THE AVIAN DIGESTIVE SYSTEM

Introduction
The digestive system of birds differs from mammals in many ways, including lack of teeth and lack of a soft palate. Birds have also developed strategies to protect them against predation that include the ability to rapidly ingest and digest food.. Because most birds need to keep their body weight at a minimum for flight they cannot store food for long periods of time. Most fruit, seed and insectivorous birds, such as the species commonly kept as cage birds, pass seeds through their digestive system in about 30 to 40 minutes, while carnivorous birds such as falcons, may easily digest a meat only meal in just a few hours. Birds that store food for long periods of time in their crops are the most susceptible to predation (pigeons and doves) and also include birds that have developed other evolutionary strategies for survival, such as the ground dwelling birds related our domestic chickens and turkeys. Camouflage, running speed and other similar behaviors help ptarmigans, pheasants, and the flightless ostriches and emus to survive.

Mechanisms of digestion:

The beak is created by the fusion of several separate keratinous (hardened epithelial cells, such as those that make up the skin) layers. The beak grows continuously to compensate for wear and tear and, despite its tough looks, is extremely sensitive. While taste of food is not very important to birds, the tongue is full of nerve endings that feel for the shape and texture of food types. As birds don't have a soft palate, swallowing of food is aided by throwing the head back, as well as by backwards angled papillae on the tongue that force the food towards the throat. Saliva acts as a lubricant to assist in swallowing and for forming the food into an oval capsule shape called a bolus that is easy to swallow and helps to move the food down the Esophagus to the crop.

The oral cavity and pharynx form a single unit, the oropharynx, connected to the nasal cavity by the triangular slit shaped choana. Eustachian tubes open by an opening behind the choana. This Eustachian slit remains open continuously to permit pressure equalization in flight.

Many birds store food in the esophagus if the stomach is full. Food is also stored in the distensible crop, just in front of the chest. The crop may also be used to process food for regurgitation to babies.
While in the crop the food softens and swells before it is passed to the stomach.

The Avian stomach is very simple consisting of a glandular proventriculus and a muscular ventriculus or gizzard. The gizzard is a muscular structure lined with a tough, abrasive keratin layer. Food may be passed backwards and forwards between these compartments Food digestion is accomplished in the glandular stomach which is lined with ducts containing glands that secrete hydrochloric acid and pepsin.

After food is finely crushed and "chewed" by the gizzard it passes by peristalsis, smooth muscle contractions, out of the gizzard into the intestines, where nutrients and liquid absorption occur.

The liver and pancreas connect to the duodenum of the small intestine and secrete bile and pancreatic juices. As in mammals, these act to emulsify fat, digest carbohydrates and proteins and neutralize the acid secretions of the stomach.

The liver has two large lobes surrounding the lower pericardium and these occupy most of the l space of the abdomen. There may or may not be a gall bladder. The liver acts to store carbohydrates and fats.
The large intestine in birds is short and relatively featureless except for the intestinal caeca, leading in to the cloaca. This is a site of water absorption. Nutrients are passed through the intestinal walls into the blood stream for utilization by the body system through the portal hepatic venous system