62. Avian digestion

Question 1

The length of the gastro-intestinal tract

Answer 1

The length of the gastro-intestinal tract in the birds is 4-6 times longer than the length of the body.
Passage time is relatively short (20 hours).

Question 2

Food intake

Answer 2

• Center of food intake is the hypothalamus
• Influenced by:
• High temperature and protein in food decreases food intake
• Low temperature and egg production increases food intake
• Beak is adapted for food-intake, no teeth, no soft palate, no parotid gland, unified oral-pharyngeal cavity.

Question 3

Esophagus

Answer 3

• Peristalsis of esophagus passes food to the crop or directly to the stomach.
• In domestic birds, the crop serves for temporal storage of food, and softening of it.
• During nestling, the crop produces “crop-milk”, which serves as baby food for some species
• Species with crop: gallinaceae, pigeons, turtle doves

Question 4

Stomach

Answer 4

Glandular stomach:
• Primary function is secretion of gastric enzymes.
• Two types of glands:
• Mucous glands - mucin
• Complex glands - HCl, pepsinogen, mucin production
Muscular stomach
• The muscular stomach in birds eating hard food is very well developed.
• The inner layer is a ceratinoid layer, consisting of secretion of gizzard glands, detached epithelial cells and fragments of food.
• Motility is synchronized with the glandular stomach:
• Gizzard contracts, then 2-3 peristaltic contractions flow along the duodenum, which is followed again by the contraction of the gizzard and then by the glandular stomach. During the first contraction of the gizzard, the content flows to the duodenum.
• Second contraction of gizzard, content flows back to the glandular stomach.
• Period of each contraction: approx 30 sec.
• Small stones in the gizzard helps to grind the hard seeds.

Question 5

Small intestines

Answer 5

Duodenum
• Related to the contractions of the stomach, peristaltic and segmented contraction.
Ileum
• Mucosa is similar to mammalian, but villi are longer and thinner.
• Fermentation occurs here. Digestive enzymes are partly of intestinal, pancreatic and microbial origin.

Question 6

Liver

Answer 6

• Double lobed, having one bile duct emptying into duodenum, and one into the gall bladder.
• Bile neutralizes the chime and emulsifies lipids, and in some species it contains amylase.
• Gall bladder species: hen, duck, goose
• No gall bladder: pigeon, guinea fowl

Question 7

Pancreas

Answer 7

• Double lobed, with three ducts that open into the duodenum

* Pancreatic enzymes digest into the lumen, enterocyte enzymes digest in the small intestines at brush border

Question 8

Cecum

Answer 8

• Paired ceca
• Two kinds of contractions: mixing and propulsive
• Mixing: less powerful, but more frequent
• Propulsive: stronger, but less frequent
• Small contractions mix the content of the ceca, stronger ones pass it along by peristalsis and antiperistalsis.
• Microbial fermentation: only volatile fatty acid absorption
• Extensive water absorption from the urea solution, which returns into this segment by antiperistalsis.

Question 9

Colon, cloaca

Answer 9

• Short and well separated from rectum
• Almost permanent anti-peristalsis passes urea from cloaca to colon/ceca, thus filling up the ceca.
• 3-4 peristalsis and 10-14 antiperistalsis occur in a minute
• To recollect important end-products of microbial digestion, birds ingest parts of their feces. They are coprophagia. This is significant because of vitamin supply.