DIGESTIVE SYSTEM OF BIRDS: PARTS AND FUNCTIONS - BIOLOGY - 2025

The digestive system of birds begins in the beak or mouth and includes different important organs and endings such as the cloaca. Although it has similarities with the digestive systems of mammals and reptiles, the digestive system of birds is characterized by having special organs such as the crop and the gizzard (Stevens & Hume, 1995).

The digestive system of any animal is of vital importance for the processing of the food that the animal consumes. Through the digestive system, birds can absorb all the nutrients their bodies need to grow, maintain, and reproduce.

Digestive system of a bird. Image recovered from: alejandrajaimeperez.wordpress.com

Since birds do not have teeth, the food they digest is mechanically and chemically broken down in the digestive system. That is, different digestive enzymes and acids are released in order to digest food and the organs involved in the process grind and mix them, guaranteeing maximum absorption of nutrients during the process.

Due to their high metabolic demands, birds must consume more food than other vertebrate animals in proportion to their size. The digestive process makes possible the release of nutrients contained in food. Likewise, it makes possible the absorption and uniform distribution of these nutrients in the bird's body.

Deep understanding of the workings of the digestive system of birds allows industries such as poultry to be sustainable. In the same way, the care of birds in captivity is made viable thanks to the knowledge of their digestive system (Svihus, 2014).

You can also see:

• Excretory system of birds: structure and elements.
• Respiratory system of birds.

Parts that make up the digestive system of birds

Morphology of a bird. Example of Vanellus malabaricus. 1-Beak, 2-Head, 3-Iris, 4-Pupil, 5-Mantle, 6-Lesser Coverts, 7-Scapulars, 8-Coverts, 9-Tertiary, 10-Rump, 11-Primary, 12-Vent, 13 -Thigh, 14-Tibia-tarsal joint, 15-Tarsus, 16-Fingers, 17-Tibia, 18-Belly, 19-Flanks, 20-Chest, 21-Throat, 22-Wattle, 23-Eyestripe. Source: Wikimedia Commons

Peak

Birds use their beaks to feed. All food that enters the bird's body passes through the beak first. Birds don't have teeth, so they can't chew food.

However, inside the beak can be found glands that secrete saliva that serves to moisten food, allowing it to be easily swallowed.

The saliva inside the beak contains digestive enzymes such as amylase that serve to start the process of digesting food. Birds also use their tongues to push food to the back of their beaks so they can swallow it (Jacob & Pescatore, 2013).

Esophagus

The esophagus is a flexible tube that connects the beak to the rest of the bird's digestive tract. It is responsible for carrying food from the mouth to the crop and from the crop to the proventriculus.

Maw

The crop is a projection of the esophagus located in the neck region of the bird. Swallowed food and water are stored in this bag until they can pass into the rest of the digestive tract.

When the crop is empty or almost empty, it sends hunger signals to the brain so that the bird takes in more food.

Although the digestive enzymes secreted in the beak initiate the digestion process, in the crop this process is quite slow, as this organ serves as a temporary storage place for food.

This storage mechanism was developed in birds that are typically hunted by other animals, but need to move in the open to find food.

In this way, birds can consume a considerable amount of food quickly and then move to a safer place to digest that food.

On some occasions, the crop can be affected by obstruction or impaction problems. This happens when the bird has not consumed food for a long period of time and suddenly ingests a large amount.

When this happens, the food can start a decomposition process inside the crop and make the bird sick. The crop can also become clogged when the bird consumes large pieces of plant material that block the passage of food to the rest of the digestive system.

An inflamed crop can also block the windpipe or air outlet, causing birds to die from suffocation.

Proventricular

The esophagus continues after the crop and connects it with the proventriculus. This organ is known as the glandular stomach of birds where primary digestion begins.

Hydrochloric acid and digestive enzymes like pepsin mix with ingested food and begin to break it down more efficiently. At this time, the food has not yet been ground.

Ventricle or gizzard

The ventricle or gizzard is an organ of the digestive system of both birds and reptiles, earthworms and fish.

It is usually known as the mechanical stomach, as it is made up of a pair of strong muscles with a protective membrane that act as if they were the bird's teeth.

The food consumed by the bird and the digestive juices coming from the salivary glands and the proventriculus pass to the gizzard where everything will be ground and mixed.

Sometimes birds can eat small rocks in their feed. These are usually softened in the proventriculus and ground in the gizzard.

Generally, the ground rocks remain in the gizzard until its size is small enough to pass through the rest of the digestive tract.

When a bird swallows a sharp object, such as a tack or stapler hook, the object can get caught in the gizzard. These objects can pierce the gizzard when your muscles begin to move rapidly.

Birds that show damage to the gizzard walls begin to suffer from malnutrition and eventually die (Loon, 2005).

Small intestine

The next step of digestion occurs in the duodenum and the nutrients released by food are absorbed mainly in the lower part of the small intestine.

The duodenum receives digestive enzymes and bicarbonate from the pancreas and bile from the liver to counteract the effect of hydrochloric acid from the proventriculus.

The digestive juices produced by the pancreas are mainly related to the digestion of proteins. Bile is an important cleansing agent in lipid digestion and absorption of fat-soluble vitamins such as A, D, E, and K.

The lower part of the small intestine is made up of two parts, the jejunum and the ileum. Meckel's diverticulum marks the end of the jejunum and the beginning of the ileum. This diverticulum forms during the embryonic stage of birds (Bowen, 1997).

Mint

The mint is made up of two blind pouches where the small and large intestines meet. Some traces of water contained in the digested food are reabsorbed at this point.

Another important function of the mint is the fermentation of the remains of food that have not yet finished being digested. During the fermentation process, the mint produces fatty acids and the eight B vitamins (thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folic acid and vitamin B12).

The mint is located very close to the end of the digestive tract, however, some nutrients available in food are still absorbed in it (Farner & King, 1972).

Large intestine or colon

Although its name indicates that the large intestine is larger than the small, it is actually shorter. The main function of the large intestine is to absorb the last traces of water present in the digested material.

Sewer

In the cloaca, digestion waste mixes with waste from the urinary system (urea). Birds generally expel fecal matter from the digestive system along with uric acid crystals resulting from the excretory system process.

Since birds do not urinate, they expel uric acid waste in the form of a whitish, creamy paste. Bird feces can indicate how healthy they are. The color and texture of the stool indicates the condition of the digestive tract.

In the cloaca the reproductive system of birds also converges. When a female lays an egg, the vagina folds over the surface of the egg, so that the cloaca can be opened without coming into contact with feces or urine (PoultryHub, 2017).

Intestinal microflora of birds

In both the small and large intestines, it is normal to find populations of beneficial microorganisms for digestion (bacteria and yeasts, among others), these small organisms are called microflora. These populations are, in part, responsible for the successful digestion of birds.

When a bird hatches its egg at birth, its digestive system is in a sterile state. When a baby bird is raised by its mother, it obtains all the microorganisms from her microflora.

When a bird is hatched in captivity, it does not have the possibility of obtaining its mother's microflora and keepers must prepare a mixture of microorganisms to mix with the bird's food.

Intestinal diseases of birds generally occur when the balance of the microflora is disturbed by external organisms. As a result, birds can suffer from enteritis or inflammation of the intestines.

Enteritis can be detected when the bird has diarrhea, consumes more water than normal, loses its appetite, is weak, has slow growth or loses weight.

References

1. Bowen, R. (September 7, 1997). Colossal. Retrieved from Digestive Physiology of Birds: vivo.colostate.edu.
2. Farner, DS, & King, JR (1972). Digestion and the digestive system of birds. In DS Farner, & JR King, Avian Biology, Volume 2 (pp. 352-359). New York and London: Academic Press.
3. Jacob, J., & Pescatore, T. (2013). Avian Digestive System. Animal Sciences, University of Kentucky.
4. Loon, R. (2005). Digesting the Meal. In R. Loon, Birds: The Inside Story (pp. 152-153). Cape Town: Struik Publoshers.
5. (February 1, 2017). Poultry Hub. Obtained from Digestive system: poultryhub.org
6. Stevens, CE, & Hume, ID (1995). The digestive system of fish, amphibians, reptiles and birds. In CE Stevens, & ID Hume, Comparative Physiology of the Vertebrate Digestive System (pp. 40-42). Cape Town: Cambridge University Press.
7. Svihus, B. (2014). Function of the digestive system. The Journal of Applied Poultry Research, 306-314.