Should Wildlife Rehabilitators Or Pet Bird Owners Add Grit To Their Bird’s Diets?
Ron Hines DVM PhD
Takeaway Points: Many birds in the wild depend on grit to process the food they eat. But birds in captivity do not require soluble or non-soluble grit when you feed them diets that contain calcium at 0.5%=500 mg/ up to 0.9%=900 mg/100 grams of diet. That can be the amount of calcium in the pellets you feed pet or “ambassador” birds, or in their other food dietary ingredients. Amounts of calcium above that level can cause kidney damage. (read here) That includes parrots, macaws lovebirds and parakeets; and probably goes for wild birds kept in zoos and rehab centers as well. Birds in the wild that are primarily fruit-eating, insect-eating or meat-eating are much less dependent on grit and do just fine during a lifetime without it. As far as wild bird rehabilitators, consider grit only for mature non-releasable “ambassadors” birds, or select a grit alternative. Let the other releasable birds find their own preferred grit size and amount once you release them.
Finches, for example, have been found to not need soluble grit when their seed mixes contain the amounts of calcium I mentioned above. Their needs are on the higher end when they are still growing or laying eggs. Fine-grained, insoluble grit is also an optional for them if it corresponds to their body size (see table at bottom of page). Access to cuttlebone or finely crushed eggshell is another alternative. Besides being a source of calcium, cuttlebone helps keep their beaks smooth and free from overgrowing – if they will use it. Premium and fortified finch seed blends usually express their calcium content on their label. Other manufacturers that are reputable should be able to furnish you with that information if you ask them.
What Types Of Birds Consume Grit?
Deliberately eating grit varies greatly between bird species. Grit eating is very common in birds that feed primarily on hard seeds, less common in birds that feed primarily on fruits, less common in birds that feed primarily on insects (such as the curve-billed thrasher in the gizzard photo below) and even less common in carnivorous birds like raptors or fish-eating birds. People find grit and small stones in the castings of raptors, but it isn’t clear that the stones ever made it down lower than their crop. The gizzards of raptors are poorly developed because meat is easily digested in the high acidity of their proventriculus. How long grit remains in the gizzard of birds is also highly variable – even within the same bird species or a single individual bird. Perhaps it remains in the gizzard’s hard inner folds until it becomes too smooth or too small to be useful. If it is calcium-rich grit such as shells, it eventually dissolves. Perhaps an occasional very fibrous meal flushes the gizzard of its grit. We really do not know. House sparrows, force-fed grit, have flushed out half of it in their stools in the first 24 hours. Yet 32% of the grit was still in their gizzards 13 days later. In red-winged blackbirds force-fed grit, 90% was eliminated during the first day. As I mentioned, most bird experts believe that birds that naturally eat soft diets are less likely to consume grit. If they do, it tends to be in quite small amounts. That goes for birds that live primarily on berries and fruit, as well as those that live on soft insects and meat or fish. (ask me for Fisher1995)
Birds consume hard (insoluble) grit as an aid to processing their food efficiently. They often seek out sharp fragments of granite, flint chips or quartz when available. When not available, fragments of snail shells, seashells, insect chitin and other hard dibries are substitutes. Those secondary soluble grit sources need more frequent replacement since they all dissolve more quickly. Secondary sources that don’t readily dissolve, like lead shot and bullet fragments, are toxic. (read here) Some birds add to that clays that contain minerals that they lack – usually salt. Birds with muscular, grinding, gizzards tend to consume more grit than those that do not have well-developed gizzards. Birds seem to “know” when they need grit and seek it out. When seasonal diets are hard and coarse, they tend to seek more out. When fruit and insects are plentiful, they have less interest in consuming grit. In the wild, seed-eating, ground-feeding birds tend to be the most dependent on large amounts of grit in their diet. Individual grit particle size is highly dependent on the bird’s body weight. When birds in the wild’s gizzard folds are filled to capacity, any excess grit that they consume just passes out in their stool. Gizard grit content is normally in rapid flux – the amount consumed equal to the amount lost – but birds suddenly deprived of grit can reduce their loss of grit and retain grit particles in their gizzards for long periods.
By increasing the food’s surface area exposed to digestive enzymes and acid in their intestines, grit improves the digestibility and nutrient absorption from food. But when a bird’s natural food is plentiful in the wild, or when that bird is in captivity and fed mechanically harvested grains or a processed pelleted diet, it will thrive without grit. One non-releasable collared dove I care for has gone 8 years with no grit provided and no weight loss or debility. In fact, grit can be harmful to birds maintained outside of their natural environments and food sources. Cage environment, stress, boredom, and lack of social interaction can result in the over-consumption of grit and result in impactions unless a very fine sand grit is given. A grit so fine that it passes quickly through their digestive tract and likely provides nothing useful regarding food digestion or absorption. I believe that for wildlife rehabilitators or cage-bird owners, if there are no visible seeds, fruit, or insect particles in the bird’s stool, it has no need for grit. There is also no known benefit in providing grit to nestlings receiving balanced commercial or home-prepared diets. (read here)
The Avian Digestive System – One Of The Tradeoffs For Flight
Flighted birds need to minimize their weight, yet retain their muscle strength. Some of that strength they preserve through a very high metabolic rate. That high metabolic rate is responsible for their high body temperatures – flight requires more energy than running. Their intricate air sac system not only lightens them, it increases their ability to absorb oxygen. Birds also have increased abilities over mammals to power their metabolism anaerobically. Their bones are hollow to decrease their weight, and, probably for the same reason, they have no well-developed urinary bladder, eliminating that weight as well. Flighted birds accumulate little body fat, eliminating that weight. Their intestinal tract is quite short when compared to mammals – probably for the same reason, to conserve weight. However, that gives food little time for complete digestion. For example, thrushes can eat blackberries, digest them, and excrete the seeds forty-five minutes later. Despite that, canaries compensate with high digestive enzyme levels and manage to absorb almost 90% of the carbohydrates (starches) and 98% of the oils in their diet. Non-flying birds and only an occasionally flying bird species have two well-developed ceca. Ceca afford them more space for nutrient absorption. But songbirds are not among them. They have relatively small ceca.
The Crop
In birds with a well-developed crop (aka ingluvies), a small amount of digestion already begins to occur there. Although a bird’s digestive enzymes play a part, much of that early digestion is by helpful bacteria that normally reside in their crop. (ask me for Pacheco2004) After that, consumed food passes down to the first chamber of the bird’s two-compartment stomach, its proventriculus.
The Proventriculus
In the bird’s proventriculus, the digestive enzymes, pepsin, and chymotrypsin (read here) and gastric acid begin to disassemble proteins in their diet into their composite amino acids before the contents pass into the bird’s muscular gizzard (its ventriculus) for mechanical grinding and more enzymatic breakdown. (read here) Meat and fish-eating birds tend to have a large proventriculus and smaller, much weaker gizzard.
How The Gizzard Works
The job of a bird’s gizzard is to grind food particles in order to decrease their size and expose their nutrients. That increased food surface area allows for more exposure to the digestive enzymes and acid provided earlier by the bird’s proventriculus. The gizzard’s two opposing and tough koilin (keratin) plate linings protect the gizzard musculature from those destructive digestive enzymes and acid and accomplish the grinding. In birds that rely on grit, the grit particles get trapped in the folds of that hard inner hard gizzard lining – doing their work much like teeth or sand paper. Gizzard size and grinding strength is plastic (modifiable). In the spring and early summer, when many songbirds take advantage of the soft caterpillar bloom, their gizzards have less work to do. When they shift back to a dry seed diet later in the summer, their gizzards can increase in weight by 25-30%. How long food stays in their gizzard depends on its hardness, fiber content and initial particle size. Grit speeds that process. Once ground to small particles, food passes into their small intestine for absorption. Some have even suggested that pigeons have tactile abilities in their gizzards that sense the number of grit stones present and the need for more. I have no idea if that is true. A passage in an avian medicine handbook frequently used by veterinarians states that food moves from a bird’s proventriculus into its gizzard in a “series of cycles”. But nowhere in the vast knowledge base on poultry is such back and forth movement ever mentioned.
I mentioned that when a bird’s desire for grit is present, but none is supplied, they often have a very strong urge to consume whatever is available. I had to deal with that when I worked at SeaWorld. The penguins ate fiberglass, grout, caulk, ear and nose rings, small portions of the artificial habitat displays, etc. Unlike natural grit that wears smooth and continually reduces in size until it passes out in the bird’s stools, I had to remove that stuff with an endoscope or surgically. The favorites of the walruses were infant pacifiers. Wild walrus also naturally eat stones. Some suggest that is for ballast rather than digestion.
Soluble Grit
There are two forms of grit that birds naturally consume. The first is soluble grit that is rapidly dissolved by the bird’s gizzard acidity. Those are the forms of grit that are high in calcium – primarily seashells and limestone. A second function of calcium-containing grit (soluble grit) such as limestone or shells is to supply added calcium. Wild grains, fruits, and berries can be an inadequate source of calcium because although they contain calcium, many of them are also high in phytates and oxalates that block calcium absorption. (read here) In captive wild or domestic bird settings, soluble grit such as ground oyster shell or feed-grade limestone (calcium carbonate) is unlikely to cause obstructions since the acids produced by the bird’s proventriculus easily digest it. When rare problems do occur, veterinarians often blame it on underlying health issues such as liver disease, kidney disease, pancreatitis, kidney failure, dehydration or general malnutrition. Limestone and oyster shells vary in their degree of hardness and therefore in their solubility. It has also been suggested that readily available grit, furnished to birds in confinement or captivity-induced “boredom” might leads to a higher than normal consumption of grit. (ask me for Gionfriddo1995)
If you are only contemplating giving soluble grit to birds in your care to lessen their likelihood of a calcium deficiency, I suggest you consider eggshell powder mixed into their diet instead. Along with crushed cancellous bone, I give powdered eggshells to growing owls and immature raptors – about a third of a teaspoon-full in 70 grams of their meat diet.
Insoluble Grit
The most common insoluble grit found in birds consists of fine, sharp particles of quartz or granite. Stomach acids have no effect on this type of grit. Once they eventually lose their sharpness and become rounded, the grit particles pass out of the bird in its stools. (ask me for Gionfriddo1994) However, when inexperienced songbirds are kept in cages or unnatural settings, they cannot be relied upon to eat the proper size or amount of grit. The second photo is of an individual insoluble grit stone that a flighted, young-of-the-year dove consumed. It resulted in a complete obstruction of the entrance to its proventriculus. That unfortunate dove consumed a piece of grit that was too large. It came to me with a broken wing that I repaired. At the time, I was supplying the many doves that I raise with insoluble grit half that size as they waited in my flights for the Texas dove hunting season to end. Perhaps that piece of grit was already consumed before I obtained the bird. Perhaps it was unintentionally left in its flight from a previous occupant. I no longer provide them with grit. Once I release them, I am confident that they will find the proper size and amount of grit that they need on their own.
Soil And Clay
Some birds travel long distances to consume clay. That is called geophagy. Some believe that the clay helps detoxify harmful constituents in these bird’s diets. Others believe it is due to a craving for calcium, still others because these birds are deficient in salt. The mineral particles that compose these clays are too small to be considered grit or a digestive aid. The parrots that live nearby the Tambopata river in Peru are the most famous of that group – but eating clay there is not confined to parrots. It’s been stated that eighty-eight percent of the birds that eat clay there are primarily fruit eating birds: parrots, native tropical pigeons, and guans. Most now believe that the birds are attracted by the clay’s high salt content. (ask me for Brightsmith2004)
Do I have Grit Alternatives?
Yes
There is no need for you to provide insoluble grit as a digestive aid if you run your bird’s diet through an old-fashioned corn grinder. Set the crank that you see in the photo above at its coarsest setting to accomplish the same thing that grit does in a large seed-eating bird’s gizzard; a bit finer for birds whose adult weight is less. An alternative to soluble grit is a sprinkle of powdered eggshells. A coffee bean grinder, such as the one in the second photo above, does that well.
If I Offer Grit, What Size And Type Of Grit Should I Provide?
If you decide to supplement your birds diets with grit, it is important that you provide the correct size grit. The grit size naturally selected by adult birds increases in an almost straight angular line (linearly) proportional to that species’ adult body weight. In one test, the characteristics of grit that influenced a bird’s choice besides size included, shape (they liked rough edges), color, hardness, and texture. House sparrows preferred brown, yellow, or white grit, while bobwhites preferred yellow and green grit. Neither of them paid much attention when the grit was dyed black.
If it is a high-calcium soluble grit such as limestone or oyster shell that you intend to provide, besides being the right size, many avian nutritionists suggest that it not be given to birds consuming pelleted or seed diets because those diets are usually already supplemented with calcium. Read the label on the seed pack or pellet that you provide. Too much calcium carbonate, the same ingredient in Tums™, oyster shell and limestone, has the potential to decrease the acidity of a bird’s proventriculus. Birds rely on that acidity to begin their digestive process. Insoluble grit, such as flint or granite, do not pose that potential problem. However, as I mentioned earlier, birds have the potential to over-consume both types of grit when kept in non-natural settings.
How Do I Purchase The Proper Size Grit?
I am told that if you purchase quality commercial insoluble grit, the grade sold for poultry chicks 3–6 weeks old (Chick grit) is about 0.2 mm in diameter. The one sold for pullets 6–12 weeks old (Grower grit) is about 3.24-4.75 mm and the one sold for pullets 12–20 weeks old and older is about 4.75-6.35 mm in diameter. You have probably bitten into a pitted olive that still had its pit. Inspect before you feed, mechanical sorters are not infallible.
The following two tables are based on two key articles. (ask me for copies of Gionfriddo1994PhD & Bennett2011). On your mobile device, the columns on these graphs may be skewed. The number before the % sign is the number that were found to have grit, versus the ones that had no grit in their gizzards. The number before the mm sign is the diameter size of 80% of the grit particles, with the rest being slightly larger or smaller. For your comparison, 22 Caliber air rifle pellets are 5.5 mm in diameter. Daisy BB gun BBs are 4.5 mm in diameter. Cuisinart Mesh strainers from Target have 3 mm square holes. Home Depot fiberglass window screen has 1 mm square holes. Popular #5 24 no-see-um porch screen mesh has 0.896 mm square holes.
Do Wild Birds Have Sources Of Non-mineral Grit?
Yes.
Yellow-billed cuckoos eat primarily large insects. This yellow-billed, South Texas cuckoo died in a car collision accident. Where I live, the soil is heavy clay, devoid of gravel or sand. This bird weighed 56.5 grams. Its gizzard, full of unidentifiable ground dibries and small insect parts, weighted 6.78 grams. There was no mineral grit in its gizzard. However, what appeared at first to be small smooth stones turned out to be very hard, grasshopper heads. Heads too large to exit its gizzard intact and passed out into its stools. Grasshopper heads are composed of mineralized chitin, similar in hardness to the claws of lobsters. In this bird, they functioned as grit.
Why would anyone spend the time, money, and energy to collect the following data? There was a very good reason. Granular insecticides, fungicides, and seeds impregnated with those or similar chemicals are commonly spread on fields by farmers. The EPA was interested in knowing what size and what color granules were least preferred by the cornfield birds that were likely to consume them by accident.
| Species Percent With Grit Max grit diameter 80%
American Crow 59% 3.4 mm American Robin 49% 0.6 mm Blue Jay 85% 1.2 mm Brown-headed Cowbird 75% 0.6 mm Cedar Waxwing 20% 0.3 mm Chipping Sparrow 95% 0.8 mm Common Grackle 58% 0.7 mm European Starling 38% 1.1 mm Horned Lark 99% 1.1 mm Hermit Thrush 83% 0.3 mm House Sparrow 99% 0.7 mm or 0.5 mm House Wren 57% 0.3 mm Indigo Bunting 80% 0.9 mm Killdeer 96% 1.7 mm Mourning Dove 86% 1.8 mm Northern Bobwhite Quail 96% 1.6 mm Northern Cardinal 70% 0.9 mm Northern Oriole 0% 0 mm Red-headed Woodpecker 41% 0.6 mm Red-winged Blackbird 74% 1.0 mm Ring-necked Pheasant 100% 2.0 mm or 6.0 mm Savannah Sparrow 100% 0.9 mm Vesper Sparrow 90% 0.8 mm Yellow-rumped Warbler 0% 0 mm Hooded Crane 100% 2.8 mm Canvasback, Redhead, Scaup 100% 1 mm to “gravel size” Band-tailed pigeon 1-8 mm, with the majority about 3 mm Rufous-collared Sparrow 0.1-3.2 mm, 42% 0.4-0.8 mm From Bennett2011.pdf: American Crow 53% 2.9 mm American Robin 44% 0.8 mm American Tree Sparrow 100% 0.4 mm Barn Swallow Insectivore 22% 1.2 –1.9 mm Blue Jay Omnivore 85% 1.6 mm Brown-headed Cowbird 52% 1 mm Chipping Sparrow 95% 0.9 mm Common Grackle 57% 0.9 mm |
| Species Percent With Grit Max grit diameter 80%
Com Yellowthroat 21% 0.3 mm Eastern Kingbird 24% 1.3 mm European Starling 36% 1 mm Fox sparrow 100% 0.5 mm Horned Lark 99% 1.2 mm House Sparrow 98% 0.7 mm Indigo Bunting 81% 0.9 mm Killdeer 93% 1.9 mm Lark Sparrow 100% 0.6 mm Mourning Dove 100% 2.3 mm Northern Bobwhite 90% 1.8 mm Northern Cardinal 73% 1 mm Red-headed Woodpecker 52% 0.9 mm Red-wing Blackbird 73% 1.1 mm Ring-neck Pheasant 100% 2.3 mm Rock Dove 100% 2.3 mm Savannah Sparrow 100% 1 mm Song Sparrow 93% 0.8 mm Vesper Sparrow 86% 0.9 mm Western Meadowlark 44% 1.4 mm Yellow-billed Cuckoo 44% 1.6 mm |
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