Avian Digestive System
An understanding of the avian digestive system helps the owner develop an effective feeding program and helps with recognizing when something is wrong so you can take necessary actions to correct the problem.
The gastrointestinal (GI) or digestive system converts food into nutrients body needs to grow and maintain itself. Food is broken down by mechanical (chewing) and chemical means (digestive enzymes). Since ‘chewing’ is less efficient for birds, they have some secondary ‘mechanical’ mechanisms in their GI system. After being released from food through the digestion process nutrients are absorbed and distributed throughout the body. The GI system begins at the beak, includes several important organs, and ends at the cloaca.
Food picked up by the beak enters the mouth; since birds don’t have teeth they don’t chew their food. They break or grind food into smaller pieces and the glands in their mouth secrete saliva which wets the food to make it easier to swallow. This saliva contains enzymes such as amylase that starts the digestion process.
The esophagus is a flexible tube that carries food from the mouth to the crop and from the crop to the proventriculus. The crop is a “pocket” in the esophagus and is located just outside the body cavity in the neck region. Swallowed food and water are stored in the crop until they are passed to the rest of the digestive tract. When the crop is empty or nearly empty, it sends hunger signals to the brain so that the bird will eat more.
Digestive enzymes secreted in the mouth began the digestion process, but very little digestion actually takes place in the crop, it’s more of a a temporary storage pouch. Since birds are “prey” animals (hunted by others) they need to move quickly to obtain food and get to safety. The crop allows birds to consume relatively large amounts of food quickly and then move to a more secure location to digest that food.
Occasionally, the crop becomes impacted, or backed up. Crop impaction also can occur when tough, fibrous vegetation is consumed or the bird ingests cotton fabric or string. If the crop becomes impacted food cannot pass to the GI system and the bird can starve. The swollen crop also can also press on the windpipe, causing the bird respiratory distress.
The esophagus continues past the crop to the proventriculus. The proventriculus (known as the ‘true’ stomach) is the glandular stomach where primary digestion begins. Hydrochloric acid and digestive enzymes, such as pepsin, begin to break the food down more significantly. At this point food really hasn’t been ground down into smaller pieces so food moves to the ventriculus where it’s ground up.
The ventriculus, aka gizzard, is a part of the digestive tract often referred to as the mechanical stomach, because this is the primary organ where mechanical digestion occurs. The ventriculus is contains two sets of strong muscles that masticate the food, and it has a thick lining that protects those muscles. When a bird eats a small, sharp object, such as a tack or staple, the object is likely to get stuck in the gizzard. With the strong grinding motion of the gizzard's muscles, sharp objects can cause damage to the gizzard wall with will eventually result in the bird loosing weight. If not resolved the bird will eventually die so this is another good reason to keep small sharp objects out of reach along with access to wires and jewelry and so on.
From the ventriculus, food passes into the small intestine which is made up of the duodenum (also referred to as the duodenal loop) and the lower small intestine. The rest of digestion occurs in the duodenum and where the released nutrients are absorbed mainly in the lower small intestine. The duodenum receives digestive enzymes and bicarbonate from the pancreas to counter the hydrochloric acid from the proventriculus. The liver provides bile by way of the gall bladder. Bile is important in the digestion of lipids and the absorption of fat-soluble vitamins (A, D, E, and K). Digestive juices produced by the pancreas are involved primarily in protein digestion.
The lower small intestine is composed of two parts, the jejunum and the ileum. The Meckel's diverticulum marks the end of the jejunum and the start of the ileum. The ceca (plural form of cecum) are two blind pouches located where the small and large intestines join. Both the small and large intestines normally are populated with beneficial organisms (bacteria, yeast, etc.), referred to as microflora. These microflora aid in digestion.
Some of the water remaining in the digested material is reabsorbed here. Another important function of the ceca is the fermentation of any remaining coarse materials. During this fermentation, the ceca produce several fatty acids as well as the eight B vitamins (thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folic acid, and vitamin B12). Because the ceca are located so close to the end of the digestive tract, however, few of the produced nutrients are absorbed.
Despite the name, the large intestine is actually shorter than the small intestine. The large intestine is where the last of the water reabsorption occurs. In the cloaca, the digestive wastes mix with wastes from the urinary system.
Parrots void digestive waste as solid feces, urine, and urates from the cloaca. The reproductive tract also exits through this area. When a hen lays an egg, the vagina folds over to allow the egg to leave through the cloaca opening without coming into contact with excrement. A bird’s poop can give the owner good insight into how well your bird is eating and its overall health; make a point to check your bird’s droppings regularly and check our our blog “Every Birdie Poops” for some important things to look for.
For further reading:
Anatomy Physiology of the Avian GIT.pdf