Avian Digestion

A. Mouth


1. The Mouth consists of the upper maxilla and the lower mandible and is called the beak
2. Lack of teeth may be an adaptation for flight, birds have lighter jaw bones and muscles.
3. The beak and tongue manipulate food. Lacking a soft palate, and the hard palate is divided by a long, narrow slit in the center which is lined with numerous projections or papillae, opening into the nasal passage. This opening and the lack of soft palate makes it impossible for the bird to create a vacuum to draw water into the mouth, therefore birds have to scoop water when drinking and then let it run down by gravity.

B. Salivary Glands
1. Birds, mammals and reptiles have salivary glands. Birds that consume dry food (grainivores, like chickens and turkeys) have well developed salivary glands. Fish eating birds like cormorants or GBH’s have poorly developed glands.
2. Saliva contains a starch-digesting enzyme known as amylase, which hydrolyses starch to maltose, and maltase which hydrolyses maltose to glucose. The salivary gland’s main purpose is to lubricate food, so that it can move down the digestive tract. These glands are exocrine in nature (i.e., they secrete outward into the mouth vs endocrine, into the bloodstream).
3. Since birds don’t chew their food the way most mammals do, the esophagus is larger in diameter to accommodate large food particle size as compared to mammals.

C. Esophagus

1. Fairly long
2. The esophagus secretes mucus which further lubricates the bolus and aids in swallowing. The opening to the trachea (breathing tube) is the glottis. In mammals, the trachea is protected from swallowing food by the epiglottis, which is a cartilaginous valve in the throat guarding the entrance to the trachea. Birds do not have an epiglottis, which is why careful feeding techniques are so important.

D. Crop
1. Size and shape varies with avian species and is a modification of the esophagus which forms a pouch that can store food when the bird is eating rapidly
2. In grainivores, it is bilobed and large
3. In owls and insectivores it is rudimentary or absent
4. Functions for food storage
5. The beginning stages of digestion start in the crop due to the action of the amylase and maltase present from the mouth.
6. Crop Milk—method of regurgitating the lining of the crop to feed young, doves and pigeons are known to do this. It is induced by prolactin which is an endocrine hormone secreted from the ant. Pituitary gland, important in mammals for its effect on the mammary gland and lactation.

E. Proventriculus – Glandular Stomach
1. secretes pepsin and hydrochloric acid (HCl) for the digestion of protein, as well as, peptidase and gastric lipase that break down fats into glycerol and fatty acids. Because the food passes quickly through the proventriculus there is little digestion of food here, but the secretions pass into the muscular stomach, or gizzard, where enzymatic action takes place.
2. Small in grainivores
3. Large and distensible in aquatic birds
4. Produces and releases gastric juices from chief cells of gastric glands
a. Pepsin (pepsinogen is the precursor)
b. HCL--Create an acidic environment to help catabolize food.
c. Mucous also secreted, functions as a lubricant

F. Gizzard —Muscular Stomach
1. or ventriculus, which is the muscular stomach, functions to crush food. It is also part of the reflux system that allows for further mixing of nutrients and enzymes from the lower GI.
2. Functions in mechanical digestion of food
3. The gizzard can exert pressures up to 500 lbs/sq inch. Food can remain in the gizzard anywhere from a few minutes to several hours depending on the coarseness of the food. The valve separating the stomach (gizzard) from the small intestine is known as the pylorus or pyloric valve.

G. Small Intestines

—is the principle site of the absorption for amino acids, vitamins, minerals, lipids and soluble carbohydrates.
3 sections
1. Duodenal Loop—primary site of chemical digestion. Minerals and amino acids which are not used or impounded when absorbed after digestion are recycled back into the small intestine where they can be reabsorbed. This increases the overall efficiency of absorption.
2. Jejunum—main site of absorption.
3. Hormones are also secreted in small intestines which regulate gastric intestinal actions
Secretions of the walls of the small intestines include:

· Peptidase ­ which break down peptides to amino acids
· Maltase ­ which break down maltose to glucose
· Sucrase ­ which break down sucrose to glucose & fructose
· Lactase ­ which break down lactose to glucose & galactose (chickens do not produce lactase and are poor converters of lactose to glucose and galactose)

H. Liver
1. bi-lobed, fairly large in birds
2. Functions to synthesize bile.
3. Bile emulsifies fat to aid in digestion
a. contains amylase so bile also helps with CHO catabolism
b. Bile activates lipase activity –breakdown of lipids to fatty acids and glycerol.
c. Also involved in metabolism of CHO, protein, and fat

I. Gall Bladder
1. Present in most avian species; absent in pigeons and parrots. In those species, the liver releases bile directly into the small intestines
2. a storage compartment that excretes liver bile when it is needed. Bile consist of several weak acids that have the following functions:

Neutralize the HCl generated in the proventriculus
Form soluble soaps with free fatty acids that help in the absorption of fatty acids
Facilitate digestion of fats by forming emulsions
Activate pancreatic lipase
Aid in absorption of fatty acids, cholesterol, and fat soluble vitamins

J. Pancreas
1. Located w/in duodenal loop
2. At least 3-lobed
3. Pancreatic secretions reach duodenum thru 3 ducts.
K. Ceca
1. Located in junction of Small and Large intestines
2. Functions in microbial fermentation of fiber and water reabsorption.

L. Large Intestines

—aka Colon
1. is a relatively short organ (only 4 -5 inches) and serves primarily to absorb water and maintain an electrolyte balance. The ileocecal valve separates the small and large intestine. Large concentrations of microorganisms reside here living on undigested food particles. A substantial part of the feces is actually dead microorganisms (up to 1/3)

M. Cloaca

1. Junction for the digestive, renal (excretory) and reproductive tracts
2. Also involved in water reabsorption and mineral balance

**Adaptations from http://icdweb.cc.purdue.edu/~neumans/avdig.html and http://asnet.tamu.edu/www/kids/chicken.htm; Dr. Christopher Bailey