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The respiratory, cardiovascular and digestive systems work together to provide the nutrients that are necessary for the bird's muscles to function.
Digestive System
Respiratory System
Circulatory System
The bird's digestive system is faster and more efficient than the digestive systems of most other vertebrates. Birds require a constant supply of nutrients to sustain their activities; however, since they fly they can't afford to store food within their bodies for long periods of time. Most birds must eat throughout the day to sustain their activities.
In the northern hemisphere the number of daylight hours decreases in the Fall. As the average daytime temperature drops the insect populations also decrease. Birds that depend upon insects as their primary food source migrate to the southern hemisphere where the longer days provide more time to forage.
Most birds digest their food very quickly. For example: shrikes can digest a mouse in about three hours. Many fruit-eating birds complete the digestive process in less than thirty minutes.
Birds are able to put on body fat rapidly in preparation for migration. Some songbirds can increase their body weight by as much as 40% in 10 days or less of intensive feeding.
Birds gulp their food and swallow it without chewing. Most birds have tongues that are short, narrow and triangular. There are few taste buds on their tongues.
The bill is used to acquire food and to crush, tear or hold food before it is swallowed. Birds use their tongue, combined with the hard palate, to discriminate between food items based upon how the food feels. Birds consume a wide variety of foods including some non-nutritive items such as sand, pebbles, and shell.
Salivary glands are well developed in birds that feed on foods that lack moisture. Aquatic birds, except ducks, have reduced or absent salivary glands.
The esophagus connects the mouth with the glandular stomach (proventriculus). In some birds, such as doves and pigeons, food is stored in the crop, an enlarged portion of the esophagus.
Birds have a two part stomach. The first stomach area, the proventriculus, is a soft-walled glandular tube. The epithelial cells in the walls of the proventriculus secrete hydrochloric acid, digestive enzymes, and mucus. The pH of a bird's stomach ranges from 0.7 to 2.3. The mucus protects the stomach from being damaged by the digestive enzymes and the acid. The acid kills microorganisms that may be in or on the food and also denatures the proteins in the food making them easier to digest. The digestive enzymes begin to breakdown large molecules such as starches and proteins.
The food passes into the second part of the stomach, the gizzard, a structure with very thick muscular walls and a hard, sandpaper-like inner surface. The gizzard performs many of the functions of mammalian teeth. It crushes and grinds the food and mixes it with the enzymes.
The food mixture leaves the gizzard and moves into the small intestine where digestion continues. The enzyme amylase reduces starch to glucose. Protease enzymes reduce proteins to a mixture of twenty amino acids. The carbohydrates, proteins and fats released by the digestive enzymes are absorbed into the blood stream along with a mixture of vitamins and minerals. The length of the small intestine depends upon the composition of each bird species' diet.
The nutrients released in the small intestine are rapidly absorbed into the blood stream. The function of the relatively short large intestine is to absorb water. The remaining waste products are excreted through the vent.
(See pages 177, 179 and 181 in Manual of Ornithology by Noble Proctor & Patrick Lynch for drawings of the thoracic and abdominal organs of a Rock Dove.)
For more information and excellent illustrations see Gary Ritchison's BIO 554/754 Ornithology course Digestion
(Printable version of the Digestion lecture)
The circulatory system distributes nutrients (oxygen, glucose, etc.) and hormones to every cell in the bird's body. It also transports wastes (carbon dioxide, uric acid, etc.) to the lungs and the kidneys.
The avian circulatory system consists of a four-chambered heart and an extensive system of arteries and veins. Although there are many similarities between the heart of a bird and the heart of a mammal, the bird's heart is about 40% larger than the heart of a similar sized mammal. The bird's heart functions more efficiently which allows it to move more blood per beat.
For more information and excellent illustrations see Gary Ritchison's BIO 554/754 Ornithology course Circulatory System
(Printable version of the Circulation lecture)
The muscle activity necessary for flight creates a very high demand for oxygen.
The bird's respiratory system transports air from the exterior into the lungs (external respiration). Cells use oxygen in the production of ATP, the cellular energy carrier, (internal respiration).
A bird's lungs function differently from a mammal's lungs. Mammals, including humans, breathe air into their lungs, move oxygen from the air to the blood in the alveoli in the lungs, and exhale the air.
Birds require two breaths for the air to complete one cycle through the lungs. The steps in this process include:
1) Inhaled air moves through the primary bronchus into the posterior air sacs (first inspiration).
2) The air moves from the posterior air sacs into the lung. Oxygen moves from the air in the lungs into the blood, Carbon dioxide moves from the blood into the air in the air sac (first expiration).
3) The air in the lungs moves into the anterior air sac (second inspiration).
4) Air is exhaled from the anterior air sacs through the primary bronchis to the exterior (second expiration).
Note that the incoming air in step one has a high concentration of oxygen and the outgoing air in steps three and four has a high concentration of carbon dioxide. The exhange of oxygen from the air in the lungs to the bird's red blood cells occurs in step two.
Oxygen diffuses across the alveolar membrane into capillaries in the lungs. The oxygen can dissolve in the blood plasma or bind to hemoglobin in the red blood cells. Avian red blood cells contain a nucleus unlike mammalian red blood cells.
For more information and excellent illustrations see Gary Ritchison's BIO 554/754 Ornithology course Respiratory System
(Printable version of the Respiration lecture)