The mouth has a horny covered tongue to facilitate swallowing of food substances to the oesophagus. There are no teeth. The lower and upper lips have been modified into sharp strong beaks for picking of food substances such as grains, fruits, worms and foliage. Salivary glands are poorly developed since they rarely secrete enzymes rather they secrete mucus for lubricating the horny covered tongue. Only few taste buds are present.
Pharynx and oesophagus
The pharynx is short and is connected to the long and narrow oesophagus posteriorly.
The oesophagus extends to the crop. The oesophagus directs food and water to the crop.
The crop is large and functions as a chamber where food is temporarily stored. In some birds such as pigeons and parrots, the epithelial cells lining the walls are broken down to produce? bird milk” which is regurgitated by both parents to the mouth of the young ones.
The stomach consists of proventriculus (true stomach) and strong muscular gizzard. The proventriculus secretes gastric juice (which contains protein digesting enzyme) and dilute hydrochloric acid. The muscular gizzard by its churning action, grinds large food substances into smaller ones for easy digestion. To aid in the grinding process, the birds swallow some coarse sand and pebbles which are stored in the gizzard.
Connecting the gizzard posteriorly is the U-shaped duodenum. The duodenum is the upper part of the small intestine. The liver and pancreas are connected to the duodenum by the bile and pancreatic ducts respectively. Bile is secreted into the duodenum by two bile ducts while pancreatic juice is secreted into the duodenum by two or three pancreatic ducts. The bile emulsifies fats and oils. The pancreatic juice contains amylase, trypsin and lipase which digest carbohydrates, proteins and fats respectively.
The duodenum (upper intestine) is connected to the remaining intestine (lower intestine). The intestine is slender and coiled.
The entrance from the duodenum to the lower intestine is muscular and ring-like called pyloric sphincter.
The pyloric sphincter regulates the quantity of food that goes to the lower intestine. Digestion of all classes of food is completed in the lower intestine by intestinal enzymes. The long and coiled intestine affords a large surface area for absorption. At the posterior end of the intestine, it joins a short rectum. The blind caeca branch off from the junction of the intestine and rectum. The rectum leads to a cloaca which discharges faeces and urine.
Note: Ducts from the kidneys and genital organs also open into the cloaca.