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Birds depend heavily on their digestive systems to stay nourished and healthy. They have high
metabolisms, and their digestive systems are faster and more efficient than other animals. Most
birds in the wild cannot afford to store heavy food materials within their bodies for long
periods, and they usually need a constant supply of nutrients to sustain activity. Most birds
digest their foods quite quickly. Nutrients are absorbed through the intestinal membranes and
into the bloodstream, principally into the vessels of the superior and inferior mesenteric
veins. These vessels then bring the nutrients into the portal hepatic system of circulation
through the liver, where many of the nutrients are processed and stored before being
distributed to the tissues of the body.
(1) Beak - Is made up of bone, covered by a hardened coating of epidermis
(outer layer of the skin). This in place of lips like in mammals. It is used for grasping,
crushing, tearing and scooping of food
(2) Mouth - Birds do not have lips and teeth. They use their tongue that moves back
and forth to swallow and to push the food down into the esophagus. Most birds’ tongues
are short, narrow and triangular, with few taste buds. Although the taste of food may be
relatively unimportant to birds, they are capable of making fine distinctions in food items
based on the feel of the food to the tongue and hard palate. There are a few saliva
glands in the mouth to add some limited moisture to the food. The saliva, unlike with
mammals, has little if no digestive properties and is mainly used as a lubricant.
(3) Esophagus (gullet) - Is to pass the food from the mouth to the stomach.
(4) Crop - The crop is for food storage. The food is moistened and softened.
Crops evolved in some birds to help the bird load up on food and then fly off and
digest it safe cover. The food moves from here to the proventriculus.
(5) Proventriculus (glandular or true stomach) - This is where the first significant
amount of strong hydrochloric acid, pepsin (digestive juices), and mucous start the process
of breaking down the food material and aids in protein digestion. The food is then passed
rapidly into the gizzard so not much digestion occurs here.
(6) Gizzard (muscular stomach) - This is where the physical breakdown occurs.
The gizzard is highly muscular and strong, with a sandpaper-like inner surface.
In most birds, the gizzard contains sand grains or small rocks to aid in the grinding
process. The gizzard is amazingly strong. It has been reported that turkey’s
gizzards can completely crush 24 walnut shells (in the shell) in less than 4 hours
and turn surgical lancet blades into grit in less than 16 hours. The gizzards of
carnivorous birds can crush large bones. It is used for grinding and mixing of
food in preparation of digestion. The internal mucosa is very thick and constantly
erated and sluffed off. Food leaves the gizzard for the duodenal loop and small
intestine.
(7) Liver - Produces bile, which is temporarily housed in the gall bladder.
The livers stores fats and carbohydrates, synthesizes many proteins, and helps filter
and neutralize metabolic waste products in the bloodstream.
(8) Gall bladder - Attached to the liver for bile storage. Bile mixes with the
food slurry into the duodenum loop and small intestines. The liver and gallbladder bile
is for starch and fat digestion.
(9) Pancreas - Secretes enzymes amylase, lipase, trypsin which helps in digestion.
Pancreatic enzymes works on protein starches, sugars and fats, and neutralizes strong
gastric acids, and constitutes the principal digestive juice. The pancreas is the chief
regulator of carbohydrate metabolism, secreting insulin and other compounds to regulate
the concentration of blood sugars.
(10) Duodenum (loop) - Digestion starts as the pancreas secretes digestive enzymes.
The duodenum forms a loop and to support the pancreas and receives ducts from this gland
and the liver (or gallbladder if present).
(11) Small Intestines - Most of the digestion and all of the absorption of nutrients
takes place.
(12) Ceca (colic caeca or diverticula) - The bacteria are thought to break down
cellulose plant fiber. It consists of two pouches that fill and empty from the same
direction.
(13) Large Intestines - The digested food empties into the large intestine.
It is considered the same as the rectum in mammals.
(14) Cloaca - The cloaca is the common chamber into which the digestive,
urinary, and reproductive tracts open. The bursa of Fabricus is located as a
diverticulum in the dorsal wall of the cloaca. The bursa contains lymphatic tissue
and has a function related to immunity and antibody production. Since birds have no
lymph nodes, this and the thymus are the primary lymphoid organs. When the bird
eliminates fecal waste from its digestive tract, the cloaca folds back at the vent
allowing the rectal opening of the large intestine to push out, closing the
reproductive opening. So then there is a minimal chance that fecal matter will
contaminate the reproductive system.
(15) Vent - The circular opening at the posterior end of the bird,
made up of muscle and elastic tissue.
Moonlight Mile Herb Farm © 2010 Susan Burek
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