Nutrition and digestive system in birds


Birds have high metabolic rates and require a large amount of energy for flight. Due to this energy consumption, they have a voracious appetite. Small birds have high metabolic rates and eat more food relatively their body weight than the large birds. For example, hummingbirds feed almost constantly during the day. But still cannot maintain high metabolic rate at night, hence, become torpid.

Classification of birds based on food

All the birds are grouped on the basis of their feeding habits. Birds are classified into two groups;

Euryphagous

These birds are omnivorous and feed almost everything which is available in season. But they have to compete with other omnivores having the same spectrum of food.

Stenophagous

These birds are specialist and feed very selectively. Their food basically has few species, and if some hazard like disease occurs in their prey, it proves very dangerous for them and most probably they become extinct.

Bill and tongue modification

Different birds feed on a different type of food. So their bill and tongue are modified according to food and food resources.
The tongue of woodpecker is barbed and is used for extracting insect’s larva from the tree bark.
Sapsuckers have a brush like a tongue used for licking the sap from the holes of trees.
Bill of the flamingo is used to strain food from the water and that of an eagle is used to tear the prey.

Digestive system in birds

The digestive system of birds consists of crop, gizzard, stomach, and intestine.

Crop

Many birds have an enlargement of the esophagus on its lower end that serves as a storage structure, it is known as a crop. Crop enables the birds to quickly ingest the large quantities of available food and then digest it after reaching safe places.
Crop in pigeon, dove and some parrots not only stores food but also produces lipid and protein-rich milk, composed of sloughed epithelial cells of the crop lining. Newborn babies are fed on this crop milk until they are able to eat grains. Insect-eating birds don’t have well-developed crop because they don’t get abundant food.

Proventriculus and ventriculus

Their stomach is divided into two compartments. A proventriculus which secrets gastric juice and initiates digestion. A muscular ventriculus or gizzard, lined by keratinized plates and grinds the food like a seed or other hard material. Some birds may swallow sand or pebble, which are lodged in the gizzard and helps in the digestion of food.
Enzymatic digestion and absorption of food occur the small intestine, and it is aided by secretions from pancreases and liver.
At the junction of the small and large intestine are present, paired ceca. Ceca contain bacteria which help in the digestion of cellulose. Birds mostly eliminate undigested food through the cloaca.
Owls form pellets of bones, fur, and other indigestible material. These are enclosed by the sloughed epithelial lining in the proventriculus and are removed through the mouth.



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