Chickens, as with most birds, use their beaks to pick up food and as they have no teeth they are unable to chew their food. A chicken’s mouth contains glands which secrete saliva which wets the food to make it easier to swallow. The saliva also contains some enzymes which start the digestion process. The chicken’s tongue is then used to push the food to the back of the mouth so that it can be swallowed.
The food then moves into the esophagus which is the flexible tube that connects the mouth with the rest of the digestive tract. It carries food from the mouth to the crop.
The crop is an out-pocketing of the esophagus and is located just outside the body cavity in the neck region. Any swallowed food 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 (in fact it was present in some dinosaurs) that were easy prey for larger animals but which needed to move into the open to find food. This enables them to consume relatively large amounts of food quickly and then move to a more secure location to digest the food they consumed. Occasionally the crop becomes impacted or ‘backed up’ (crop impaction, also referred to as crop binding or pendulous crop).
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 food 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 the Latin term meaning before.
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. Consumed food and the digestive juices from the salivary glands and the proventriculus pass into the gizzard for grinding, mixing, and mashing, which is aided by the small stones or grit the bird consumes. 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/grit are weakened by the acidic environment created in the proventriculus and then are ground into tiny pieces by the strong muscles of the gizzard.
The small intestine starts at the exit of the gizzard with the duodenal loop and ends at the ileo-caecal-colon junction. Food in the duodenum is neutralised by the addition of more enzymes excreted by the pancreas. These enzymes break down protein. Also added here is bile produced by the liver and stored in the gall bladder, bile aids in fat digestion. The products of digestion are absorbed from the small intestine and carried to the liver primarily for remanufacture into body tissue or to provide energy.
The caeca are two blind-ended tubes which provide space for fermentation. Here undigested food particles are subjected to microbial breakdown. The caeca normally contain a mustard to dark-brown froth which is excreted about once every day.
This part of the gut is very small in the chicken and serves as a further absorption site especially for water.
The cloaca or vent is a chamber common to the digestive and urogenital systems. It is responsible for the elimination of faeces, urine and the passage of eggs or seminal fluid.