"As scarce as hen's teeth," is an apt aphorism for something nonexistent, because a chicken has no teeth. Chicken feed is broken down by saliva from the moment it enters the bird's mouth. The chicken's tongue pushes the feed toward the back of the mouth so it can slide down the esophagus—a tube leading to the crop, where the feed is temporarily stored. When the crop is full, you can see it bulging at the base of a bird's neck.

Occasionally the crop becomes impacted. Crop impaction may occur when feed is withheld before worming, causing chickens to eat too much too fast afterward. A crop may also get impacted in a bird free ranged where little is available to eat except tough, fibrous vegetation. Even if the bird continues to eat, nutrition cannot get past an impacted crop. The swollen crop may cut off the windpipe, suffocating the bird. Crop impaction is unlikely to occur in properly fed birds.

From the crop, feed is moved to the proventriculus or true stomach, where enzymes further break it down. It then moves along to the gizzard, or mechanical stomach—an organ with strong muscles, a tough lining, and a collection of small stones or grit for grinding up grains.

A chicken fed only commercially prepared mash or pellets does not need grit. If, however, it eats grains without having access to grit, its digestive efficiency will be impaired. When a bird eats a small, sharp object such as a tack or staple, the object is likely to lodge in the gizzard and, due to the strong grinding motion of the gizzard's muscles, may eventually pierce the gizzard wall. As a result, the bird will grow thin and eventually die—a good reason to keep your yard free of nails, glass shards, bits of wire, and the like.

From the gizzard, feed passes into the small intestine for absorption. The upper portion of the small intestine, called the duodenum, forms a loop. Enclosed by the duodenal loop is the pancreas, which secretes enzymes to aid digestion, bicarbonate to neutralize acids, and hormones to regulate blood sugar.

Between the duodenal loop and the lower small intestine is connected the liver, an organ that secretes green bile (or gall). Attached to the liver is a transparent pouch, the gall bladder, which stores the bile until it is needed to aid in the absorption of fats in the intestine.

Branching off between the small and large intestine are two blind pouches, called the ceca (one is a cecum), having no known function. The ceca empty their contents two or three times a day, producing pasty droppings that often smell worse than regular droppings. The frequency of cecal droppings, and their appearance among regular droppings, tells you the chicken's digestive system is functioning normally.

The last portion of the intestine, the large intestine or rectum, is relatively short. It absorbs water from feedstuffs passing through.

Crop Impaction Crop impaction, also called crop binding or pendulous crop, appears in a mature bird as a distended, sour-smelling crop filled with feed and roughage. When pressed between your fingers, the crop feels hard. If left untreated, the affected bird becomes emaciated and may eventually die due to impaired digestion. Possible causes include injury, fungal infection, improper fermentation of crop contents, or insufficient rations causing the bird to eat litter or fibrous vegetation that packs the crop until it loses muscle tone and cannot empty itself. Crop impaction is a condition, not a disease that spreads from bird to bird, although if it is a result of inadequate access to feed, several birds might be affected. To treat crop impaction, disinfect the skin over the crop, slit through skin with an extremely sharp blade (such as a utility knife), pull the skin aside, and slit through the crop so you can clean out the contents. Isolate bird and keep the wound clean until it heals. Preventing crop impaction quite simply involves providing proper rations and plenty of clean, fresh water. If you withhold feed preparatory to deworming, moisten the first feed offered after the deworming treatment.

Both the small and large intestine are normally populated by beneficial bacteria, referred to as microflora (micro meaning small and flora meaning plants). Microflora aid digestion and enhance immunity by guarding their territory against invading microbes.

An intestinal disease occurs when the balance of microflora is upset or the normal microflora are overrun by too many foreign organisms (usually eaten in contaminated feed or water). 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. Enteric diseases tend to be complex.

Combinations of, and interactions in the intestine between, various organisms—worms, bacteria, protozoa, viruses, and natural microflora—determine the severity of an enteric disease. Successfully treating enteritis requires knowing what organism, or combination of organisms, is causing the illness. Normal microflora may be kept in balance through the use of a probiotic, especially after chickens have been medicated with an antibiotic, which destroys the gut's good microflora as well as the bad.

The large intestine ends at the cloaca, where the digestive, reproductive, and excretory tracts meet. The cloaca has three chambers:

• the fecal chamber, at the end of the rectum
• the urogenital chamber, where the excretory and reproductive systems come together
• the vestibule, into which the cloacal bursa empties.

In the fecal chamber, a final bit of moisture is absorbed from feedstuffs leaving the body, which are then expelled through the vent (the outside opening of the cloaca), in the form of droppings. In a healthy chicken, feed passes through the entire digestive system in three to four hours.

The excretory system consists of the kidneys (lying along the chicken's back) connected to the cloaca's urogenital chamber by means of tubes called ureters. The kidneys filter and remove wastes from the blood. In humans these wastes are expelled in urine. A healthy chicken does not excrete urine, but expels its blood-wastes in the form of semi-solid uric acid, called urine salts or urates.

Urates may be improperly metabolized due to water deprivation, excess dietary protein or calcium, or certain diseases. As a result, droppings may contain more than the usual amount of urates (as occurs in the disease spirochetosis), the urates may accumulate as pasty deposits in the joints (as happens in articular gout), or urates may collect as crystals that block the ureters (as is the case of infectious bronchitis). Normal urates appear as a white, pasty cap on a blob of droppings.

Knowing what's normal can help you recognize and take action when the digestive system goes awry. Frequent bouts with a particular digestive disorder, for example, may indicate a need for improved feeding or better sanitation.