seems to tell pet birds to keep some grit in their gizzard". The original 1977 book had a list of food dangers that included the overconsumption of grit. It's interesting that the newer book does not have such a list, and instead says specifically that free-choice grit does not cause problems to birds on a balanced diet.  It looks like Dr. Lafeber decided that the alarmist view of grit was wrong. I can find no indication that the Lafeber company ever sold grit, so there's no element of financial gain in Dr. Lafeber's position.

Dr. Lafeber senior passed away in 2001 and his son (another Dr. Lafeber) now runs the company. At some point in the 2000's the company's attitude seems to have changed, and a web search for lafeber grit now turns up numerous pages of bird care sheets saying only that birds don't require grit and it puts them at risk for impaction.  (see Other book quotes)

7. Observed behavior in the wild

Parrots may not require grit but many of them are known to eat it anyway.  The parrot species that have been observed eating grit in the wild include budgies, cockatiels, eclectus, various cockatoo species (including corella, galah, sulfur-crested, and Major Mitchell),  swift parrot, orange-fronted parrot, night parrot, various Neophema species, many-colored parrot, princess parrot, red-capped parrot, red-cheeked parrot, Port Lincoln parrot, western rosella (and possibly other rosella species), Mulga parrots, African grey parrot, and lorikeets. There are probably others, but these are the species whose grit use has been mentioned on the internet and in books. Observations are not available for every parrot species, but one Australian study made the general statement that "Birds eat grit (particularly the parrot and the dove)".

Observations of grit-eating behavior seem to primarily involve Australian parrots.  I don't know whether this is because Aussie parrots eat more grit than other species, or because it's easier to see them doing it.  Parrot species in other locations have been observed eating soil (geophagy), mostly at South American clay licks like Tambopata. Four or five unspecified parrot species were observed eating soil in New Guinea, and rosy-faced lovebirds were observed doing it in Namibia. It appears that African grey parrots regularly engage in soil eating. Soil is a more complex material than plain stone, and birds who eat it may be using it as a source of 'conventional' grit or may be using it for an entirely different reason. It's thought that the South American sites are used primarily as a source of sodium (Brightsmith et al, Forbes), and the New Guinea study thought that binding toxins was the primary purpose.  But one study stated that "There is also evidence that geophagy provides grit to many weak-billed birds", and it's possible that this could be a motivation for parrots in at least some cases.

Wild parrots have been observed eating a variety of other nondigestible items including charcoal (meaning burned wood from wildfires) and plant material including bark, raw wood, and dead leaves. Wild Gouldian finches are also reported to occasionally eat charcoal (Anderson). Wild South American macaws even feed chunks of bark to babies in the nest, and it was hypothesized that this might help strengthen the nestlings' crops. The purpose of this behavior has not been established, so we don't know what we're taking away from our birds when we don't let them do it. There doesn't seem to be an official hypothesis for the woody material, but several sources indicate that hard plant parts and other hard materials can be used as a substitute for stone grit. Research has identified some beneficial effects that stone grit and other hard material may have on the digestive tract in addition to its food-grinding use, but this has not been tested. It's also thought that bark and/or sap might provide nutrients or medicinal compounds. It's conceivable that woody materials might serve the same (currently unknown) purpose that charcoal serves; they're all different forms of wood.

It's thought that the charcoal might neutralize toxins or reduce acid in the digestive tract. Charcoal does neutralize toxins but it isn't selective and will also absorb nutrients. In addition, too much charcoal can cause constipation.  But the birds seem to think that the benefits outweigh the disadvantages, since they crave it and seek it out. Majewska et al found that charcoal reduced the mortality of turkey chicks and improved their weight. It was a strong natural adsorbent for gas, bacterial toxins, and mycotoxins, and also a source of various elements occurring in natural proportions with an adequate nutritive value (whatever that means). I don't know how much of a problem gas is, but bacterial toxins and mycotoxins can obviously be major issues that can occur with any kind of natural diet.

(see Wild Behavior)

8. Observed behavior in captivity

There are two types of grit-eating activity in captive birds. There's authorized grit consumption, where the owner provides sensible quantities of clean grit for the bird to eat, and there's unauthorized grit consumption, where the owner doesn't provide anything so the bird uses a "do it yourself" approach and eats any kind of gritty material it can get hold of. Unauthorized grit can include relatively benign material like floor dirt, tile grout, concrete, and stone/brick/stucco fireplaces, or dangerous materials like lead weights from curtains, pewter figurines, small bits of broken glass, and shiny jewelry. One of the arguments for providing good-quality grit is that it helps prevent birds from eating dangerous stuff as a grit substitute. Our birds don't have to go scrounging for whatever they can find if they already have a grit source.

9. Pro-grit experts

The "never feed grit" position is so simplistic that it doesn't need further elaboration, and in any case it seems to mainly consist of "everybody says it's bad".  An informed pro-grit opinion  requires nuanced thinking and analysis. I found reasoned pro-grit statements on the internet by a number of respectable experts. Their comments are summarized in the Pro-grit experts section of the footnotes, in order to be consistent with the presentation of other internet sources.  The comments vary from person to person, but all support the idea that grit is beneficial and it makes sense to provide limited amounts of it to parrots. The list includes the following:

Rosemary Low, an aviculturalist, author, and parrot conservationist who has written 25+ books on pet parrots and many articles on the subject.

EB Cravens, the 'natural birdkeeping' guru.

Australia's former representative to the World Parrot Trust, Mike Owen

Statements from the Lafeber company that are no longer available on the Lafeber website, but have been preserved elsewhere.

Avian veterinarians and highly experienced breeders.

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10. Conclusion

Parrots may not require grit, but it's part of the natural diet of many species, particularly Australian birds. Grit may increase the digestive efficiency of parrots, and may have other physical benefits that we are not currently aware of.  The information available to the general public does not support the idea that grit is dangerous to healthy adult parrots who don't have a mineral deficiency, and there are respectable sources who advocate the sensible use of grit with these birds.  Online examples provide useful precedents, so prudent owners who offer grit to their bird aren't blindly gambling with the bird's health. You do have to use common sense and a reasonable degree of caution of course.

The anti-grit position seems to be based on anecdotes and assumptions which may not be correct. Some avian vets and the unwashed masses have accepted the idea that grit is dangerous, but people with advanced degrees in biology apparently have not.

Birds vary in their appetite for grit; some want a lot, some want a little, and some aren't interested at all; in any case, it's up to you to set the limits. It is very helpful to learn about what your bird's species does in the wild to help guide you on this issue and other diet,  health and behavior-related issues.

In the end, the decision on whether or not to use grit is a matter of personal preference.  Your parrot can survive without grit but might survive more optimally with it; but the decision to offer grit may have risks attached to it, like anything else in life.  The decision will depend partly on how much confidence you have in your own ability to use grit wisely. This article has attempted to present evidence-based information on the issue, and the rest is up to you.

 

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11. Footnotes

Quotes from Avian Medicine, 1994 medical text by Ritchie Harrison & Harrison. This book is available for free online viewing at  Avianmedicine.net, and many thanks to the authors for that. Chapter 1 has a section on grit on page 13 of the pdf document. It says

"Whether or not to provide soluble shell grit and insoluble coarse sand grit to a bird is controversial. This practice is viewed with disfavor in the United States, especially if given free choice, which may lead to over-consumption and obstructive gastritis. In Australia, grit is frequently offered to companion birds with few ill effects; however birds fed formulated diets are unlikely to need either insoluble or soluble grit. As a compromise, a cockatiel-sized bird can be offered five grains of grit biannually; a cockatoo-sized bird can be offered a half-teaspoon of grit biannually." ;

It goes on to discourage the use of cuttlebone as unnecessary due to the widespread availability of formulated diets (pellets) and possibly toxic, which I think is an indicator of a conservative thought process. It also says "Charcoal may be consumed by a bird when it is offered; however it has been shown that charcoal can cause a vitamin B deficiency and it should not be offered on a regular basis".

The indirect message seems to be that grit appears to be safe in small amounts. The amount that they suggest seems absurdly small to me, but it is geared toward birds eating a large percentage of pellets.  "Biannual" means twice a year, not every two years.

There's another section on grit in chapter 3, page 15 of the pdf document. This is the chapter written by Dr. Randal Brue of the Kaytee corporation. It says:
"Grit is not required in the normal, healthy psittacine or passerine bird. Grit, defined as a granular, dense, insoluble mineral material (generally granite or quartz) is required in birds that consume whole, intact seeds. Examples of  birds that require grit are pigeons, doves, free-ranging gallinaceous species, and Struthioniformes. These species naturally eat whole grains as a varying protion of their diet. Because of the inert nature of the fibrous coating of many seeds (particularly corn, peas), digestive enzymes are relatively ineffective against them. Grit in the ventriculus acts to grind the whole seeds, thereby providing a substrate on which the digestive enzymes can act. Psittacine and passerine birds normally remove this fibrous hull, allowing the ingested portion to be easily acted upon by the digestive enzymes. It is likely, however, that in the case of a bird with a pancreatic dysfunction or other problems involving the physical digestion of food, grit could provide a benefit by enhancing the surface area for digestive enzymes to act. There have been numerous examples of birds not having grit for 15 to 20 years and still not showing any signs of decreased performance or poor digestion. Amazon parrots that did not receive grit for over five years still maintained high digestibility of ingested sunflower seeds, showing the unimportance of grit in the healthy bird. There have been numerous reports of birds, especially with health problems and depraved appetites, consuming copious quantities of grit and developing crop or gastrointestinal impactions. Considering the small chance of benefit and the potential risk, ad libitum feeding of grit should be avoided."
There are two types of grit, soluble and insoluble, but this quote does not consider soluble grit. Pro-grit sources consider both types to be beneficial. Psittacines may not require grit, but many will eat it if given the chance whether they're in the wild or captive, so apparently they derive some benefit from it. "Ad libitum" basically means "all you want", and recent sources generally do not recommend an all you can eat grit buffet.  

Chapter 19 mentions on page 15 of the document that “Crop impactions can occur in birds provided ad libitum grit”.   

Chapter 31, page 3 of the pdf says “In some cases, feeding a small quantity of grit may improve digestion and aid absorption, but should be supplied only in low quantities to prevent gastrointestinal impaction”.  

The next page of that same chapter mentions grit several times, most of it a repetition of earlier statements. One important piece of new information is that

“Studies in poultry indicate that the addition of grit increases the digestibility of feed by as much as ten percent, but similar studies have not been performed in companion birds. Given that obesity is more of a problem than maldigestion in companion birds, increasing the digestibility of a formulated diet that exceeds suggested nutritional requirements is probably unnecessary. Charcoal that is used in some grit mixtures may interfere with the absorption of vitamins A, B2, and K and contribute to deficiencies of these compounds."

Let's repeat that: scientific studies have demonstrated that grit improves the digestibility of food, in chickens at least. They don’t say what kind of food the chickens were eating.  

A feeding ecology study of wild cockatiels found charcoal in 29% of crops, so it’s part of their natural diet even though Ritchie Harrison are dubious about it. Charcoal consumption seems to be popular with Australian parrots and other Australian bird species, maybe even more popular than grit consumption.

Also in Chapter 31, the end of page 4 running into page 5 notes that crop impaction is common in North America but uncommon in Australia, for reasons unknown. One possibility is that in North America, it’s more common for a grit-deprived bird to gorge on grit when he gets the opportunity, while Australian birds are more likely to have had grit all along so they have no desire to overindulge. A more likely possibility is that grit and crop impaction are being blamed for a problem that was actually caused by something else.  The discussion of grit ends by discouraging the feeding of soluble grit (which is not described by that name).

Chapter 37, page 9 of the pdf says
“The administration of three to five appropriately sized pieces of grit may help in the removal of metal particles from the ventriculus by reducing their size and facilitating passage, particularly when used in conjunction with psyllium (hemicellulose).”  In other words, grit helps birds get rid of inappropriate objects that they’ve swallowed.

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Quotes from Clinical Avian Medicine and Surgery, 1986 medical text by Harrison & Harrison.

This forerunner to Avian Medicine had a more subdued, neutral tone. It suggested limiting grit but did not speak out against it.

On page 18 the book says "In the author's opinion, sufficient grit for the efficient functioningof the ventriculus in psittacines is accomplished by providing 10 to 12 appropriately sized grains two to four times a year.  Some digestive disturbances and impactions have been associated with overconsumption of ad lib grit. Because grit does not dissolve in the ventriculus, oyster shell and other dissolvable substances do not serve as grit products." 

Page 70 says "The presence of grit or small stones aids in grinding; however grit may not be essential for food digestion."

On page 154 it says "Ventricular obstruction is especially common in budgerigars that have consumed large quantities of grit.  The passing of whole, undigested seeds is often associated with this latter condition."  This statement doesn't make sense to me.  If the ventriculus (gizzard) is obstructed (blocked) then how is it possible for whole seeds to pass through?  It's common for sick birds to pass whole undigested seeds through the digestive tract whether they have grit in the crop or not.  It seems likely that the birds were passing whole seeds because they were sick but retained the grit in their gizzard, and the grit was blamed for a problem caused by something else.

Page 220 talks about X-rays and says "The ventriculus (gizzard) is usually the organ most easily identified in birds that receive grit.  Common disease conditions involving the gizzard include impaction/obstruction from foreign material or grit, luminal masses, and toxicosis from heavy metals."  The question remains of course of whether grit impaction is a real phenomenon, but the text points out that the presence of grit in the gizzard makes it easy to identify this organ in X-rays.

Page 336 talks about passing whole seeds and says "Whole seeds can also be passed in response to mineral oil gavage for grit obstruction. Conversely, not having access to any grit may produce these symptoms." So... according to this book, whole seeds may be passed if the bird is forced to swallow mineral oil as treatment for grit obstruction. That sounds like the result of the oil, not the grit. Conversely, the bird may pass whole seeds if it doesn't have any grit in the gizzard. That sounds like a digestion problem that a little grit might solve. There are no complaints about passing whole seed with moderate grit consumption. 

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Quotes from Clinical Avian Medicine, 2005 medical text by Harrison & Lightfoot.

This successor to Avian Medicine also had a more neutral tone.

Page 112 says "Grit is small non-dissolvable rock. The necessity of grit in the diet is debatable. Some birds, such as pigeons, fowl, canaries and finches, appear to need the availability of grit. In psittacine species, an occasional grit particle is harmless but it is not necessary for healthy maintenance of pet parrots."

Page 415 says "The role of grit in avian digestion is an interesting one. Insoluble grit may lodge in the gizzard and add to the maceration of the food, particularly in species that do not dehusk the seed before swallowing it, eg, pigeons, and galliforms like quail. It is controversial whether birds deliberately seek insoluble grit to aid in digestion or whether its ingestion is incidental to eating digestible foods or soils containing minerals and trace elements.

Grit is absent from the stomachs of nectarivorous birds, which also have poorly developed gizzards." 

Page 867 says "All gallinaceous birds should have access to grit, when not fed strictly an artificial diet. The grit container should be emptied and refilled regularly because birds select only stones that are suitable for their body mass. Pellets or complete rations have an adequate supply of calcium and should not be supplemented with lime or crushed shell."

Page 867 on the management of canaries, finches and mynahs says "Soluble grit sources, such as cuttlefish bone (Sepia spp.), oyster shell, limestone (calcium carbonate), marble (crystalline limestone) and gypsum (calcium sulfate), are used as calcium supplements and are usually completely digested by birds. Insoluble grit consists of items such as sand, quartz and granite and can lead to health problems (eg, impaction of the crop, proventriculus and gizzard) if it is overconsumed. Beach sand is shell (and soluble), but contamination with salt excludes its use."  

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Quotes from Comparative Avian Nutrition, 1999 textbook by Prof. Kirk Klasing of UC Davis.

Page 99 says "Grit is commonly trapped in the longitudinal folds of the [gizzard] cuticle and aids in grinding seeds. Grit consumption is correlated with the coarseness of the seeds in the diet and is lowest when birds are fed ground grains."  Pellets are primarily made of ground grains.

Page 239 says "The availability of calcium from large particles (grit) is dependent upon the digestive physiology of the bird. Most granivorous and herbivorous birds have the capacity to maintain large particles in their gizzard, where they can be slowly eroded over a period of days." Most parrots are classified as granivores.

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Other book quotes more or less in order of publication; different books by the same author are grouped together. These are the more interesting comments from the dozens of books I looked at.

Experiences with my Cockatiels (1950 edition) by Mrs. E. L. Moon - the various editions of this book are a fascinating look at the olden days of bird breeding

(Page 20) The following grit formula has helped so much to prevent egg binding by supplying the needs of laying hens. Don't waste it by spreading on the floor. If you want sand on the floor, use common sand or ordinary commercial grit, but keep a dish of this formula before them all the time. It is a little trouble to make up but can be made in one-half this amount if preferred:

1 lb. fine oyster shell
1 lb. powdered oyster shell
¾ lb. medium fine charcoal
1½ lb. clean sand or fine chick or pigeon grit 
5 oz. ground bone meal
4 oz. ground limestone
4 oz. iodized salt
2½ venetian red
¼ of an old soft brick pounded fine
2 oz. Conkeys Yo.

Yo is a yeast-oil food supplement and obtained from A. E. Conkey Co., Cleveland 5, Ohio. Venetian Red is mineral earth often used to color cheap barn or implement paint on the farm. It is used for the tonic effect of the iron it contains..

Your hardware or feed store can furnish most of these ingredients. This formula makes over five pounds and will last a year for one pair.

(Page 46) There is on the market now peeled millet, which is invaluable for hand feeding. It may be ground while the birds are small, and later added whole to their mush, if you have a large hole in the feeder - it won't come through a glass canary feeder. You can also add bone charcoal, which is better than ordinary charcoal for hand feeding. If you have no proper feeder and can't make one, your baby birds can be taught to eat out of a teaspoon, letting them bite on the edge of the spoon, but the feeder is much easier. When you feed whole millet, a tiny pinch of grit should be added to ONE feeding every other day. Use care that they don't get too much grit. 

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Cockatiels (1963) by Nancy Curtis - Proof that the pro-grit side also made some very dubious claims.

(Page 27) A bird has no teeth and the process of pulverizing and digesting the food is a complicated one. Without gravel, grit, oyster shell or charcoal, or a combination of these, a bird would soon become constipated and die. It is a tragic death to watch. Hence, grit of this kind (there are many on the market) must be kept available at all times. The grit tray is such an integral part of a bird's diet that he is more cautious of keeping it clean than any of his other food. 

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Tender Loving Care for Pet Birds (1977) by T.J. Lafeber DVM (veterinarian and owner of the Lafeber birdfood company)

(Page 13) Grit has long been considered by many aviculturalists as an essential ingredient in the bird's diet. Some question has been raised on this subject. Experience has shown that pet birds that hull their seed digest their food equally well with or without grit..

Seed eating birds seem to have a physiological need for grit in their gizzard. It is natural for them to seek and eat grit. Overeating can irritate and obstruct the gastrointestinal tract. This occurs when the bird is suffering from a deficiency or other ailment.

Grit should be presented in a grit-mineral mixture (grit and oyster shells or grit and minerals). Since grit remains in the gizzard for long periods, a few grains daily are sufficient.

(Page 18) In trying to compensate for a nutritional deficiency, a bird is apt to overindulge in grit. This could easily be compared to pouring sand down the kitchen sink. The effects are obvious in the sink. In the bird, it results in either an irritated intestinal tract or a complete obstruction, which would be fatal.

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Let's Celebrate Pet Birds (1989) by T.J. Lafeber DVM (veterinarian and owner of the Lafeber birdfood company)

(Chapter 6) Birds have a superior digestive system partially due to their gizzard. Non-existent in other animals, the gizzard has the power to crush and grind pieces of food into a smooth creamy paste. In this form food can be rapidly digested, thus allowing the bird to process relatively large volumes of food daily.

Grit aids the grinding process. The small pieces of sand, tiny pieces of rock or even granite provide the gizzard with ''teeth" to help chew the food.

While not essential in pet birds who hull their seeds, or are on manufactured diets, nature seems to tell pet birds to keep some grit in their gizzard.

In chickens, it has been shown that the size and strength of the gizzard is related to the hardness of the food and the presence of grit. With a well-developed gizzard, a stronger, healthier and larger digestive system occurs. The feeding of grit to pet birds may have these same advantages.

Commercially, grit is sold in pet stores as sand, fine pieces of granite or in grit mineral mixtures.

Birds on deficient diets when trying to find nutrients lacking in their diet will often overeat grit. Excessive grit can irritate and even obstruct the gastro-intestinal tract.

Grit placed in a dish or sprinkled on the floor - to be eaten free choice - presents no problem to birds on balanced diets.

Chapter 12 of this book does not mention grit, but it contains an interesting description of a bird's high-efficiency digestive system.  The book can be viewed online for free at Netpets.  

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The T.F.H. Book of Cockatiels (1979) by Wilfried Loeding

(Page 13 ) Another source of danger to [wild] cockatiels is motorists. The birds fly onto the road to pick up bits of gravel which they need for their digestion. Usually, however, they manage to take off before an approaching car gets too close.

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Lovebirds (1984) by Matthew M. Vriends PhD (ornithologist)(interesting because of the comments about wood chewing)

(Page 37) Included in the daily diet should be twigs or small branches of willow, hazel, peach, and elderberry to enable our lusty gnawers to gnaw to their hearts' content. An additional benefit is that some of the saps have high nutritional value. When the red coloring on the brids' plumage may appear to be fading, branches with pine needles attached are recommended to restore the original coloring.... Another element that should not be lacking in the diet is grit.

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Lovebirds, Cockatiels, Budgerigars: Behavior and Evolution (1987) by J. Lee Kavanau (professor emeritus, UCLA Department of Ecology and Evolutionary Biology) (This lengthy book has more than 1,400 references, most of them scientific publications)

(Page 285, describing a breeding study) Sharp-edged grit or gravel was provided in ordinary food trays and supplemented with bird mineral mix (but some parrots will ignore it unless it is scattered on the floor). The grit trays were cleaned of seed hulls or shells ("topped off") or renewed every 1 or 2 weeks, as necessary. A new supply of mix almost always is the objet of great attention and vigorous competition among Peach-faced Lovebirds, and not much less so among some Budgerigars. Though grit or gravel (or oyster shell) is not absolutely essential for granivorous birds, because hard seeds make a fairly good substitute for grit), it is highly desirable. Grit greatly aids the trituration (grinding) of food in the rhythmically-contracting muscular stomach and increases the latter's motility. It also makes digestion in this organ and in the small intestine (chemical and enzymatic breakdown) more efficient. For example, in the domestic fowl, grit increases the digestibility of whole grains and seeds by about 10%. It is recommended that minerals be supplied separately from grit, inasmuch as mineralized grits could result in excessive mineral intake, with possible kidney damage.

[Crocodilians also swallow stones and other hard objects ("gastroliths") that become resident in the muscular stomach and doubtless aid in the trituration of coarse food.]

Unlike most granivorous birds, parrots of this habit typically not only remove husks, they also fragment seeds mechanically with the mandibles. In the case of fig seeds (Ficus ovalis) only a very fine crack is made in the hull by Orange-chinned Parakeets (Brotogeris jugularis). The muscular stomachs of graminivorous [feeding on grass or the seeds of grass] parrots have a particularly well-developed epithelial koilin ("keratinoid") lining. The muscles of the muscular stomach in many species are fairly readily modifiable to meet the needs (hardness of food) of the diet. Accordingly, if a bird manages to survive long enough (without, or with inadequate, grit) for its ventricular musculature to adapt, it may become independent of grit.  This was true of the lovebird, "Jaws", which lived for years without grit. Removal of the muscular stomach, in fact, has little influence on digestion if only soft food is given.

Minerals and trace elements, some of which also can be obtained from certain types of grit, serve as dietary supplements. It was unnecessary to spread grit or gravel on the enclosure floors for the birds of this study, as some claim. If grit is made available to sick birds, it should be monitored closely. The reason for this is that such birds sometimes overeat grit, leading to impaction [the author cites a 1984 publication by Dr Harrison as the source of this information]. Caution also must be exercised in feeding parrots table scraps, such as pastries, as insufficiency of roughage in the diet can lead to wasting or atrophy of the gizzard and gut muscles. [Source: Bird Diseases by Arnall & Keymer, TFH Publications]

Commercial distributors assert that a charcoal supplement aids digestion and combats hyperacidity, through "sweetening the stomach." I know of no experimental evidence for this conclusion. Boosey, in his remedy for crop sickness, includes powdered charcoal together with 1 part of bicarbonate of sad and 2 parts of bismuth bicarbonate, but he presents no evidence for its effectiveness. On the debit side, charcoal adsorbs vitamins A, B2, and K from the intestinal tract, thereby creating vitamin deficiencies. Hart recommends against including it in grit for Budgerigars.  The most telling contra-indication is that charcoal is not used by veterinarians in treating birds. 

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Hand-Rrearing Parrots and Other Birds (1987) by Rosemary Low (ornithologist)

A question is often asked whether it is necessary to give grit to seedeating parrots while they are being hand-reared. Species which normally consume much grit, such as Cockatiels, can be given a little fine grit with the food twice weekly, after they are about three weeks old, as some birds do feed grit to their chicks. I have never given grit to the larger, omnivorous parrots.

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The Complete Book of Parrots (1988) by Rosemary Low 

(Page 67) Do parrots need grit? The answer is that some do and some do not. There are many different factors to take into account. In the wild, seed-eating birds of many species - not only parrots - swallow pieces of grit, shell, or small stones. These end up in the muscular stomach or gizzard (proventriculus) into which food passes after it has left the crop.  In the gizzard the food is ground up before the nutrients are assimilated into the body. This is achieved by the food's being rubbed against grit or other small hard items.  If such items are not present, hard items of food may leave the bird's body undigested. (Note, however, that undigested particles of food in the feces may have other causes, such as malfunction of the pancreas.)

To assess whether your birds need grit, consider the following factors:
1. Diet - birds such as lories and Hanging Parrots, whose food consists almost entirely of soft items, do not need grit; indeed, the gizzard is poorly developed.
2. Birds kept in outdoor aviaries may be satisfying their need for small hard particles in the gizzard by swallowing small pieces of stones from the floor, , by gnawing the wall and, I suspect, by swallowing small pieces of wood gnawed from perches or branches.  I know of a pair of Hyacinthine Macaws, kept at liberty, that consumed  large amounts of limestone, especially while rearing a chick. Birds kept in cages have no access to such items and should be offered grit at least once a month, although it may be ignored most or even all of the time.
3. Certain species seem to have a greater need for grit than others; for example, most cockatiels and budgerigars consume grit with great enthusiasm. It may be that ground-feeding species need grit the most, because their diet usually consists of a greater proportion of seed than that of tree-dwelling parrots. A ground-feeding Australian cockatoo, well known for its habit of picking up grit from the roads, is the Galah or Roseate. Many birds, probably mainly young ones, are killed by vehicles as a result.

Different types and sizes of grit are available, prepared specifically for cage birds. For example, one type consists mainly of crushed oyster shells, perhaps with the addition of charcoal, and another type is know as mineral grit and contains various particles that provide ingredients such as calcium, iodine, and lime. As most seed diets are deficient in minerals, grit can play an important role in supplementing the diet. The grit should be provided in small containers that are emptied and refilled regularly.

Grit should be limited for sick birds and those that have formerly been deprived of it and might take too much.

One valuable function of grit is that it can break down a foreign body of reasonable size that reaches the gizzard.  This was demonstrated to me in a seven-and-a-half-week-old macaw chick that I was hand-rearing. When it and its sibling were small, I was in the habit of using a plastic spoon to feed them, which is perfectly acceptable for young chicks. I retained the habit too long and one day a predictable accident occurred. The spoon broke in the chick's mouth and it swallowed a jagged piece that was probably about 1 cm square (.15 sq in).

Not only was this chick especially dear to me, it it also happened to be a Blue-throated or Caninde Macaw (Ara glaucogularis)[a rare species]. I was aghast at my stupidity. However, the piece of plastic never appeared in the feces and I felt there was a good chance it would stay in the gizzard. I therefore fed the young macaw a little grit once a week, and he suffered no ill effects.

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Cockatiels! Pets-Breeding (1990) by Nancy A. Reed

(Page 76) The use of gravel and grit has become a controversial subject lately. Some breeders believe it may be one cause of their birds "going light." Theoretically, birds that husk each seed should not need an abrasive to help digest the kernels. My large parrots that I have now had over fifteen years do not have any gravel mixture offered them, for the simple reason that all they do is dump the mix and mutilate the container. Yet they remain in top condition.

However, I do continue to feed my Cockatiels the gravel mix. (Some breeders substitute oyster shell for grit.) To date, I have never experienced problems with birds "going light," and the Cockies relish their daily offerings of the mineral supplement. Having talked with many people, I believe the gravel problem may mostly involve Budgerigars. However, some Cockatiel breeders have discontinued feeding gravel with no adverse effects. I personally choose to continue the supplement, both because my birds and enjoy it and have had no problems and because I am too Scotch to throw out the two-year supply that I have on hand. After that, I shall probably discontinue its use.

(Page 146, on setting up for breeding) A larger dish than usual should be used for the gravel-and-grit mix, as the birds will be devouring this source of minerals for developing eggs and young.  Some breeders add oyster shell to their mixture, but I feel there usually is a sufficient amount already in the mix, and too much can cause shells to be too hard for hatching chicks to pip through..

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The Complete Bird Owner's Handbook (1994) by Gary A. Gallerstein, D.V.M. This book was published at about the same time as Avian Medicine and reflects the same general attitude. The entire thing looks inaccurate however.  I have seen no evidence that grit-feeding was borrowed from poultry practices rather than being based on the birds' natural habits.  It was known that chickens and turkeys can digest hard dry grain without grit long before this book was published. I've seen no other claims that birds overeat grit out of boredom, and it's questionable whether crop impaction with grit is actually a real phenomenon in adult birds. Elsewhere in the book, the author says "commercially formulated diets can be fed as the sole source of nutrition", an idea that is now unpopular.

(Page 59-60) Grit consists of small pieces of sand and stones once thought to be crucial for digestion of food in the bird's gizzard. The notion of feeding grit to pet birds was "borrowed" from poultry. Chickens and turkeys eat whole seeds and require small bits of sand to grind off the coating of the seed in order to digest it.  Parrots, on the other hand, crack their seeds before they eat them. This eliminates the need for "grinding stones" in the gizzard. Smaller softbill species, such as canaries and finches, do require extremely small amounts of grit. Two pieces per week is probably adequate for these birds.

There is a danger in overfeeding grit to birds. From boredom or sickness, birds sometimes eat too much grit and can develop an impaction in the digestive tract. Sand-covered cage liners should not be used. Birds can eat this sand as well even if it's not called "grit."

Note: Grit is not necessary for parrots.

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Cockatiels as Pets (1998) by Howard Richmond

This need for minerals is recognized in another product prepared for birds, called “grit”.  Grit is a combination of digestively insoluble particles of gravel and other particles, such as crushed oyster shell, charcoal, mineral salts, and the like, all of which yield their mineral content in the course of digestion.  At present, the practice of offering gravel (and therefore grit) to cage birds is controversial, because gravel has lately been implicated in digestive illnesses.  The complicated questions involved will not soon be resolved. However, it can be said that here and there cockatiels are now living apparently healthy lives, both with access to gravel and without it.  But while it’s uncertain whether the insoluble particles we call gravel are beneficial to a bird’s digestion, there is no question about the necessity of minerals. Your cockatiel does require these, in whatever form they are offered: cuttlebone, mineral blocks, completely digestible grit, or powdered mineral supplements.

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The Cockatiel Handbook (1999) (previously published as The New Cockatiel Handbook, 1989) by Matthew M. Vriends PhD (ornithologist)

(Page 61) A good grit mixture is absolutely essential for cockatiels. Insoluble grit assists the gizzard muscles in the grinding of food, such as seed. There are various grit mixtures available. Sea sand and ground granite are insoluble in the stomach and act as grindstones until they are worn small enough to be passed through the system. (Soluble grit, including oyster shells, cuttlebone, and sterilized eggshells are quickly assimilated into the bird's body.)

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BSAVA Manual of Psittacine Birds (British Small Animal Veterinary Association) (2007) by Nigel Harcourt-Brown & John Chitty (editors)

In parrots, the gizzard is extremely muscular and has internal and external adaptations for grinding food with grit.

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Cockatiels: A Guide to Caring for Your Cockatiel (2006) by Angela Davids -- my personal nominee for "worst modern advice about diet I've seen in a print book". Not that I've seen too many; the internet age seems to have put a severe dent in the publication of bird-care books.

In the past, owners covered the floor of the cockatiel cage with a layer of grit, often consisting of ground oyster shells or a similar material high in calcium, in the belief that ingesting grit would help the bird grind seeds. But cockatiels and other parrots hull their seeds before swallowing them, so there really is no need to add grit to the diet. In fact, young cockatiels and sick birds can ingest too much grit and end up with clogged crops and subsequent infections and digestive problems. Additionally, there are sufficient quantities of calcium in pelleted diets and in special foods for laying hens, so deficiencies are not common. If you believe your bird needs more calcium, purchase a cuttlebone (which is the “spine” from the shell of a cuttlefish) from a pet store, and clip it to the side of your cockatiel’s cage.

Elsewhere in the book it says “Veterinarians recommend a diet of 85 to 90% pellets and 10 to 15% seed and other treats (which can include healthy human foods like vegetables and pasta)." My comments: Calcium deficiency is actually quite common as demonstrated by the large number of egg binding cases caused by it. Some vets think a diet of 90% pellets is not good for cockatiels and 30-50% is more appropriate. Leaving it up to the pet owner to decide whether the bird needs more calcium is a bad idea; most pet owners aren't qualified to make that determination, and authors shouldn't assume that everyone feeds a diet of 90% pellets.  It makes more sense to provide cuttlebone, mineral block, or soluble grit, and let the bird decide whether it's needed.

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A Guide to Cockatiels and Their Mutations as Pet & Aviary Birds (2007) by Dr. Terry Martin BVSc & Diana Andersen (an Australian publication; many people there are still pro-grit in the 21st century)

(Page 36) It is debated whether grit is necessary in the diet, as very little appears to be consumed. However, insoluble grit aids digestion by grinding up seed in the gizzard and is particularly important for birds kept in pet cages or aviaries without access to sand and dirt floors. Soluble grit is digested by the acid in the proventriculus and therefore not used for grinding purposes. The soluble form is a good source of minerals and calcium - essential for general wellbeing and successful breeding. 

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Cockatiels (2008) by Thomas Haupt - another pro-grit book published in the 21st century, or almost in the 21st century depending on whether you go with the first edition or the second edition (the first edition was published in German and English in 1998). I don't have information on the general European attitude toward grit, but perhaps they are less anti-grit than Americans tend to be.

(Page 33) In nature, birds absorb minerals by eating some earth. As a substitute, you should give your cockatiel bird grit. This is a mixture of different kinds of rock, seashells, oyster shells, cuttlefish, and limestone. You can also buy a mineral powder that you mix in the bird's food. 

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Internet sources  

Pro-grit experts

Rosemary Low: Minerals and Grit, of Vital Importance - on the website of the World Parrot Trust. She says it's a myth that grit is harmful, and that owners who do not offer grit may be withholding substances of unknown value. She points out the dangers of birds eating inappropriate grit substitutes, and says that our birds should be the judges of whether or not they need grit. 

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EB Cravens: Views on Mineral Grit for Parrots - on the website of the World Parrot Trust. He says he has observed both wild and captive birds eating grit; that he hasn't been able to find anyone who personally observed a parrot larger than a cockatiel who died of crop impaction from grit; that caution should be used with fledglings; and that sand, soil, and clay are integral parts of the healthy psittacine diet.

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EB Cravens: Ask an Expert - on the website of the World Parrot Trust. He says

"truth is, field studies have shown that psittacines of all kinds do go to the ground and ingest grit for many reasons--they even feed it to the chicks in the nest. My breeder parrots, all of which have access to the ground, will begin to ingest soil and sand and crunchy substrate one to two weeks before the laying stage and continue to eat grit well through the first weeks of chick feeding.


"The key for the pet owner is what species you are keeping---certain parrots like cockatiels, lovebirds, princess and other ground foraging species (including cockatoos) will take more grit than eclectus, capes, lorikeets and the like. Soft food eaters need less grit than do seed eaters such as budgerigars.


"Finally, it must be emphasized that the addition of grit to the domestic parrot diet can be done very safely if one gives small salt and pepper amounts of clean bird grit to the diet once every two or three weeks."

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Holistic Bird grit article - This article displays comments from a forum discussion. It's kind of hard to tell where one person leaves off and another begins, but if you look carefully you'll see the name at the end of each bit. First off is a negative opinion by Patrick Thrush. His training is in psychology/sociology and his claim to fame in the bird world is that he is the full spectrum lighting guru. Personally I don't take him too seriously on diet issues since he said elsewhere that birds can use vitamin D2 in plant sources, which is contrary to what the experts think - they say birds use D2 poorly and need D3, which is very rare in plants. The D2 in plants is mostly found in fungi, which are not normally used as a food plant.  There don't appear to be any known food plants that contain a meaningful amount.

The next comment is by Mike Owen, who was Australia's representative to the World Parrot Trust at the time.  He reports his personal observations with both captive and wild birds and says they do eat grit and problems with grit are nonexistent in Australia. His most significant statements are posted below.  Carolyn Swicegood (a proponent of natural foods for birds) takes a middle of the road approach, noting the risks and saying that free choice (unlimited) grit might not be wise, which is a reasonable position. Next is a naturopath who mentions the nutritional benefits of silica aka sand.

Last of all is Susanne Russo, a very experienced cockatiel breeder widely known as srtiels on cockatiel forums and the webmaster of the International Cockatiel Resource.  She says her birds have been eating sand and feeding it to their babies for years with no ill effects. I contacted Susanne to confirm that these were her statements, and she provided the additional information that she was talking about her cockatiels not some other species, and that the local sand (in Florida) contains traces of wind-blown salt from the ocean, which might be a factor in the birds' desire for the sand.

Here are the most significant statements from Mike Owen's section.

"Almost all Australian parrots have access to grit... we NEVER, I repeat, NEVER, see any problems with grit. My vet, President of the Australian chapter of AAV, has only ever seen two instances of impacted crops in 15 years of practice.::

"One was a tiel with a crop full of human hair from over preening his owner, and the other a lorikeet which had gone crazy with mango (understandable) and had a crop full of mango fibres. It is one of the great mysteries of aviculture why North American parrots are apparently so willing to get their crops full of grit and suffer impaction, while Australian birds never get this problem."

"I can assure those doubting that wild parrots intentionally consume grit that, from my own observations, they do deliberately pick up and swallow grit. I have watched flocks of 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."

"What is surprising is the large amount of grit usually to be found in the gizzard of an autopsied wild parrot. A Rosella for instance might have up to 50 grains of grit in the gizzard. These range in size from perhaps white French millet size, down to almost microscopic size - presumably reflecting the amount of wear that the grain has undergone. At a recent Parrot Convention held in Grafton that I attended, one talk was by an avian vet, and was a demonstration of how to autopsy a parrot. The "victim" was a road kill Galah, and it's gizzard had a pile of grit in it - almost half filled!"

"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."

"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."

"Perhaps, as some later replies have alluded to, an absence of grit might lead to a long term digestive system problem due to inefficient absorption of nutrients due to inefficient grinding of seed. Such a deficiency may never show up as a primary cause of early death, but may result in a depressed immune system, and result in a bird having an earlier death than it should have."

"The grit used by Australians tends to be whatever is handy. As long as it is small (about millet size) then it doesn't seem to matter, if it is quartz, crushed volcanic rock or any other rock. Some use river sand, others beach sand, and others quarry crushings... crushed shell grit is useful only as a calcium source, it dissolves too quickly, and is too soft, to be of value in gizzard grinding."

"While I am happy and relaxed about giving my birds unlimited access to grit, I neither encourage nor discourage anyone else to provide grit. It is a choice that has to be made by each individual, like the choice to feed pellets or seed (or neither!). And, as always, if in doubt - DON'T."

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Dr. Scott Echols is a board certified avian veterinary specialist with a high level of science-based knowledge about avian nutrition.  He owns the Nutrition for Pets Facebook group and is active there, answering members' questions about pet nutrition. In this thread he responded to a question about grit [you have to join the group to actually see the thread]:  

"Next question, do pet birds need grit? The quick answer is no. But there are caveats. In general grit can aid the digestion of tough and/or fibrous foods. Wild type diets include non-digestible foodstuffs in addition to the nutritive components. Not everything a wild bird eats is actually edible and nutritious.

"However, there is another benefit to grit in wild bird diets. Grit can help toxic items pass- especially some forms of heavy metal. This has been studied and proven in waterfowl that filter feed through mud and silt. They often pick up fine metal (and presumably other potentially toxic non-nutritive) particles as a normal part of feeding. Grit can help the bird naturally remove those toxic particles. So in some situations, grit may help pet birds (passerines like canaries, parrots and more) that have ingested select solid (metal form, or other) toxins.

"There was at least one comment about grit causing impactions. I would say this is very rare. More likely the bird gets dehydrated (typical with any disease), there happens to be grit in the intestinal tract (very common as many birds are fed grit) and is found in a clump leading to the conclusion that the grit caused the impaction. In 20 years of practice (and lots of necropsies), I have never diagnosed what I would consider a grit impaction and cause of death... Same thing with crop impactions, I have never diagnosed it caused from grit. However, I have seen a number of crop impactions with grit in chickens almost always secondary to Marek's disease (which caused damage to the nerve that operates the crop)."

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Avian Veterinarian Dr Ross Perry is a holistic vet in Sydney, Australia. He wrote Chapter 1 of Avian Medicine, including some comments on grit (see Quotes from Avian Medicine). The book as a whole takes a very conservative approach to the issue, and the comments in Chapter 1 reflect this. But outside the restrictions of the book, Dr Perry takes a more balanced and naturalistic view of grit. He had this to say on the ICR Facebook page:  

Although I wrote conservatively one of the quotes from Avian Medicine, parrots and cockatoos in my care regularly, at least weekly, are offered a selection of mixed soluble and insoluble grits. Because of pollution of the ocean with various heavy metals, whereas I used to collect shellgrit and cuttlebone from the beach, boil it and dry it, it might be prudent to use some crushed natural limestone or tiny shells from ancient times available in some rock deposits. As for insoluble grit, a hammer and a piece of sandstone, or some fine aquarium sand gravel, can go a long way.

Heavy metal poisoning with partial paralysis of the gut, and bornavirus infections have often not been recognised in the past, in my opinion, and might be at the basis of many so called grit impactions. Yet it has been shown that feeding insoluble grit in limited quantities to parrots and cockatoos with metal fragments in their gizzards is more effective in displacing and perhaps breaking down the metal fragments than giving peanut butter and metamucil... I ended up doing both!

Grit might not be neccessary for parrots and cockatoos solely on pelleted diets and soft greens, However, I advocate providing some food at least in its natural form including seeds of grains, gum nuts, wattles, grevilleas etc even if most of the diet is pellets. Hence I provide grits!

Apart from lorikeets I cannot remember necropsying a wild Australian parrot or cockatoo and noting an absence of insoluble grit in its gizzard. (I can hear some smart A saying they must have all died from grit impaction.... I don't believe that for a moment.)

Also note that those who have done many necropsies on cockatoos, parrots, finches and canaries (as well as poulty and many other birds) know that the normal gizzard is a thick muscular organ with a thick inner leathery koilin layer in direct contact with the gut contents in that region. It is perfectly designed for grinding insoluble grit and seeds so that the latter become a paste or emulsion before they exit into the duodenum. The smaller the seed fragments are ground before entering the duodenum, the bigger their surface area for digestion in the duodenum rather than fermentation lower in the gut.

Canaries and finches normally have sand grit particles in their gizzards too. On the other hand nectivorous lorikeets have a relatively soft flabby gizzard and a thin non-leathery koilin layer and don't usually have insoluble grit in their gizzards. Palm cockatoos and other black cockatoos have huge beaks not for swallowing large seeds but for chipping away and slicing large seeds into tiny fragments before swallowing them. I don't remember whether they normally have grit in their gizzards yet I imagine yes. Sulphur crests, little and long billed corellas, galahs, Major Mitchell cockatoos and cockatiels all as wild birds have grit in their gizzards.

I choose to be guided by Nature as my best teacher. When the proventricular gizzard junction becomes diseased as with "Megabacteria " avian gastric yeast infection, or the gizzard becomes diseased with gizzard worms or a penetrating piece of wire or bornavirus infection as examples, the grinding action of the gizzard becomes impaired and an immediate clue when observed to such disease processes is the recognition of visible seed fragments in the faeces. (Microscopic examination by an observant specialist willl often detect microscopic clumps of undigested food earlier in the disease process).

However, the most common cause of undigested seed fragments in the droppings of budgies and cockatiels and various parrots can simply be addressed by providing a teaspoon of crushed sandstone or river sand with insoluble grit particles of a range of sizes suitable for the gut of the species of bird... insoluble grit deficiency. In simplistic metaphorical terms the teeth of cockatiels and budgies and most parrots are not in their mouths, they are in their gizzards, and easily replaceable with more insoluble grit when they are worn out or become so small that they easily pass into the duodenum.

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Avian Veterinarian Louise Bauck: Diamonds Aren't Forever - tells an amusing story of a cockatiel that ate a large diamond from a man's earring in a quest for grit and passed it without incident three weeks later. She talks about the dangers of unauthorized grit substitutes and suggests giving safe and healthy grit to pet parrots to provide a safe outlet for the bird's natural desire to eat grit.

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Dr. Hanson on JustAnswer.com - This one shouldn't be taken too seriously. An avian vet with 30+ years of experience who also bills himself as an ornithologist and aviculturalist makes overconfident assertions about things that haven't been proven - no one actually knows why wild cockatiels eat charcoal, just that they do it, nor does everyone agree that grit is essential. Then Patricia, a bird behaviorist (not a diet-related field obviously) tries to overcome the assertions of a vet with a PhD by linking an article on Winged Wisdom, written by a person with unknown credentials. It's overkill on both sides, although it does look like Dr Hanson knows a lot more about the physiology of grit eating than Patricia does.

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Southeast Texas Avian Rescue - reproduces a now-vanished article from the Lafeber website, along with some introductory comments about their own grit-feeding practices.

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EclectusParrots.net - not truly an expert opinion since the author's identity is unknown. Sensible comments echoing the experts who have already been cited.

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Essential Parrot - also not an expert opinion, but a 2012 blog entry by "Clara" with a simple analysis of the pros and cons of grit.

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Impaction and crop function

Bird Health with Dr. Rob Marshall - a general discussion on crop impaction in poultry.

Wikipedia gizzard article - says "gizzard stones or gastroliths and usually become round and smooth from the polishing action in the animal's stomach. When too smooth to do their required work, they may be excreted or regurgitated."

Discussion on the Backyard Chickens forum - says "The grit gets trapped in the folds of muscle inside the gizzard, and as food (grain) works it's way into the gizzard from the crop, it is ground up into a form that the chicken's intestines can break down further and digest... If there is not enough grit, the particles of food can become trapped in the folds of the gizzard instead of the grit, and when it spoils it causes problems."

Chicken Health for Dummies - recommends offering crushed granite grit as part of the cure for crop impaction.

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Wild behavior

If you go to Google Scholar and search for parrot grit, the results show snippets of text with these words in it. But when you click on the link, most of the time you get an abstract that does not mention the grit, and it’s expensive to view the whole paper. Unless stated otherwise, the references below show text snippets from the list of results (which may not be a complete thought or a complete sentence), not material from the abstract.

Notes on the diet of the critically endangered orange–fronted parakeet (Cyanoramphus malherbi) on Maud Island
Luis Ortiz–Catedrala & Dianne.H Brunton, New Zealand Journal of Zoology, 2009
From the actual abstract: Orange-fronted parakeets consumed fruits and leaves of 14 plant species as well as non–dietary items such as bark sticks and grit. Of dietary items, 96% were on plant species and 4% invertebrates. From the Google Scholar search results: Other parrot species have been reported consuming bark and grit but the specific function of these in parrots is unclear (Gilardi et al. 1999; Symes & Perrin 2003).

Interspecific and locational differences in heavy metal levels in four species of birds near Sydney, Australia
J Burger, M Gochfeld - Environmental monitoring and assessment, 1999
Birds eat grit (particularly the parrot and the dove), while other species consume dirt regularly, if inadvertently, while feeding.

The Swift Parrot
KA Hindwood, M Sharland - Emu, 1964
Sand and grit, apparently taken as an aid to digestion, have also been found in stomachs.

The Diets of Three Species of Parrots in the South of Western Australia.
JL Long - Wildlife Research, 1984
From the full version of the paper: Both charcoal and grit, but mainly charcoal, occurred in the gizzards of all three species. Together they made up 13.7% of the total dry weight [of the crop contents] for the red-capped parrot, 19.8% for the western rosella, and 20.1% for the Port Lincoln parrot at Balingup. At Wickepin they accounted for 45.4% in the western rosella and 29.8% in the Port Lincoln parrot.

That's quite a difference in grit and charcoal use between the two locations. Looking at the paper's charts on food consumption, the most noticeable difference between them is that the Wickepin birds ate a lot of casuarina, which wasn't on the food list at all for Balingup.  Maybe they need some extra help to process casuarina. The Casuarina Control Coalition says that the plant has potent chemical defenses in all its component parts, including the fruits and seeds.

The use of dry weight makes it hard to figure out what the "true" percentage of the diet is for the grit and charcoal, since stone doesn't have any water weight to lose and charcoal would have little or none, while the actual food has plenty.  But the amount of grit and charcoal is obviously fairly significant. I wish they had given specific percentages for grit and charcoal individually, but they said it was mainly charcoal so I have to assume that charcoal was more than half of the combined total. It's interesting that the red-capped parrots who liked to feast on cyanide-laden apple seeds ate less grit/charcoal than the species that weren't so keen on apple seeds. 

Another piece in an Australian ornithological puzzle–a second Night Parrot is found dead in Queensland
A McDougall, G Porter, M Mostert, R Cupitt, S Cupitt… - Emu, 2009
X-ray analyses revealed no seed in the bird's digestive system although mid-density stones or grit, possibly ironstone, was found in the gizzard

Parrots of the World (Forshaw) page 238 - "Crops from [Mulga Parrots] collected in south Australia contained scraps of charcoal, fine grit, vegetable matter and numerous seeds (Lea & Gray)."

Feeding Ecology of the Cockatiel, Nymphicus-Hollandicus, in a Grain-Growing Area
D Jones - Australian Wildlife Research, 1987
[The LFB website has a writeup on this paper here. The following quote is from the actual paper. The birds in the study had a diet of 80% unripe sorghum which is not the natural feeding pattern and may have affected their grit consumption.]

Pieces of charcoal 0.5-4.0mm in length were found in 29% of all crops, and small pieces of minerals, mainly quartz, in 13%.  Only four crops contained non-food items other than grit. There were mainly small pieces of dense woody material, probably fragments of the tree trunk, bark, and leaves, and were of negligible weight. One crop contained a large collection of non-food material, including bark, charcoal, a very large number of pieces of quartz and two fragments of unidentified insect exoskeleton.  This was the only crop to contain any insect remains.

Another case where charcoal was more plentiful in the crop than stone grit. Absorbing dietary toxins is the reason that's often assumed for eating charcoal. But I have doubts about how toxic the diet of wild cockatiels is, and charcoal can interfere with the absorption of certain vitamins (A, B2 and K). So I was never too sure that the bird actually got a net benefit from it. But the study by Majewska et al indicates that they do, and it's not just with plants that are naturally loaded with cyanide and other poisonous chemicals. It's also effective against bacterial toxins and mycotoxins. It seems to me that these toxins might be a bigger threat to wild cockatiels than the chemical defenses of the food plants. The natural diet of cockatiels is mostly grass seed, and there doesn't seem to be any species of grass that's considered toxic. But there can be some nasty stuff that grows on grass, like ergot, and charcoal may be useful for counteracting it. 

Cockatiels in the Wild - Nonscientific observation of wild cockatiel behavior which says "During the dry seasons they eat the seeds of various grasses and other plants, and they pick up some sand to aid digestion."

Tambopota Macaw Project: Birdtalk Magazine article - "Evolving a Parrot's Diet" is an easy to read article describing the crop contents of wild macaw chicks, which included pieces of bark fed to them by their parents.

The Wildlife Protection Foundation: Tambopata Macaw Project - Another plain-English discussion of the macaw diet study, including feeding bark to chicks. 

Effects of Diet, Migration, Breeding on Clay Lick Use by Parrots in Southeastern Peru. (link leads to complete paper)
Donald Brightsmith, Ph.D. - 2004 - American Federation of Aviculture 2004 Symposium
[Not directly grit-related, this paper discusses the seasonality of clay lick use. It appears that the clay licks are not used as a calcium source during egg formation. Usage peaks when there are babies in the nest, suggesting that the parent birds are obtaining sodium for their chicks.]

Parrot colpa and geophagy behaviour from the El Gato region of the Tambopata-Candamo Reserved Zone, Amazonia, Peru. (link leads to complete paper)
KA Hindwood, M Sharland - Emu, 1964
There is also evidence that geophagy provides grit to many weak-billed birds (Best & Gionfriddo 1991)

Activity, Behaviour and Interactions of Parrot Species at a Peruvian Clay Lick [sorry, there isn't a functional link]
EM Shaw - macawproject.org
... This study presents some of the first detailed empirical data on the behaviours of eight parrot species at a Peruvian lick ... Some reasons cited for soil consumption - obtaining grit for the mechanical breakdown of food, buffering of gastric pH, and treatment for diarrhoea - are largely discounted for parrots (Brightsmith & Arambur˙ 2004, Diamond et al. 1999, Gilardi et al. 1999).
 

Diet and foraging behaviour of the Rosy-faced Lovebird Agapornis roseicollis in Namibia
H Ndithia, MR Perrin - Ostrich-Journal of African Ornithology, 2006
... and several parrot species in South America (Gilardi 1987) and Australasia (Diamond et al. 1999). The function of soil ingestion was not clear, but Diamond et al. (1999) presumed that the grit assisted with the mechanical grinding of ingested food.

Geophagy in New Guinea birds
CT Symes, SJ Marsden - PsittaScene, 2003
From the abstract: We report geophagy for the first time for New Guinea birds: four or five parrot species... The ingested soil was much too fine-grained to be useful as grit; it contained only modest levels of all 14 minerals analysed; it lacked buffering capacity; and there was no evidence that it protected against diarrhoea. Instead, the soil's high measured cation-exchange capacity, high content of cation-binding minerals and binding of large quantities of tannic acid and quinine suggest a different hypothesis: that geophagy in this case served to bind poisonous and/or bitter-tasting secondary compounds in ingested fruits and seeds.

Grey Parrots of the Congo Basin Forest - The most scientific of the African grey sources. It says "not only do Greys come to the clearing to forage on vegetation but also to engage in geophagy, or soil-eating."   A chemical analysis was done on the soil and it appears that a bachelors degree thesis was written on the subject. I have been unable to contact the author for more information, but Africangreys.com reports that the soil was rich in minerals.     

Parrots of the World (Forshaw) page 288 - "Chapin noted that bits of quartz are occasionally found in the stomach contents [of Grey Parrots] and to get these the parrots must come to the ground."

Seneca Park Zoo: Congo African Grey Parrot - "African greys can commonly be seen in the wild on the ground at waterholes where they ingest mud and minerals. This is thought to be an adaptation to compensate for increased pesticide and toxin levels in their food – the soil eaten absorbs the toxins that they would otherwise ingest." Their assumption about the reason for the soil-eating is questionable, but the observation is expected to be sound.

Companion Parrot Online: So Very Grey - "Scratching and digging is a common behavior for African greys and doesn’t always have to do with nesting behavior. In the wild, greys feed on calcium rich grasses that grow in shallow pond-like depressions in forest clearings. It seems that a great deal of the scratching and digging is primarily a feeding behavior. Bob and Liz Johnson from Florida sent me a video of their large planted aviary and the many birds that live in it. I noticed dirt flying out of the ground with no explanation until I saw a small patch of red backing out of the hole. The red turned into an African grey. It backed out of the hole and then looked into it as if to check his work. Then he headed back into the hole and disappeared except for more flying dirt."  We don't know what the bird was searching for in this anecdote, but apparently he wasn't finding it.

Parrothouse: Grey Matters - "Greys in the wild often feed on the ground and dig in the soil."

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Medical Case Studies of birds with grit in the crop, two in the US and one in Tasmania. For two studies, the abstracts do not mention grit, the full paper is not available without a subscription, and the quoted material is what google displayed in the search results.

Grit Impaction in 2 Neonatal African Grey Parrots (Psittacus erithacus erithacus)
Thomas P. Ryan DVM, Dipl ABVP (Avian) - Journal of Avian Medicine and Surgery, 2002
Two 3-week-old African grey parrots (Psittacus erithacus erithacus) were each presented with an impacted crop. An inappropriate diet fed to the parents and a displaced parental instinct for feeding mineral and grit to the offspring were postulated to be the causes of the crop impactions. Surgical intervention relieved both impactions and recoveries were uneventful.

Dysplastic koilin causing proventricular obstruction in an eclectus parrot (Eclectus roratus)
R De Voe, L Degernes, K Karli - Journal of avian medicine and surgery, 2003
... The next morning, the parrot's clinical condition appeared slightly improved. ... The ventriculus and diverticulum contained a moderate amount of grit, seed, and barium sulfate (Fig 2). Barium sulfate and a scant amount of seed were present in the proventriculus. ...

Grit was NOT the cause of death.

Renal disease in captive swift parrots (Lathamus discolor): Clinical findings and disease management
BD Gartrell, SR Raidal, SM Jones - Journal of avian medicine and surgery, 2003
... gout, and an inflamed duodenum; the kidneys were pale and swollen with urate tophi present (Fig 2). The gizzard contained quartzite grit and occasional ... An adult male swift parrot (bird D) showed an acute weight loss of 8 g in 1 week ( 11% of body weight); mild generalized...

Grit was NOT the cause of death.

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Other studies

The 177-page thesis in the link below seems to pretty much represent the current state of the art in our general knowledge about grit, and the entire work is available for public viewing. It never mentions parrots but talks about many other species, and the general principles are likely to be applicable to many parrot species.  Selected quotes are printed here.


Evaluation of factors influencing grit use by birds
Gionfriddo J. P.; PhD thesis, 1994, Iowa State University
The value of the grit particles found in birds' gizzards has been recognized for more than two centuries... Grit use is widespread among birds, and the value of grit in increasing avian digestive efficiency has been demonstrated.  Grit also is known to provide supplementary calcium and other minerals which may be critically important to granivores and other species with low-calcium foods...

Within a species, an inverse relationship sometimes exists between mean grit size and the number of grit particles in the gizzard, indicating that birds consuming smaller grit generally use more particles. Because birds use grit to improve mechanical grinding of food in the gizzard, the value of (and need for) grit should vary with diet. Several authors have noted greater grit use when diets consist of hard, coarse foods such as seeds and other plant material...

The amounts and characteristics of grit in bird gizzards depend, not only on selection of grit particles by the birds, but also on retention of at least some of those particles in the gizzard. Retention of individual grit particles is influenced by the rate at which grit is ingested. When birds have free access to grit, they may consume and eliminate considerable amounts daily. On the other hand, birds suddenly deprived of grit can reduce their output of grit and retain particles in their gizzards for long periods.  Other factors, including grit size and diet, also may influence grit retention in the gizzard. Some particles may be retained longer than others because of their size.  Diet can affect retention in several ways. For example, coarse, hard diets may increase the grit ingestion rate and thereby reduce retention.  Hard diets also may reduce grit retention by accelerating grit particle disintegration and elimination.

Avian grit use also is influenced by diet. The ultimate (functional) cause of many grit-use shifts probably is seasonal dietary changes that produce variation in the value of grit. Field studies have documented seasonal diet and grit-use changes in several avian species. Hogstad reported that grit use by Bramblings was much greater when they consume seeds than when they shifted to soft insect larvae. As Dunnocks changed their diet in late summer from insects to seeds and insects, their grit use increased significantly. A similar association between increased grit use and greater consumption of hard (usually plant) foods also has been documented in other research...  Experiments with Willow Ptarmigan showed that birds consuming coarse food (twigs and buds of willow and birch) ingested and excreted 2-4 times as much grit as birds fed pelleted food, and that ptarmigan kept on a constant diet maintained a constant grit intake throughout the year. 

The retention of individual grit particles in birds' gizzards is highly variable. Under certain conditions, such as when birds are denied access to grit, retention may be very long, even >1 year. On the other hand, when birds have daily access to abundant grit sources, they may continually replenish grit in their gizzards. In the latter instance, many grit particles may be retained only briefly in the gizzard, passing completely through the digestive tract in a few hours.

Grit generally is found in the gizzards of most species that eat plant parts and many that eat insects. In the present study, we found grit in the gizzards of 62 of 90 species... Grit use often varies with such factors as the age of the bird, diet, sex, and reproductive status, and the characteristics and availability of suitable grit particles. The large amounts of grit we observed in gizzards of Ringneck Pheasants, American Tree Sparrows, and House Sparrows are not surprising because these species feed on the ground, mainly on seeds... Our finding no differences in mean grit size, mean grit shape, and mean grit surface texture among birds consuming different diets indicates that different foods do not require different types (sizes, shapes, surface textures) of grit for adequate digestion... the availability of grit probably plays an important role in determining the characteristics of grit in birds' gizzards.

As the benefits of grit use became more widely known, the practice of providing grit to birds became increasingly widespread. Feeding grit to poultry is generally considered a wise economic practice...  Several authors have contended that some birds require grit for digestion and would weaken and die if deprived of it [gallinaceous birds and Ring-Necked pheasants are mentioned as birds possibly requiring grit; however it is established that chickens and bobwhite quail (both gallinaceous) do not require grit].  Although grit use may be highly beneficial to birds, it does not seem to be essential to the survival of birds receiving adequate nutrition. Studies of poultry have shown, for example, that although grit use hastens and improves digestion, it is not essential to survival, growth, or egg production.  Moreover, birds whose gizzards have been removed may live indefinitely, although they may show a reduced ability to digest coarse foods... For dinosaurs and crocodilians, the relative importance of stomach stones as ballast versus grinding agents is uncertain. Siegel-Causey suggested that stomach stones first were used by reptiles because of their value as ballast, and later their use was retained in avian basal groups because of the (previously secondary) digestive benefits.

Powerful muscular contractions of the gizzard crush and grind food items against the dorsal and ventral grinding plates (greatly thickened portions of the cuticle that lines the inner surface of the gizzard). The presence of grit particles in the gizzard is thought to improve the efficiency of this process by providing hard, moving grinding surfaces within the food matrix... A second function commonly attributed to grit use it the supplementation of minerals, especially calcium, in the diet... Several other possible functions of avian grit use have been proposed.  The presence of grit in the gizzard may enhance digestion by further stimulating the secretion of digestive fluids or by facilitating the action of such fluids.  It also may help by stirring and mixing the digestive enzymes and food particles in the digestive tract or by slowing the rate of food passage. Finally, grit may be ingested as a source of trace elements needed by birds.  Several researchers have reported that grit causes changes in the condition of the digestive organs. Tagami and Kuchii stated that grit use may have favorable physical or physiological effects on intestinal tract tissue.  Gizzards of domestic chicks fed grit are often larger and heavier than those of chicks deprived of grit. Grit use is probably not necessary for proper gizzard condition, however.

The value of grit as a grinding agent and nutritional supplement greatly depends upon its mineral composition, which determines grit hardness, solubility in the avian digestive tract, and nutritional value. For this reason the composition of grit particles is sometimes a major determinant of avian use. Most studies have found mainly quartz grit in the gizzards of wild birds... Because of its hardness, quartz might be expected to be more efficient than limestone as a grinding agent in the gizzard.  In experiments with chickens however, both limestone and quartz achieved the same beneficial effect (improved digestion of feed) while they remained intact in the gizzard. 

Mathiasson concluded from field and experimental evidence that the type of food eaten has an immediate effect on the process that regulates grit retention in the gizzard. He suggested that tactile receptors in the gizzard mucosa receives information on the number of grit particles present, and that the consistency (hardness or softness) of the food in the gizzard determines the rate at which grit particles contact these receptors.  The number of particles retained therefore depends on the hardness of the food... To what extent the gizzard is selective in the retention of individual grit particles is not clear... Whether the excessive accumulation of grit in the gizzard is prevented by periodic evacuations of the gizzard or by a slower, more constant turnover is unknown... Grit disintegration in the gizzard may be substantial. 

Birds often consume non-food items other than stones and rock fragments. These items are sometimes retained in the gizzard where they seem to function as grit substitutes. Among the materials found in birds' gizzards and reportedly serving as grit were hard seeds, insect parts, small snails and shells, shell fragments, fossils, lead shot, bones, teeth, and coral. Grit availability and the specific food habits of a species seem to determine to what extent hard seeds are used as a substitute for grit particles. Even when grit is available, however, some birds use hard seeds instead.

Grit-use patterns in North American birds: the influence of diet, body size, and gender
James P. Gionfriddo and Louis B. Best - The Wilson Bulletin, 1996
We investigated avian grit use by examining the gizzard contents of 1440 birds collected from 12 states. Grit was present in gizzards of 62 of 90 species and varied greatly in number and mean particle size. Gizzards of granivorous birds contained more grit particles than those of insectivores, omnivores, and frugivores. Grit particle characteristics (mean size, shape, and surface texture) did not differ among birds consuming different diets. Mean grit size increased linearly with the common logarithm of the bird body mass.  Within avian species, grit-use patterns did not differ by gender. Grit use is widespread among birds, and diet strongly influences the amount of grit used by birds.

Grit use varied seasonally and with diet in wild sparrows.  Captive sparrows replaced their grit supply every five days on average.
Grit use by house sparrows: effects of diet and grit size
Gionfriddo J. P.; Best L. B.; - The Condor, 1995
Free-ranging House Sparrows (Passer domesticus) were captured with mist nets in central Iowa from August through March, 1990-1993, and their gizzard contents were used to compare grit use by sex, season, and diet. Males and females did not differ in mean grit amounts or sizes (overall mean size = 0.5 mm) in their gizzards. Gizzards of birds captured during March and August contained more grit than those of birds captured during September through February (x = 674 vs. 477). Gizzards containing >75% animal material (insects) had more grit than those containing >75% plant food (x = 681 vs. 531). Aviary experiments then were conducted with captive House Sparrows to evaluate the effects of diet and grit size on grit choice and retention. When birds were given grit particles 0.2-1.4 mm in size and either soft animal food (canned dog food) or hard plant food (wild bird seed), grit in gizzards of birds on the two diets did not differ in mean number or size. When birds were given both animal and plant food and either small (0.2-0.4 mm) or large (1.01.4 mm) grit, gizzards of birds consuming small grit contained 5 times more particles than those of birds consuming large grit (x = 275 vs. 51). In experiments evaluating grit retention, most grit in gizzards was replaced within five days. Grit replacement rates were unaffected by diet, but birds given only hard, plant food averaged more grit per gizzard than those given only soft, animal food (x = 538 vs. 205). Gizzards of House Sparrows given only small grit consistently retained grit longer and contained more particles (x = 853 vs. 174) than those of birds given only large grit.

Identification, distribution, and function of gastroliths in dinosaurs and extant birds with emphasis on ostriches (Struthio camelus)
Oliver Wings - Ph.D. Thesis, The University of Bonn, Germany, 2004
The results of a study on German (n=135) and South African (n=212) free-ranging farm ostriches ( Struthio camelus ) indicate that ostriches ingest stones of greatly varying size. Adult animals typically hold one kilogram of stones in their stomach. On average, each stomach contained several thousand gastroliths with a grain size >1 mm. The mean gastrolith mass is very similar in both populations and constitutes about 1% of the mean body mass. Gastrolith mass is not significantly correlated with gender, age, season, and food contents by weight. Quartz is the predominating mineral type among ostrich gastroliths.

Studies of the grit requirements of certain upland game birds
Lester J. McCann - The Journal of Wildlife Management, 1939
My comments: This study on ringnecked pheasants and bobwhite quail found that when grit was removed from the birds' diet, they increased their retention of the grit that was already in their crops. However they also lost weight.  The entire paper is not available at the link, and the introduction to the paper does not state the final conclusion the way a modern abstract does.  So we don't know what else was found.

Influence of sand and grit on the performance of turkey poults fed on diets containing two concentrations of protein
J.A. Oluyemiab, A. S. Arafa & R. H. Harms - British Poultry Science, 1978
1. Growth rate and the efficiencies of food and energy utilisation were investigated with large white poults. They were fed from 1 to 3 weeks of age on diets containing either 240 or 300 g crude protein/kg diet, with no filler, 25 g washed builders’ sand/kg or 25 g chick-size grit/kg.
2. Sand and grit increased growth and improved the food and energy conversion ratios. None of the interactions of crude protein content with filler was statistically significant.
3. It is speculated that grit and sand improved the grinding and the digestion of the food particles.

This study found that digestion improved when insoluble stone grit was provided to cockerels fed a diet of whole oats, whole wheat, and cracked corn. The size and texture of the grit didn't matter. The complete paper is available.
The influence of size and surface condition of grit upon the digestibility of feed by the domestic fowl
R.E. Smith - Canadian Journal of Animal Science, 1960
Two experiments were carried out with growing cockerels to study the effect of size and surface condition of grit upon digestibility. The criterion employed to evaluate these effects was the apparent digestibility of the individual proximate constituents. Comparisons were made between six sizes of grit ranging in diameter from 0.6 mm. to 5.0 mm., and between four grits of different surface conditions. All grits utilized in this investigation significantly improved feed digestion. However, neither the size nor the surface condition of the grit influenced this response. Smaller sized grits, and grits displaying smoother-type surfaces were not retained within the gizzard to the extent of the rougher, larger sized grits; consequently a greater consumption of the former grits was experienced.

This study found that both soluble and insoluble grit improved the digestibility of food. The complete paper is available.
The influence of soluble and insoluble grit upon the digestibility of feed by the domestic fowl
R.E. Smith and T.M. MacIntyre- Canadian Journal of Animal Science, 1959
Two experiments were carried out with growing cockerels to study the effect of soluble and insoluble grits upon digestibility. The criterion employed to evaluate these effects was the apparent digestibility of the individual proximate constituents. Comparisons were made between soluble calcitic limestone, insoluble quartz, silica sand and combinations of these grits.These trials have demonstrated that grit improves feed digestion, particularly of whole grain feeds. In addition, both types of grit were found to be similar in their ability to initiate and maintain this beneficial response, as long as they remain present as distinct entities in the gizzard. It was observed, however that calcitic limestone was not retained in the gizzard nearly as long as the insoluble quartz grit and consequently a greater consumption of the former may be expected.

On the effects of using plant materials as a type of 'grit' for chickens.
Effects of oat hulls and wood shavings on digestion in broilers and layers fed diets based on whole or ground wheat
H. Hetland, B. Svihus & ┼. Krogdahl - British Poultry Science, 2003
1. An experiment was conducted to study the effects of inclusion of oat hulls in diets based on whole or ground wheat for broilers. Effects of wood shavings on layers were investigated in a further experiment.
2. Inclusion of oat hulls in wheat-based broiler diets did not affect weight gain. Feed conversion efficiency (FCE), corrected for insoluble fibre contents, was improved by oat hull inclusion.
3. Gizzard size increased with inclusion of oat hulls, whole wheat, wood shavings and grit.
4. Starch digestibility was significantly increased by inclusion of oat hulls for broilers, and by wood shavings for layers.
5. Wood shavings and whole wheat did not affect bile acid concentration of gizzard contents. However, the total amount of bile acids in gizzard increased with access to wood shavings due to an increase in the weight of gizzard contents, indicating an increased gastroduodenal reflux.
6. Fibre concentration was considerably higher in the gizzard contents than in the feed.
7. Duodenal particle size decreased with access to grit. No effect of hulls or whole wheat inclusion was found, indicating that all particles are ground to a certain critical size before leaving the gizzard.

On grit's effectiveness for removing metal objects from a budgie's gizzard.
Comparison of Treatment Protocols for Removing Metallic Foreign Objects From the Ventriculus of Budgerigars (Melopsittacus undulatus)
Corina Lupu , DVM, Dipl ABVP (Avian) and Stephanie Robins , BSc - Journal of avian medicine and surgery, 2007
To compare the efficacy of treatment protocols recommended to aid passage of metallic foreign objects from the ventriculus of birds, a 1-mm metal sphere, made from solder wire, was placed into the crop of each of 44 budgerigars ( Melopsittacus undulatus). After survey radiographs confirmed the spheres were lodged in the ventriculus, birds were divided into 6 groups. Each group received 1 of 6 different treatment protocols: psyllium with grit, acidic drinking water, fine grit, coarse grit, cathartic emollients (peanut butter and mineral oil), and a control group. All birds were treated simultaneously with a chelating agent, dimercaptosuccinic acid (DMSA), to prevent heavy-metal toxicosis. Successive survey radiographs were used to monitor elimination of the spheres from the digestive tract. Of all protocols tested, birds treated with either fine or large grit had the shortest mean elimination time of the metal spheres. These results indicate that administration of grit particles, either fine or coarse, appears to be effective in hastening the passage of metallic foreign objects from the ventriculus of budgerigars.

Grit helped prevent gizzard lesions in chicks. But if lesions were already present, grit made the problem worse.
Relation of grit to the development of the gizzard lining in chicks
H. Bird, J. Oleson, C. Elvehjem, E. Hart, J.Halpin - Poultry Science, 1937
Thickening of the gizzard lining, similar to that encountered in previous studies of nutritional crater lesions, but distinct from the latter, was found to be prevented by feeding grit or by using coarsely ground ingredients in the ration. When the abnormal thickening occurred in the absence of crater lesions, i.e. on a diet adequately supplied with the anti-gizzard-erosion factor, growth was less satisfactory than on this diet supplemented with grit. On the crater producing diet, however, growth rate was not consistently improved by the addition of grit, and it was thought that this might be due to the bad effect of the grit on chicks already suffering from crater lesions.

Avian Digestive Tract Simulation to Study the Effect of Grit Geochemistry and Food on Pb Shot Bioaccessibility
M. Martinez-Haro, M.A. Taggart, A.J. Green, R. Mateo - Environmental Science and Technology, 2009
My summary: They used a simulated gizzard to test the effect that different kinds of grit had on the absorption of toxic lead from lead shot eaten by waterfowl, and found out that the type of grit made a difference. With silica grit there was a more acid environment in the gizzard with a resulting higher concentration of dissolved lead (which is bad). Calcium grit resulted in a less acid environment and less dissolved lead. This is consistent with the common real-world practice of using Tums (calcium carbonate tablets) to reduce acid indigestion. The calcium may also reduce the bioavailability of lead by enhancing its precipitation into lead carbonate, and by promoting higher dissolved calcium levels in the intestine, which competes with the lead for absorption.

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