Image from John Anderson, Dept. of Animal Sciences, The Ohio State University. Used with permission.
In most livestock, teeth function to grind feed into smaller particles. Birds must pass feed usually whole into the esophagus because they do not have teeth. Therefore, particles of poultry feed should be small enough to pass through the esophagus. Feed passes from the mouth and through the precrop esophagus to the crop. If the proventriculus and gizzard are full, feed is stored in the crop. Feed is also moistened in the crop.
Feed passes from the crop through the postcrop esophagus to the proventriculus. The proventriculus (a glandular type of stomach) secretes acid and enzymes. The acid and enzymes are mixed with the feed to start the digestive process. The feed then passes to the gizzard (a mechanical type of stomach). The gizzard has very strong muscular walls that grind the feed. Depending on the type of feed ingested, poultry may also swallow small rocks. These rocks aid in the grinding of harder feed particles in the gizzard.
Feed passes from the gizzard into the small intestine, where additional enzymes are added and digestion occurs. The small intestine also serves to absorb digestion products (proteins, carbohydrates, and fats). You should notice in the photo above that the first part of the small intestine loops around the pancreas (called the duodenal loop). The pancreas secretes digestive enzymes into the small intestine. The small intestine of a mature chicken is more than 4.5 feet in length, which is necessary to provide the surface area required to absorb digested feed.
Two blind pouches called the ceca (singular: cecum) are attached at the junction between the small intestine and large intestine. Microorganisms capable of breaking down fibrous material live in the ceca. However, this is not a significant part of the digestion system in modern birds. Scientists believe that the ceca may have played an important role in the digestion system of ancestors of modern birds.
The large intestine
of a mature chicken is relatively short, about 4 inches in length. The
large intestine stores undigested waste material and absorbs water from
the material. The large intestine connects to the cloaca,
which is where the digestive, urinary, and reproductive systems meet.
Uric acid is mixed with feces and passes out of the vent.
The vent serves also as the point where eggs pass out of the bird body.