A knowledge of chicken anatomy, and what the parts normally look like, will also help you to recognise when something is wrong and take the necessary actions to correct the problem.
The digestive tract of any animal, including chickens, is important in converting the food the animal eats into the nutrients their body needs for maintenance, growth, and production (such as eggs). Once food is eaten, it must be broken down into its basic components. This is done through both mechanical and chemical means.
Mechanical action typically involves chewing, but since chickens don’t have teeth other mechanical methods are used and will be discussed later.
Chemical action includes the release of digestive enzymes and fluids from the stomach, pancreas and liver.
Once the nutrients have been released from food during digestion, they can be absorbed and distributed throughout the chickens body.
The digestive tract is also referred to as the gastro-intestinal or GI tract. Which ever term is used, in chickens it begins at the mouth and ends at the cloaca and has several important organs in between
Chickens, as with most birds, obtain feed with the use of their beak. Food picked up by the beak enters the mouth. As previously mentioned, chickens do not have teeth so they are not able to chew their food.
The mouth does contain glands which secrete saliva which wets the feed to make it easier to
swallow. The saliva also contains some enzymes which start the digestion of the food eaten. The chicken’s tongue is then used to push the feed to the back of the mouth so that it can be swallowed.
The esophagus is a flexible tube that connects the mouth with the rest of the
digestive tract. It carries food from the mouth to the crop and from the crop to the proventriculus.
The crop is an out-pocketing of the esophagus and is located just outside the body cavity in the neck region. Any swallowed feed and water is stored in the crop until it is time to pass it on to the rest of the digestive tract. When the crop is empty, or nearly empty, it sends hunger signals to the brain so that the chicken will eat more.
Although salivary glands of the mouth secrete the digestive enzyme amylase very little digestion
actually takes place in the crop – it is simply a temporary storage pouch. The crop evolved for birds that are typically hunted by other animals but which need to move to the open to find feed. These birds are able to consume relatively large amounts of food quickly and then move to a more secure location to
digest the food they consumed.
The esophagus continues past the crop to connect the crop to the proventriculus. The proventriculus (also known as the ‘true stomach’) is the glandular stomach where digestion begins. As with human stomachs, hydrochloric acid and digestive enzymes (e.g., pepsin) are added to the feed here and digestion begins. At this point, however the food has not yet been ground up. The term ‘proventriculus’ is used since it comes before the ‘ventriculus’ or gizzard, with ‘pro’ being a Latin terming meaning before.
Gizzard / Ventriculus:
The gizzard, or ventriculus,is a part of the digestive tract unique to birds. It is often referred to as the ‘mechanical stomach’. It is made up of two sets of strong muscles which act as the bird’s teeth (see Figure 4 above). Consumed feed and the digestive juices from the salivary glands and the proventriculus pass into the gizzard for grinding, mixing, and mashing.
When allowed to free-range, chickens will typically eat small stones. These stones remain in the gizzard until they become ground into pieces small enough to pass through to the rest of the digestive tract. The stones/pebbles are weakened by the acidic environment created in the proventriculus and then are ground into tiny pieces by the strong muscles of the gizzard.
Chickens fed only commercially prepared feed do not need stones. If, however, whole grains are fed, it is necessary to provide small pebbles, typically given as grit. Grit is a commercial product made up of small stones. It should not be confused with limestone or oyster shell which are given to laying hens as a source of calcium for their egg shells.
Chickens kept on pasture will also require supplementation with grit, though many of them may consume enough pebbles when they forage.
When a chicken eats a small, sharp object such as a tack or staple, the object is likely to get stuck in the gizzard. Because of the strong grinding motion of the gizzard’s muscles, these sharp objects may eventually put a hole in the gizzard wall. Chickens with damaged gizzards will grow thin and eventually die – a very good reason to keep your poultry houses free of nails, glass shards, bits of wire and the like.
The small intestine is made up of the duodenum (also referred to as the duodenal loop) and the lower small intestine. The duodenum receives digestive enzymes and bicarbonate (to counter the hydrochloric acid from the proventriculus) from the pancreas and bile from the liver via the gall bladder.
The digestive enzymes produced by the pancreas are primarily involved in protein digestion. Bile is a detergent that is important in the digestion of lipids and absorption of fatsoluble vitamins (vitamins A, D, E and K). The remainder of the digestion occurs in the duodenum and the released nutrients are absorbed mainly in the lower small intestine.
The lower small intestine is composed of two parts, the jejunum and ileum. The Merkel’s Diverticulum
marks the end of the jejunum and the start of the ileum.
The pancreas plays important roles in both the digestive and hormonal systems. It secretes hormones into the blood system that are important in the regulation of blood sugar.
In the developing embryo the yolk sac supplies the nutrients needed for it to develop and grow. Right before hatch, the yolk sac is taken into the navel cavity of the embryo. The residual tiny sac is the Merkel’s Diverticulum
The material remaining in the yolk immediately after hatch is able to supply the feed and water needs of the newly hatched chicken. This is why it is possible to ship chicks long distances without adverse affects, as is done when chicks are purchased online and shipped via the postal service.
Ceca (plural form; singular = cecum): The ceca are two blind pouches located where the small and large intestines join. Some of the water remaining in the fecal material is reabsorbed here. Another important function of the ceca is the fermentation of any remaining coarse materials. In doing so they produce several fatty acids as well as the eight B vitamins (Thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folic acid and vitamin B12).
Because the ceca are located so close to the end of the digestive tract, however, very little of the produced nutrients are absorbed and available to the chicken. The ceca empty their contents two or three times a day, producing pasty droppings that often smell worse than regular droppings. Cecal droppings typically have a mustard to dark brown in color. The number of times cecal droppings are ‘pooped’, as well as their color and texture, tell you that the chicken’s digestive tract is functionally normally.
Large intestine (also known as the colon)
Despite the name, the large intestine is actually shorter than the small intestine. The large
intestine is where the last of the water re absorption occurs.
In the cloaca there is a mixing of the digestive wastes together with wastes from the urinary system (urates). Fecal material is usually voided as digestive waste with white uric acid crystals on the outer surface (i.e., chickens do not urinate/pee). The reproductive tract also exits through this area but when a hen lays an egg the vagina folds over to allow the egg to leave through the vent without coming into contact with the feces or urine.
Intestinal Micro flora
Both the small and large intestine are normally populated by beneficial bacteria, referred to as micro flora (‘micro’ meaning small and ‘flora’ meaning plants). This population of micro flora are important since they aid in digestion. Intestinal disease normally occurs when the balance of normal microflora is upset or the normal microflora is overrun by too many foreign organisms. The result is enteritis or inflammation of the intestines, producing symptoms that include diarrhea, increased thirst, dehydration, loss of appetite, weakness, and weight loss or slow growth.
When the damage to the intestinal tract is severe it is typically referred to as necrotic enteritis. ‘Necrotic’ means ‘dead tissue’ while ‘enteritis’ refers to an inflammation of the intestinal tract. Necrotic enteritis is a problem in many different types of production systems.
So where do these ‘beneficial’ bacteria come from? When chicks hatch their digestive tracts are virtually sterile. If raised by a mother hen, they would obtain the beneficial microflora by consuming some of their mother’s fecal material.
This is not possible in artificial incubation and brooding. Probiotics are a collection of the normal beneficial microflora that would inhabit a chicken’s digestive tract. By spraying it in the shipping boxes or supplying it in the first feed the chicks receive the ’good’ bacteria that they need to fight off infection by pathogenic bacteria, such as salmonella.