Fri. Nov 25th, 2022

Digestive system of pet birds: Birds lack teeth, so their food must be swallowed whole. Birds have a storage organ known as crop, which is located at the base of the neck.  From here, food passes down to the proventriculus, where the digestive process starts, before entering the gizzard, which is equivalent to the mammalian stomach. Nutrients are then absorbed through the wall of the small intestine.

The digestive system of plant-eaters differs in various respects from that of the predatory species. Vegetable matter is less nourishing than meat, so plant-eaters generally need longer digestive tracts to process the large quantities of food they must consume in order to obtain enough nourishment. In addition, digesting plant matter poses certain difficulties. The gizzards of the seed-eating species such as many finches (Fringillidae) have especially thick muscular walls, which serve to grind up the seed.


Pigeon digestive system
Pigeon Digestive system (Courtesy:

The oesophagus is large in diameter, particularly in birds that swallow large meals. Swallowing is accomplished by oesophageal peristalsis, appears to be aided by the extension of the neck.


Most but not all birds have a crop, which varies from a simple expansion of the oesophagus to one or two oesophageal pouches. Depending on the state of contraction of the stomach, food being swallowed is diverted into the crop and later propelled into the stomach by waves of peristalsis in the crop.


The proventriculus is a standard part of avian anatomy and is a rod shaped organ, located between the oesophagus and the gizzard of most birds. It is generally a glandular part of the stomach that may store and/or commence digestion of food before it progresses to the gizzard.


The pancreas plays a vital role in accomplishing both of these objectives, so vital in fact that insufficient exocrine secretion by the pancreas leads to starvation, even if the animal is consuming adequate quantities of high quality food.

In addition to its role as an exocrine organ, the pancreas is also an endocrine organ and the major hormones it secretes – insulin and glucagon – play a vital role in carbohydrate and lipid metabolism. They are, for example, absolutely necessary for maintaining normal blood concentrations of glucose.


The gizzard is a disk shaped, very muscular and in many birds contains small stones that facilitate grinding of foodstuffs. Also known as muscular stomach.


The large intestine consists of a short colon and, typically, a pair of caeca. Short villi extend into the lumen of the colon, unlike what is seen in mammals. The cloaca is an expanded, tubular structure that serves as the common opening of the digestive, reproductive and urinary systems, which opens to the outside of the bird as the vent.

As in mammals, absorption of water and electrolytes is the primary function of large intestine of birds. Antiperistalsis that originates in the cloaca is a prominent pattern of motility in the avian colon and has been suggested to assist not only in filling the caeca, but to flush urine from the cloaca into the large gut for absorption of water. In some birds, the caeca appear dispensable and can be removed without apparent harm. In other species, the caeca are important sites for fermentation, and the volatile fatty acids generated from microbial digestion of cellulose contributes significantly to energy demands.


The liver is the largest gland in the body and performs an astonishingly large number of tasks that impact all body systems. One consequence of this complexity is that hepatic disease has widespread effects on virtually all other organ systems. At the risk of losing sight of the forest by focusing on the trees, we will focus on three fundamental roles of the liver:

  • Vascular functions, including formation of lymph and the hepatic phagocytic system.
  • Metabolic achievements in control of synthesis and utilization of carbohydrates, lipids and proteins.
  • Secretory and excretory functions, particularly with respect to the synthesis of secretion of bile.

The latter is the only one of the three that directly affects digestion – the liver, through its biliary tract, secretes bile acids into the small intestine where they assume a critical role in the digestion and absorption of dietary lipids. However, understanding the vascular and metabolic functions of the liver is critical to appreciating the gland as a whole.


The spleen plays important roles in regard to red blood cells (erythrocytes) and the immune system.


Birds have a small intestine that seems very similar to the small intestine of mammals. A duodenum, jejunum and ileum are defined, although these segments are not as histologically distinct as in mammals. The proximal small intestine receives bile from the liver and digestive enzymes from the pancreas, and the absorptive epithelial cells are decorated with essentially the same battery of enzymes and transporters as in mammals.

The small intestine is the portal for absorption of virtually all nutrients into blood. Accomplishing this transport entails breaking down large supramolecular aggregates into small molecules that can be transported across the epithelium. An exception to this statement is seen in herbivores, where large amounts of short chain fatty acids are absorbed at other sites.

By the time ingesta reach the small intestine, foodstuffs have been mechanically broken down and reduced to a liquid by mastication and grinding in the stomach. Once within the small intestine, these macromolecular aggregates are exposed to pancreatic enzymes and bile, which enables digestion to molecules capable or almost capable of being absorbed. The final stages of digestion occur on the surface of the small intestinal epithelium.

The net effect of passage through the small intestine is absorption of most of the water and electrolytes (sodium, chloride, potassium) and essentially all dietary organic molecules (including glucose, amino acids and fatty acids). Through these activities, the small intestine not only provides nutrients to the body, but plays a critical role in water and acid-base balance.


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