The Role of Gizzards in Bird Digestion

Birds have evolved to extract the life-sustaining nutrients they need from a wide variety of foods, using a complex system of digestive organs. You probably know that our feathered friends don’t have teeth, but have you ever wondered how they break up their food for digestion?

Avians have a two-part stomach, complete with a section known as the gizzard, which grinds their food. This organ has evolved to replace the teeth and powerful jaw muscles of their flightless dinosaur ancestors.

We don’t often get a look at a bird’s inner workings, although you may have seen this tough, silvery organ in the giblets of a store-bought hen. Chickens use their well-developed gizzards to grind up tough seeds, but they are not the only birds with this important organ. All avians have a gizzard, although its size and strength vary depending on a bird’s diet.

Here, we take a closer look at the avian gizzard and explore its structure, function, and role. Read along to learn all about this unique digestive organ.

Anatomy and Location of the Gizzard

Position in the Digestive Tract

The gizzard, or ventriculus, is located in the thoracoabdominal cavity, just before the duodenum of the small intestine. En route to the gizzard, food must pass through the beak and oral cavity, esophagus, and glandular stomach or proventriculus before reaching the gizzard.

Physical Structure

The gizzard is a roughly disc-shaped organ characterized by strong, muscular walls. Food enters the gizzard through the cardiac opening and exits toward the intestines through the pyloric opening.

Its inner chamber is lined with a tough, horny material called koilin. This abrasive material is worn down through use and replaced as a glandular secretion that hardens after contact with stomach acid.

In birds like Geese, the koilin is shaped into rigid opposing discs or pads on either side of the ventriculus that grind together.

In birds like Geese, the koilin is shaped into rigid opposing discs or pads on either side of the ventriculus that grind together - Canada Goose foraging

Function of the Gizzard in Digestion

Mechanical Digestion

You could say the gizzard has the same role as our molar teeth because it physically pulverizes food into smaller particles. Instead of jaw muscles and hard materials like dentin and enamel, this organ uses powerful muscular contractions and a tough koilin lining to grind up food.

Birds also swallow grit, which could be small pebbles or other hard materials like snail shells. These ‘gizzard stones’ collect in the ventriculus and grind together to increase the mechanical efficiency of the gizzard.

Chemical Preparation

The primary role of the ventriculus is to prepare food for chemical digestion. By pulverizing and breaking up the stomach contents (chyme), the gizzard increases the surface area of the food to allow much greater contact with digestive enzymes in the intestines.

Gizzard Health and Dietary Influence

Impact of Diet on Gizzard Size and Strength

Evolution shapes birds and their organs to match their lifestyle, habitats, and diets, and the gizzard is no exception. A Turkey, for example, requires a powerful gizzard to crush acorns and other tough foods. Such a well-developed organ would be wasted on a Hummingbird with its mostly liquid diet.

Similarly, birds that eat tough insects, mollusks, and crustaceans need muscular gizzards to crush their prey, while species that eat soft invertebrates, fish, or fruits do not require strong grinding action to digest their food.

Common Gizzard Ailments

Bird gizzards can be affected by a variety of health problems. Impactions, or blockages in the gizzard, can cause illness and death in birds, particularly after eating poor-quality food, foreign bodies, or artificial material.

Birds are also vulnerable to various diseases caused by fungal infections (e.g., gastric yeast, candidiasis, etc.), bacterial infections (e.g., Clostridium), viral infections (e.g., adenovirus), and parasites (e.g., nematodes).

Ducks have fairly large and muscular gizzards, to help digest their food

Gizzard Stones and Their Role

Selection and Use

Birds swallow small pebbles (also known as gizzard stones or gastroliths) and other hard materials to boost the grinding power of their muscular stomach. Some grit is ingested accidentally during feeding, but birds certainly swallow grit purposefully. Grit ingestion is typical across a wide range of bird families and species, particularly in birds that eat seeds and other hard-shelled foods.

Birds select grit based on its availability, size, and composition. Smaller species may swallow material as fine as sand grains, while Ostriches ingest stones as large as an inch in diameter!

Grit may also release vital minerals like calcium, which is especially important for egg-laying females. Sometimes, the hard, indigestible parts of a bird’s meal are also kept in the gizzard to act as grit.

Stones' Replacement Cycle

Gizzard stones are taken in through the oral cavity and excreted through the cloaca or regurgitated. They become smaller with use and are smoothed by wear and abrasion, so they must be replaced periodically.

Grit may remain in the gizzard for weeks or months or be replaced frequently, depending on its availability. Birds may retain stones of specific sizes for longer than others, although retention times vary between species and, perhaps, individual diets.

Bearded tit eating grit from a table

Comparative Analysis Across Bird Species

Variations Among Species

The structure of the gizzard varies across bird species, depending on the physical properties of their food. Mechanical digestion isn’t as crucial for meat-eaters, and it isn’t much use to nectar and fruit-eating birds, so the ventriculus tends to be smaller and less muscular in these species.

Seed eaters and birds that eat hard-shelled insects have evolved particularly powerful gizzards to grind up their tough food. These birds also have high concentrations of myoglobin in their gizzard muscles to increase the strength and frequency of contractions.

Evolutionary Adaptations

The avian gizzard has evolved in response to edentulism (tooth loss) as a means of mechanical digestion. Its placement in the body has also shifted the center of mass away from the head, which has obvious benefits for balance and flight. Even with these anatomical improvements, an overly muscular gizzard is unnecessary in some species.

Birds of prey still have this organ, although it has evolved to perform a separate but equally important function. Owls and raptors collect indigestible feathers, fur, and bones in the gizzard to form a pellet, which they regurgitate before it enters the intestines.

The gizzard is also a remarkably changeable organ that can respond to changes in diet over a short period. A fascinating study on a migratory shorebird called the Red Knot showed that their gizzard weight decreases dramatically after a change to softer foods on the breeding grounds but can reverse to accommodate a diet of hard-shelled prey in the winter.

A study on migratory Red Knots showed changes to gizzard weight based on the different prey taken - it decreased when eating softer foods at breeding grounds

Summary

The muscular ventriculus, together with the help of gizzard stones and grit, helps birds make short work of foods as tough as seeds, nuts, and mussels, all without crushing molar teeth.

It is just one link in the avian digestive system, but the gizzard has allowed a dramatic shift toward toothlessness and the incredible power of flight.

Of course, not all birds need to grind hard shells to digest their food, so the gizzard has evolved in size and strength to suit the needs of each species. Even among individuals and across seasons, the gizzard may adapt and change to match a bird’s diet and role in the ecosystem.