Learn All About the Digestive System of Birds

In this article we will describe the structure and function of the various parts belonging to the Digestive System of both poultry and wild birds and analyze the digestion of poultry food into its constituent nutrients. The metabolism of carbohydrates, fats and proteins is closely related to the poultry digestive system.

When birds feed, they do not tend to chew their food, nor do they tend to digest food in the same way as people or more familiar animals, so the question becomes:

How do birds eat? And how does the Digestive System of Birds work, trying to understand the different types of digestive organs in the body of birds and their feeding process can help birders to get to know much better the best foods for birds and why a healthy diet is important for all birds specifically.

Eating Behavior of Birds

Observing birds when and how they eat is the first step in learning more about their feeding habits and how they digest their food.

Birds usually feed most actively in the morning and evening, refueling after a long night and stocking up for the next night, but they will also eat at any time of the day. To understand the digestive system of birds, observe how the birds consume different foods and observe how they behave before, during and after each meal.

How do birds digest their food?

The digestion of birds is a process of different steps which begins with the search for food and ends when it expels the non-digestible residue from the bird’s body.

Finding Food

Birds have different dietary preferences and tend to find food in different ways, but all birds are great opportunistic feeders and will often try varying amounts of food. Bird species that are more aggressive will protect their most preferred food sources, and some of them will store food for future occasions. Once a bird has found its food, it can begin the process of feeding and digestion.

Chewing and Swallowing

Birds possess a beak that is specialized to help them bite, because they cannot chew like people. Instead, all birds usually swallow food whole, or if it is too large and uncomfortable to swallow directly, they will proceed to break it into different smaller pieces.

Some birds may go so far as to tear or crush foods such as fruits or certain prey, or at other times they will use their beads to have to break up harder pieces such as nuts or large seeds. In some cases, birds will proceed to bang their food against a rock or branch to help break it into pieces, and birds may even use their claws to contain the food while they break it up.

To swallow the food, birds turn their heads back to move the bite to the back of the throat, and by means of their tongues help maneuver the food into a good position to make it easier to swallow. Saliva also plays a part in this as it makes the food easier to swallow.

The Digestive System of Birds

The Digestive System of any type of animal is very important in order to convert the food that the animal ingests into the nutrients that its body needs for further growth, as well as for maintenance and production such as egg production, i.e. in the case of laying hens, eggs.An animal’s body tends to break down food by a process, chemical mechanism.

In various types of animals, the mechanical action involves chewing; however, because birds do not possess teeth, their bodies use other mechanical actions. Chemical action tends to include the release of digestive enzymes and fluids from various parts of the digestive system. After being released from the food during digestion, all nutrients are absorbed and spread throughout the animal’s body.

The Digestive System of domestic fowl is very simple yet efficient compared to many other species, such as cattle. In the process of evolution, those types of birds that developed simple but effective digestive systems were better able to fly and therefore survive, because their digestive system is simple and therefore makes them lighter in weight.

It is very necessary that the diet provided for all birds be of high quality and easily digestible for the purpose of simplicity in the structure and function of their digestive system. This is especially very important so that the birds can achieve the expected productive performance.

The Functions of the Digestive System of Poultry

Several of the organs that make up the Digestive System of Poultry. According to the bill, food tends to move through a tube which is called the esophagus in the direction of the crop, which in that place stores the excess food so that the bird can digest it slowly. Subsequently, the food passes to the proventricular zone, which is the first part of the stomach, where it tends to be softened by gastric acid, mucus and other digestive juices.

The second part of the stomach, the gizzard, crushes the food into smaller pieces, often with the help of sand, such as sand or small stones that the bird has previously swallowed to perform this type of process. If the food in particular is hard, it may move between the proventriculous and gizzard several times for more efficient digestion.

Once the food is sufficiently broken down, it is moved to the small intestine, where the liver and pancreas proceed to help with the absorption of nutrients. Next is the large intestine, which is too short for most birds. Where the small and large intestines join and are the caeca.

Parts of the Digestive System of Birds

For example, we can take the case of the chicken, which has a very typical avian digestive system. In chickens, the digestive tract, which is also known as the gastrointestinal tract, begins in the mouth, where it includes various organs that are very important and ends in the waste. Next we will indicate which are the parts of the digestive system of the birds, in this case that of the hens, which is the most common of all.

The BeakThe salivary glands usually run along the hard palate, groups of glands fuse together to form a mass of glandular tissue beneath the epithelium. Lymphoid tissue is located in most of these glands.

The salivary glands are:

• Maxilla: Located in the roof of the mouth.
• Palatine: Located on each side of the nasal opening at the roof of the mouth.
• The Atero-Pterygoid Glands: Located in the roof of the pharynx on each side of the common opening for the Eustachian tubes. The Eustachian tubes connect the middle ear to the mouth and their function is to equalize the air pressure on each side of the tympanic membrane in the ear.
• Anterior submandibular glands: They are in the angle formed by the junction of the beaks or the upper and lower jaws.

The posterior submandibular glands.

• The lingual glands: which are located in the tongue.
• Cricoarytenoid glands: They can be found around the glottis.
• The small gland found in the corner of the mouth.

Esophagus

The esophagus consists of a flexible tube that connects the mouth to the rest of the digestive tract. It carries food from the mouth to the crop and from the crop to the proventriculus.

The crop

The crop is a waste product of the esophagus and is located just outside the body cavity in the neck region. Food and water that are swallowed are stored in the crop until they tend to pass into the rest of the digestive tract. When the crop is empty or nearly empty, it sends hunger signals to the brain so that the bird eats more.

Although digestive enzymes secreted in the mouth started the digestion process, very little digestion occurs in the crop; it is simply a temporary storage bag. The crop evolved for birds that are normally hunted by other animals but need to move in the open to find food. These birds can consume relatively large amounts of food quickly and then move to a safer place to digest that food.

Occasionally, the crop tends to be impacted or backed up. This type of problem, called crop impingement, crop jamming or pendulous cropping, can occur when a bird goes too long without ingesting food and subsequently eats too quickly when food becomes available again.

Direct crop impaction can also occur when a bird moves freely through a pasture of tough, fibrous vegetation or eats long pieces of twine. With crop impaction, even if a chicken continues to eat, feed cannot pass the affected crop. The swollen crop can also block the trachea, causing the chicken to suffocate.

Proventriculus

The esophagus continues beyond the crop, connecting the crop to the proventriculus. The proventriculus is also referred to as the true stomach is the glandular stomach where digestion primarily begins.

Hydrochloric acid and digestive enzymes, such as pepsin, are added to the food here and begin to break down more significantly than enzymes secreted by the salivary glands. At this point, however, the food has not yet been ground: this organ is called the proventriculus because its location in the digestive tract is before the ventriculus, where the food is ground.

Ventriculus or Gizzard

The ventriculus, or gizzard, is a part of the Digestive System of Birds, Reptiles, Worms and Fish.Known as the mechanical stomach, the gizzard is made up of two sets of strong muscles that act like the bird’s teeth and have a thick lining that protects these muscles. The food that is consumed and the digestive juices from the salivary glands and proventriculus pass into the gizzard for grinding, mixing and maceration.

When allowed to free-range, chicks typically eat small stones. The acidic environment in the proventriculus softens the stones, and then the strong gizzard muscles crush them into small pieces. The stones remain in the gizzard until they are ground into pieces small enough to pass through to the rest of the digestive tract.

Grit, a commercial product that is composed of small stones, can be used as a supplement to poultry feed. Birds fed only commercially prepared feed do not need grit. Chickens eating whole grains or chickens kept on pasture that do not consume enough pebbles with the forage generally require grit supplementation. Sand should not be confused with limestone or oyster shell, which is given to laying hens as sources of calcium for their egg shells.

When a hen eats a small, sharp object, such as a tack or staple, the object is likely to get stuck in the gizzard. Due to the strong grinding motion of the gizzard muscles, such sharp objects can poke holes in the gizzard wall. Chickens with damaged gizzards become thinner and eventually die. Preventing this situation is a good reason to keep a poultry farm free of nails, glass shards, pieces of wire, etc.

Small Intestine

The small intestine is one of the parts of the poultry digestive system and consists of the duodenum, which is also called the duodenal loop, and the lower small intestine. The rest of the digestion tends to occur in the duodenum, and all nutrients that are released are absorbed mainly in the lower small intestine.

The duodenum receives digestive enzymes and bicarbonate in order to counteract hydrochloric acid from the proventriculus of the pancreas and bile from the liver via the gallbladder. Digestive juices are produced by the pancreas and are mainly involved in protein digestion. Bile is a detergent that is important in the digestion of lipids and the absorption of fat-soluble vitamins including A, D, E, and K.

The lower small intestine is composed of 2 parts, the jejunum and the ileum. Meckel’s diverticulum marks the end of the jejunum and the beginning of the ileum. Meckel’s diverticulum tends to form during the embryonic stage of the bird.

The Meckel’s diverticulum

The caeca in plural form of caecum consists of 2 blind pouches which are located where the small and large intestines meet. Part of the water that remains in the digested material tends to be reabsorbed there. Another important function of the caeca is the fermentation of any remaining gross material.

During this fermentation, the caeca tends to produce several fatty acids as well as the 8 B vitamins:

• Thiamine
• Riboflavin
• Niacin
• Pantothenic Acid
• Pyridoxine
• Biotin
• Folic Acid
• Vitamin B12

However, because the cecae are very near the end of the digestive tract, few of the nutrients produced are absorbed and available to the bird.

Large Intestine the Colon

Despite the name, the large intestine is actually only shorter than the small intestine itself.The large intestine is where the ultimate reabsorption of liquid, i.e. water, takes place.

The Sewer

In the case of the chicken, the cloaca, the digestive wastes of the poultry digestive system tend to mix with the wastes of the urinary system, the urates. Generally, birds void fecal material such as digestive wastes with uric acid crystals on the external surface, i.e., chickens do not urinate.

The color and texture of chicken fecal material can become indicative of the health status of the chicken’s digestive tract: the white, pasty material that coats chicken fecal material is uric acid, the avian form of urine, and is normal.

The reproductive tract also exits through this same area. When a hen usually lays 1 egg, the hen’s vagina folds to allow the egg to exit through the cloacal opening without coming into contact with feces or urine.

Intestinal Microflora

Both the small intestine and the large intestine are usually populated by beneficial organisms such as bacteria, yeasts, etc., which are referred to as microflora, the word micro meaning “small” and flora meaning “plants”. This microflora is what aids in digestion.

The Liver

The liver is a bi-lobed organ that lies ventrally i.e. below and posterior to the back of the heart and is closely related to the proventriculus and the spleen. The lobe on the right side is the largest. The liver is dark brown or otherwise chocolate in color, except in the first 10 to 14 days when it may be found quite pale due to absorption of lipids i.e. fats from the yolk as an embryo.

It weighs approximately 50 grams in adult birds. The capsule, or glissosis, is the membrane that covers the liver and is thinner than that of mammals.

The gallbladder is located in the right lobe below the spleen. There are 2 bile ducts that emerge from the right lobe and one of these originates from the gallbladder and the second provides a direct connection from the liver to the small intestine. A system of ducts connects the right and left lobes.

Bile

The liver consists of a series of sheets of tissue that are 2 cells thick, with a sinusoid on each side of the sheet. Bile is produced by the cells. The blood vessels, when they come in contact with these sinusoids, are closely associated with them to thereby facilitate the transfer of material from one system to another.

The tiny channels are called canaliculi which have the task of collecting and transporting bile that are associated with the cells in the tissue sheets. These canals eventually join together to form the bile ducts; one of them goes directly to the intestine and the other to the gallbladder before it connects to the small intestine.

The Pancreas

This organ has 3 lobes that occupy the space between the 2 arms of the duodenal loop.From 2 or 3 ducts pass the secretions of this organ at the distal end of the duodenum through papillae common with the ducts of the gallbladder and liver.

The structure is similar to that of the mammalian pancreas and consists of a special secretory tissue for pancreatic juice, as well as other groups of cells that are called the “islets of Langerhans”. These are mainly associated with hormone production. In poultry, the cells of the islets of Langerhans are less well defined than those of mammals.

Behavior when drinking liquid

There are 3 general ways in which birds drink liquid:

• Using gravity itself
• Sucking the liquid
• Using the tongue.

The liquid fluid is also obtained from food.

Most birds cannot swallow by the “sucking” or “pumping” action of peristalsis in their esophagus that people usually do, and drink in such a manner by repeatedly lifting their heads after filling their mouths to allow the liquid to flow by gravity, a method usually described as “slurping” or “tilting upward”. The notable exception is Columbidae; in fact, as described by the expert Konrad Lorenz in 1939.

One recognizes the order by the unique behavioral characteristic, namely, that when drinking water is pumped out by means of peristalsis of the esophagus which occurs without exception within the order. The only other group, however, which shows the same behavior, is the Pteroclidae, which is placed close to the pigeons with this characteristic undoubtedly that they are very old. (see article: Agapornis Personata).

Although this rule generally still stands, since that time, certain observations have been made of some exceptions in both directions.

In addition, specialized nectar feeders such as the sunbirds called Nectariniidae and hummingbirds that are of the genus Trochilidae drink using fluted or trough-shaped protrusible tongues, and parrots of the genus Psittacidae carry water.

Many seabirds have glands near their eyes that allow them to drink seawater. Excess salt tends to be removed from the nostrils. Many desert birds obtain the water they need from their food. The elimination of nitrogenous wastes such as uric acid reduces the physiological demand for water, since uric acid is not very toxic and therefore does not need to be diluted in as much water.

Flight Adaptations of the Digestive System of Birds

Birds consume high-energy foods such as insects, seeds, fruits, meat and nectarine. The digestive system of birds is extremely efficient in absorbing energy from small amounts of food at a very fast rate. Birds have a gizzard that is composed of 4 muscular bands that tend to act to rotate and crush food by moving food from one area to the next within the bird’s gizzard. (see article: Exotic Birds).

Depending on the type of bird species, the gizzard may contain small pieces of sand or stone that the bird has previously swallowed in order to aid in the grinding process of the food consumed. Many birds have a muscular pouch along the esophagus which is called a crop. The crop serves the function of softening the food and regulating its flow through the system by temporarily storing it.