Teeth play a fundamental role in the digestive system of a human, chewing food into pieces small enough to be swallowed and pass out of the stomach. But without teeth, how do birds process any large or hard food items?
By swallowing stones, which then collect in a digestive organ called a gizzard, food is ground until it reaches a manageable size to move through a bird’s digestive tract.
But exactly how do stones help a bird digest food? Keep reading to understand more about the unique features of a bird’s digestive system.
Some birds - but not all - sometimes swallow rocks and stones. These stones are not eaten as food by confused avians, but instead serve an important function in a bird’s gizzard helping to grind up food, which makes it easier for it to move into the digestive tract.
While all birds have a gizzard, not all birds eat stones to aid digestion. Birds with diets that consist largely of hard-shelled grain and seeds do need extra help to process these foods, and will regularly eat grit, stones, rock fragments or pebbles from the ground.
These stones remain in their gizzards, rubbing against each other and against the partly digested food as it reaches their stomach. This friction causes the larger pieces of food to be ground down into a small enough state to pass into the bird’s intestines and for all nutrients to be absorbed and distributed around its body.
Keep reading to learn more about the role stones play in a bird’s digestive system, and whether other animals also rely on this process to help digest the food they consume.
A pair of ostriches foraging for food on the beach
Birds with diets that contain items that are particularly hard to break down, such as whole prey, nuts and seeds with tough outer shells, and some tough plant parts, need to grind their food into smaller pieces so it can pass through their digestive system. To aid the digestion process, they swallow sharp stones or grit particles with rough surfaces. These become lodged in a muscular part of the stomach, called a gizzard.
So what are the components of a bird’s digestive system and how do they work together?
The gizzard is a thick-walled organ found in the digestive system of all birds, sometimes referred to as a secondary stomach. Stones and grit that are swallowed by birds collect in the gizzard and are known as gastroliths.
The gizzard is where any undigested food that has not been fully broken down by the stomach and gastric juices ends up, and is pulverized there by the friction action of the swallowed stones, or gastroliths.
The stones, pebbles, grit, or lumps of rock that become lodged in a bird’s gizzard, to aid digestion, are known as gastroliths. Over a period of time, these stones become smooth and rounded from constantly being rubbed together.
Grey Heron eating a fish
During digestion, the muscles of the gizzard contact, which causes any stones that have been ingested to rub against each other. This friction grinds the food eaten by the bird into smaller, more manageable particles. This process helps the broken-down food to pass more easily into the small intestine where digestion continues.
Gastroliths remain in the bird’s gizzard, becoming smoother and rounded with time from this constant ‘tumbling’ process, akin to pebbles being tossed on a beach against each other.
As these stones lose their sharpness, they become less effective at pulverizing the undigested food, so they are then coughed out and replaced with more stones with rougher edges.
Golden Eagle with a recently caught fish
Birds that have a diet higher in fiber tend to eat fewer - if any - stones. Birds that eat seeds, grains and nuts with hard outer husks regularly eat stones and grit as without them, digestion of these foods would be impossible.
Gastroliths help digestion in plant-eating birds by breaking down the cellulose in plant matter.
Carnivorous birds that swallow large prey items such as small mammals or other birds rely on the stones in their stomach to churn against the food they have eaten until it has been ground into pieces small enough to be moved further along the digestive tract.
The size of the stones ‘eaten’ by birds can be as small as grains of sand and shingle to larger cobblestone-sized rocks.
Birds that eat hard-shelled seeds need to eat stones, as digestion would be near impossible without them
Over time, gastrolith stones swallowed by birds become smoother and less effective and need to be replaced. Birds vomit up the smooth, well-tumbled pieces of rock once they are no longer useful, and new, sharper stones are swallowed.
Birds with diets that contain a lot of hard grains, seeds and nuts do swallow stones whole. These stones then pass through the esophagus to reach the gizzard, where they come to rest. Smaller pieces of rock, grit and shingle are also eaten.
You may see chickens pecking at the ground on spots where there isn’t even a trace of food, and they’re likely consuming grit for this purpose.
Chickens pecking at the ground, consuming grit
Gizzards are not unique to birds. Many other living creatures that do not have teeth to chew and grind food also have gizzards, including crustaceans, turtles. earthworms, grasshoppers, and some fish.
Despite having teeth, some reptiles have a similar muscular organ that aids digestion of hard and indigestible foods. Crocodiles and alligators have fearsome teeth, but these are used for trapping and killing their prey, rather than for grinding and chewing food, so they swallow large stones to aid with digestion.
Mammals with gizzards are rare, but some do exist. Pangolins, for example, have no teeth and rely on their gizzard to digest the food they eat.
Dinosaur fossils have been recovered, complete with smooth stones within their digestive systems, and evidence indicates that they also swallowed rocks to aid the digestion process.
Pangolins have no teeth and rely solely on their gizzard for digestion
All birds have gizzards, from ostriches to tiny wrens, but the gizzard’s importance to their digestive system and the amount of work it needs to do varies between species. Birds that rely on soft foods, e.g. fruits or nectar, have a thin-walled gizzard and do not need the help of grit to aid the digestion process.
Stone-eating is common among birds with diets that are primarily based on seeds, grain or nuts, such as chickens, grouse, turkeys and quails, as well as ducks, emus and doves. Doves eat grains without removing the outer husk, so rely on eating grit to break these hard shells down for digestion to continue.
Owls rely on the action of the gizzard to break down large items of prey that have been swallowed whole. Bones, feathers, teeth and fur collect in the gizzard, where they are pulverized by the stones through the strong contractions of the gizzard muscles.
Any remaining liquid passes into the owl’s small intestine, and the non-digestible leftovers are squeezed together into what’s known as a pellet, and then regurgitated.
Owls rely on the action of the gizzard to break down large items of prey that have been swallowed whole
No, humans do not have gizzards, and gizzards do not form any part of the human digestive system. Humans chew the food that they eat into smaller pieces using their teeth. Food moves from the mouth into the esophagus before passing directly into the stomach.
For birds, the gizzard forms a secondary chamber of the stomach. Large or hard undigested food items remain there until they have been processed and ground up enough to move into the intestines where digestion continues.
Humans have teeth to break food down into manageable pieces before it reaches the stomach, so there is no need for a gizzard. All food that is eaten moves through the esophagus to the stomach where gastric juices break it down further so the body is able to absorb and move any nutrients to where they are required.
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