Digestive System Of        Birds: Comparing Groups With Diagram 

Question: Describe the digestive system of birds.

                 

                           Or

Write about the Digestive system of different bird species.

In their early evolution, most of the birds were carnivorous feeding principally on insects. 

A large group ( nearly 1/5th of the birds ) feed on nectar. Some birds are omnivorous  ( often termed euryphagus), or wide-eating species that will eat whatever is seasonally abundant. They also compete with numerous other omnivores.

The oesophagus or "narrow eating" species had to pay their lives if food was scarce for any season ( disease, adverse climate etc. ).

The rate of metabolism in birds is very high, with immense food requirements and rapid digestion.

Bird food is of high caloric value with minimum indigestible waste. Consequently, the digestive system of birds is compact but effective and adds to the list of specific Characteristics of Aves.

Birds lack teeth and therefore they cannot break down food by chewing it.

* Note: For your knowledge (the blue font note is for added information, and may not be included in the answer.)

* (Insects were already well established in variety and numbers, on earth, before Birds made their appearance. The only partial competitors at that time were amphibians and reptiles.

But with the advantage of flight, the birds could access them above the earth and store them at places inaccessible to their earth-bound tetrapod peers.

The other animals ( like worms, molluscs, crustaceans, fish, frogs, reptiles, mammals as well as other birds ) went on to be added to the list of bird foods.

The digestive system of birds consists of :

Many birds have specialised structures to help digest food, such as,

Beaks :

The horny beak or bill, and associated mouthparts show a wide range of modifications in carnivores, piscivores, insectivores and carrion eaters as well as in birds that feed primarily on fruits, seeds, pollen, nectar leaves or roots.

The bill of flamingos contains marginal projections which provide a filter for feeding on small invertebrates or in the case of the lesser Flamingo, blue-green algae and diatoms. 

• Strong pointed beaks in crows,
• highly specialised in flamingos, hornbills and toucan and, 
• straight hard chisel-like in woodpecker.

In the Pelican and some passerine species, the floor of the mouth cavity is distensible and serves for food storage. 

Tongue :

The tongue shows as many modifications as the bill but it contains muscle only in a few birds such as parrots.

In woodpeckers, it can be protruded to an astounding length to harpoon grubs. Fish-stabbing birds ( e.g. herons ) use the tongue to push impaled prey off the beak.

Salivary glands :

Salivary glands are usually present and well developed (exception e.g. snake bird Anhinga anhinga ). The function is primarily for mucigenous lubrication. They secrete an adhesive material in swifts and woodpeckers and are said to separate amylase in some species.

Adaptation for Woodpecker 

1. By its hard chisel-like beak and long tongue it delivers powerful rapid blows to build nests or expose the burrows of insects in the tree trunk.

2. Its skull is especially thick to absorb the shocks during blows.

The gastrointestinal tract of the Goose Anser anser resembles that of the chicken except, that it has a longer and more tubular crop, a longer midgut and a longer caeca.

 Digestive System of Birds: Comparison Diagram

Oesophagus :

The Avian oesophagus tends to be long and wide and is usually dilated into a unilateral, bilateral, or spindle-shaped crop.

Crop : 

The crop is located at the lower end of the oesophagus.

 In the crop, the food is stored and moistened before it moves further into the digestive tract.

During the nesting season, in some species of birds (e.g. in the digestive system of pigeon), the breakdown of cells in the crop produces a special secretion of " milk " that is rich in protein and fat. The parent bird regurgitates this substance to feed their young ones, providing them with the nutrients needed. 

In the chicken, the crop is isolated by a sphincter that only opens after the gizzard is filled.

( The crop then fills and undergoes periodic rhythmic contractions that empty it over the next few days. )

Stomach :

From the crop moistened food moves into the stomach. The form that the bird's stomach takes depends on the bird's feeding habits. 

The meat or fish-eating birds have an expandable stomach that can store large amounts of soft food. ( the crop and the distal oesophagus of hoatzin, function as a fermentation organ.)

( In birds, the glandular proventriculus and muscular ventriculus do the function of the stomach i.e. proventriculus does the gastric secretions ventriculus help in trituration, maceration and pumping. In predacious carnivores - hawk, petrel, heron or gull, tend to have a highly distensible proventriculus, but in galliform, passeriform and certain other species, it appears to function only for secretion of gastric juice during the passage of food into the gizzard. )

Gizzard ( Ventriculus ) :

A gizzard or Ventriculus is a spindle or cone-shaped structure. It forms part of the stomach.

The birds that eat insects or seeds have this muscular organ lined with koilen ( a horny material consisting of protein and carbohydrates, periodically moulted in many species ) called a gizzard that helps in the mechanical breakdown of food by grinding it ( trituration ) and swallowed grit increases its efficiency although it is not essential.

 In many species of birds, the gizzard contains small pieces of stone and gravel that the bird has swallowed. 

The food particles are ground in between thick muscular walls of gizzard and gravel so that they become easy to digest.

The relative size and musculature of gizzard vary in different bird types. The single layer of circular muscle is involved in most carnivores and frugivores

In fruit-eating tanager, gizzard has been reduced to an insignificant band.

Among Australian parrots, the gizzard of lorikeets, ( feeding largely on nectar and pollen )is much less muscular than that of granivorous and frugivorous parrots.

( Function of gizzard in birds : 

The gizzard is considered to be a compensatory organ for the lack of teeth in birds but it also serves as a site for,

•  food storage,
• acid- pepsin digestion of protein
• propulsion of food into the intestine. 

In some birds like owls or kingfishers and shrikes, the gizzard also serves as a filter for less digestible parts  such as bones and fur, of the prey which are later formed into pallets and regurgitated and ejected through the mouth.)

In penguins, grebes, pelicans and many storks,  ducks, geese and rails, an additional chamber, called the pyloric stomach is present whose function may be related to filtration as in the case of grebes.

 

Midgut ( Small intestine ) :

Food moves from the stomach to the small intestine.

It consists of mainly duodenum and ileum where the breakdown of food is completed and food is absorbed into the body. There is wide variation in the number and arrangement of intestinal loops. 

Two bile ducts from the gallbladder or liver and two or three pancreatic ducts empty into the duodenum or the first part of the intestine. The liver is relatively large and bilobed.

The intestine also receives three separate ducts from the pancreas.

( For weight reduction many birds lack gallbladder and drain directly into the intestine by way of the bile duct. )

Hindgut  :

 

The hindgut of birds, where the intestine joins a short but dilated rectum, consists of the caeca.

Caeca :

The caeca originated as a sphincter or valve and separated from the remaining parts of the hindgut by valves in many species. 

Caeca are usually paired, but in, 

• In herons, it is single.
• In hummingbirds, swifts, some pigeons and woodpeckers, they are absent.
• They are absent in both embryo and adult parrots and budgerigars,
• small in case of predators such as hawks,
• highly developed in omnivores and herbivores (  but not necessary in granivores ),
• well developed in grouse and rhea.

 The avian caeca is classified into four types ( by Michel 1901 ) no

1. Primitive: Well-developed and thin-walled, e.g. duck, goose, grebe and loon.
2. Enlarged: With a greater amount of lymphoid tissue ( for cellulose digestion ), e.g. galliform species.
3. Lymphoepithelial: With little digestive function (e.g. pesseriform species.
4. Functionless, vestigial or absent, e.g. penguin, hawk and budgerigar.

The caeca are histologically similar to the intestine except for lymphoid tissue. 

In the caeca, the symbiotic bacteria digest fibrous food.

The rest of the hindgut of birds is short and straight and often referred to as the rectum.

  

( In the case of horned screamer ( Aakima cornula ) and ostrich, a long segment of colon is included,  ten times the length of caeca. ) 

The intestine also receives three separate ducts from the pancreas

Rectum : 

The rectum is short because the faecal matter is relatively small and almost dry due to the reabsorption of water in the rectum.

 

( As the flying animals cannot afford to be weighed down by faecal luggage it is expelled out as fast as possible. )

Cloaca : 

The cloaca receives the rectum and openings of urinary and reproductive ducts.

Digestive waste leaves the body through the cloaca.