Poultry (chickens, geese, turkeys etc) have a distinctive digestion system, which has some clear differences to the digestion of ruminants or other monogastric animals. Their digestive system is comparatively short, so each food particle remains only approximately 6 hours in the digestive system. For ruminants, the time is almost 48 hours, and 24 hours for pigs. In such a short time the chicken must be able to absorb as much nutrients as possible from its feed. It's also interesting to note that the digestive system of a chicken develops very fast: a chick can digest fibers as well as an adult chicken.
Poultry have no teeth or soft palate. They peck food with their beaks, and especially chickens have no problem eating hard or stringy material such as dead mice, earthworms or broken eggs. All food particles are swallowed whole without chewing. The saliva has no active amylases so no digestion happens in the mouth or esophagus. Some poultry keepers routinely cut the beaks of the chicks, which causes extreme pain when pecking, and can cause problems eating. This practice is highly unethical, even though it is effective in preventing chickens from pecking each others in frustration.
After swallowing, the food slides down the esophagus into the crop. Chicken esophagus is approximately 35 cm long, covering 17 % of the length of the entire digestive tract. It secretes mucus which lubricates the esophagus. The crop is not a stomach, it is simply a small sac used to store and moisten feed (some wild birds feed their young by regurgitating food stored in their crop). The crop secretes enzymes which digest starch and proteins, and has some microbial activity as well.
After the crop the moist, partially digested food mass enters the proventriculus (gladular stomach). Like crop, the proventriculus is found only in birds. The proventriculus secretes HCI, pepsin and mucus, which start the actual enzymatic digestion. Food travels trough the proventriculus very fast: it is covered in gastric juices, but doesn't stay in the proventriculus to be digested. For actual digestion and mechanical breaking food enters the actual stomach, called the gizzard. The gizzard is surrounded by strong muscles, which contract and break food particles. The muscles are set in two pairs, thick and thin pairs of muscle. Muscle contractions also mix the enxymes well with the food mass. The internal wall of the gizzard has koilin fibres, which stick out from the wall like tiny teeth, further aiding in breaking food particles.
Liver and pancreas excrete important digestive enzymes. The relatively large liver of a chicken has two segments. Bile is excreted directly from the left segment of the liver to duodenum (beginning of the small intestine). The right segment has a duct to the gall bladder, which again has a duct to duodenum. Pancreas are located near duodenum, and excretes lipolytic, protelytic and amylolytic enyzmes. It is interesting to note that chickens produce no lactase, because their nutrition does not include milk derivatives. Chickens also lack the enzymes needed to digest cellulose, hemicellulose and beta glucans. Commercially available enzymes can be added to chicken feed to enhance digestibility.
After the gizzard the food enters the small intestine, which is short compared to mammals. Most nutrients are digested and absorbed in the small intestine. The intestine has two kinds of glands: intestinal glands secrete lipolytic, protelytic and amylolytic enyzmes, and glands of the mucuous membrane secrete maltase, isomaltase, peptidase, saccharase and palatinase. The small intestine of chickens is divided to two, duodenum and ileum, compared to the three-part intestine of mammals. Ileum alone is 120 cm long, and comprises nearly 60 % of the length of the entire digestive system.
Chickens have two caeca (singular: caecum), which assist in digesting fibers and non-starch polysaccharides. Caeca are approximately 8 cm long. Caeca are full of microbes, and they are located in the junction of the small and large intestine. Sometimes in feed digestibility trials the caeca are surgically removed, so the intestinal digestibility can be measured without interference from the caeca. In other digestibility trials a fistulae is surgically inserted to the end of the small intestine. This way samples of the feed can be collected before it is digested in the caeca. Both methods enable to animal to be kept alive after the trial, and possibly used in another feed trial later. The most common trial method is to slaughter the chickens for collecting of feed samples from the digestive tract.
The large intestine is very short, and ends in a cloaca. Together they are about 6 cm long (5 % of the length of the digestive tract). The oviduct and uric acid are also secreted to the cloaca, so both feces, uric acid and eggs come out from the cloaca, which ends in the anus.
Retention times in each part of the digestive tract. (c) unknown