This section has been written as a guide for the Aviculturalist/Keeper to give an understanding of how the Digestive Systems and various food types are inter-related. There are a number of references to be found via the search function for those that wish to delve more deeply into the subject.
Two main factors have influenced the evolution of the various forms of the Avian Digestive System.
The beak or bill, as used when referring to Waterfowl, is a bony structure with a covering composed mainly of Keratin. The Keratin layer is replaced as it is lost through normal wear and tear. There are endless shapes of beaks, because of the number of functions it is used for (Bock W J 1966). Capture of food, tearing, cracking seeds or nuts, climbing, preening and display. Food types have also played an important role in the structure of the beak. There have been many studies on this subject and some of these can be found in the references section.
Unlike mammals the tongue is not composed of muscle layers, but it's movement is controlled by the bone and musculature of the Hyoid Apparatus. Psittacidae (Parrots) do have some musculature in their tongues, which gives added flexibility for manipulating seeds.
The shape and length of the tongue gives an indication of the chosen food types eaten. Long, narrow tongues have evolved to act as probes, brushes or spears. Examples are to be found in Woodpeckers and Lorikeets/Lories. The tongues of Pigeons/Doves and Passerines tend to be smooth and short and their only function is to aid swallowing. Many fish-eating birds have short, needle-like papillae to aid the capture of their slippery prey.
This is a thin walled tube-like structure that runs from the mouth to the Stomach (Proventriculus). The walls have longitudinal folds that allow the oesophagus to expand, so enabling the swallowing of large food items. There are also many mucous glands that allow for lubrication of the food to aid its passage to the stomach. The mucous glands are larger and more numerous in seed eating species, where the food tends to be of a drier nature and harder to swallow. The oesophagus can also have a food storage functions in some species, such as Cormorants, Penguins, Owls and Woodpeckers.
The structure and function of the crop has evolved into varied shapes and sizes, as a result of the various food types eaten. It can vary in shape from a simple swelling of the oesophagus, a pouch or diverticulum through to a complex double sided structure as found in Pigeons. A true crop has a muscular sphincter or valve that controls the entry and exit of food. True crops are more often found in Granivorous species, where it acts not only to store food, but also soften it prior to digestion. False crops are either swellings of the oesophagus or diverticulae of various sizes, that are used for short term storage.
In most birds this is to be found as two structures.
The Small Intestine is a long tubular structure, which is divided into three areas - Duodenum, Jejunum and Ileum. Histological examination of three areas shows very little difference in morphology. The three areas are more or less defined by "landmarks" along its length. There is very little variation in the structure of the Small Intestines between species, as the food is of a fairly uniform consistency on arrival.
Duodenum - This portion leaves the gizzard and then loops around the Pancreas, from where digestive enzymes are released. The Bile Ducts from the Liver also join in this area.
Jejunum - This is the portion of intestine that runs from the Duodenum to a point where the remnants of the Yolk Sac (Vitelline Diverticulum) are to be found.
Ileum - This portion runs from the Vitelline Diverticulum to a point adjacent to the caeca and the beginning of the rectum.
The caeca (if present) are located at the Ileo-rectal junction and are almost always in pairs. Larger well-developed caeca are to be found in species with a high fibre diet, such as herbivores and some omnivores (Clench & Mathias 1995). Microbial fermentation occurs, allowing the breakdown of complex carbohydrates that have resisted enzymatic processes in other parts of the digestive tract. It has also been found that Nitrogen absorption, Water retention and immunosurveillance can occur in various species according to food type and habitat.
The rectum is a short tube like structure that connected the intestines to the three-chambered sac-like structure known as the Cloaca or Colon. In avian species the rectum has no developed functions other than to allow the passage of undigested food (faeces) into the Cloaca. The Cloaca serves as a storage organ for faeces and Urine/Urates arriving from the Kidneys via the Ureters. The reproductive organs also have ducts opening into the Cloaca. Waste products are expelled through a sphincter at the base of the Cloaca.
The physiology of the digestive processes in birds is very similar to that found in mammals. The following is a simplified description of these processes. More detailed information can be found in the reference section and also where appropriate under the species section.
There is one notable difference with mammals in that many species of birds have the abiltily to reverse the flow of the digesta through the alimentary tract. These reflux movemnts can occur between the Proventriculus/Gizzard, Duodenum/Gizzard and the Rectum/Caeca. The first two are to allow more mechanical breakdown of the digesta and extra time for enzymatic actions in the acidic conditions to occur. This is somewhat similar to the process found in Ruminants - "Chewing the Cud" - where the digesta is reflux into the mouth for more mechanical breakdown, but the enzymatic processes are carried out by bacterial fermentation in the rumen. The Caeca, when present, serves this purpose in avain species - see section on bacterial processes.
Due to the fact that the beak, mouth and tongue are designed for food collection/capture very little in the way of enzymatic action takes place here. The saliva is produced as a lubricant to aid the passage of food into the Oesophagus, although Salivary Amylase is found in some species that de-husk seed. Amylase is able to aid the breakdown of some Carbohydrates, such as Amylose & Glycogen, into the disaccharide Maltose.
As with the mouth very little in the way of digestive processes take place here, with the exeption of the Hoatzin. The Crop and Oesophagus primary function is for storage. Research (Soedarmo 1961 & Ziswlier/Farner 1972) suggest that some breakdown does occur due to the action of salivary amylase or microbial action.
As the food passes through the Proventriculus it enters an acidic (pH 2-3) environment where Protein digestion can begin aided by the enzyme Pepsin. Due to this, complex protein molecules are broken down into simpler Polypeptide chains. This is a relatively slow process and the reversing of the digesta flow gives more time for the processes to occur, with more mechanical action and Acid/Pepsin secretions. This action also begins the breakdown of large Lipid (Fat) molecules (Duke 1986).
As food enters the duodenum enzymes are released from the intestinal walls and from the Pancreas.Bile salts also enter the digestive tract at this point. The Pancreatic secretions also buffer the acidic pH of the digesta raising it to more neutral conditions. The optimal range of the intestinal and pancreatic enzymes is in the range ph 5.6 - 7.2 (Hurwitz & Bar 1968). Protein digestion continues under the influence of Trysin/Chymotrypsin breaking down the poly-peptide chains into amino acids. Carbohydrates/disaccharides are further broken down into monosaccharides, mainly Glucose. Amylase, Maltase and Chitobiase are among the enzymes responsible for this. Lipids/Fats are broken down into Glcerides, Fatty Acids and Alcohols under the action of such enzymes as Lipase, Phospholipase and Esterase. As the digesta continues it's journey down the digestive tract these products are absorbed through the intestinal wall and into the bloodstream. The digesta become more viscous due to the loss of soluble carbohydrates and water absorption. Undigested plant material such as Cellulose, Pectins and other fibrous material, also help the thickening process. Water,Vitamins and minerals are also absorped into the bloodstram from the intestines.
Many species of birds do not have a functional caeca and any remaining digesta and waste products pass into the rectum. The main function of the caeca is the microbial fermentation of vegetable based materials, that the bird was unable to enzymatically digest. Caeca are found almost exclusively in herbivores and omnivores. The caeca has two other functions, water retention and Immunosurveillance. Very little enzymatic digestion takes place in the caeca, but nutrients are absorbed very well. For more information on microbial fermentation, please see section on Micro-organisms and Probiotics.
Some absorption of electrolytes, Fatty Acids,Water and Glucose takes place in the rectum, particularly in those species without developed Caeca. Waste products are passed out of the body via the Cloaca.
A brief outline of bacterial morphology may help to understand some of the terms used in the scientific publications. Fungal and Viral micro-organisms have their own classifications, but as they have very little, if any, action on digestive process, will not be included in this discussion.
Bacteria are primarily classified on their shape and the two main structures are rods and cocci(spheres). In this case the word "cocci" is pronounced "cock-aye", but when referring to the Protozoan parasite it is pronounced "cock-sea". Rods can vary in shape from small ovoid structues to elongated structures occurring in chains. Cocci can occur singly, or in pairs (diplococci) or chains.
The terms Gram Positive (G+) and Gram Negative (G-) are used widely when describing bacterial classification. These terms are derived from the action of the cell wall to the Gram staining method. Bacteria that are termed G+ are purple and G- pale red/pink when viewed though a light microscope at approximately 1000x magnification. The final structure used in the classification of bacteria is the presence of an Endospore. These are only found in members of the Bacillus and Clostridia families, although with scientific progress other families may be discovered in the future.
| Shape | Gram Stain | Endospore | |
|---|---|---|---|
| Lactobacillus sp. | Rod | +ve | -ve |
| Bifidobacterium sp. | Rod | +ve | -ve |
| Clostridium sp. | Rod | +ve | +ve |
| Staphylococcus sp. | Cocci | +ve | -ve |
| Streptococcus sp. | Cocci | +ve | -ve |
| Enterococcus sp. | Cocci | +ve | -ve |
| Enterobacter sp. | Rod | -ve | -ve |
Many species of bacteria(50+) have been identified from the Gastro-intestinal tract by laboratory culturing techniques. Recent developments in the identification of bacteria by DNA, RNA and PCR techniques suggest that this may only be the tip of the iceberg, as many so far unculturable species have been discovered. The main function of the bacteria within the Gastro-intestinal tract is the fermentation of mostly plant material, which takes place in the caeca and to some extent the rectum. The bacterial population also has functions with small scale Vitamin synthesis, Stimulation of the Immune System and Pathogen suppression.
The Gastro-intestinal tract of a healthy bird will contain many differing species of bacteria. Disease, stress or improper diets can have a profound effect on the numbers and proportions of these bacteria. The principal behind probiotics is that they can be used to replace or enhance the bacterial population, preferably once the cause of the problem has been sourced and treated. For example, there is no point in giving probiotics when a course of antibiotics is still being given.
Probiotics are available in various forms from simple naturally occurring living cultures, such as Yoghurt to complex Competitive Exclusion products, containing many species of bacteria. Probiotics can be obtained in liquid, gel and powder form. The liquid and gel forms have a short shelf life and a careful check should be kept on the "Use by" date and storage instruction.
Powdered forms are either supplied in plastic tubs or sealed foil packets. They have long shelf lives unopened, but because of their hygroscopic nature can deteriorate quickly if kept in a damp environment. Manufacturers instructions should be strictly adhered to.
There is no ideal probiotic for each species of bird and as most of the probiotic development was performed on poultry and to some extent racing pigeons, a little thought must be given as when to use them. For example a probiotic produced for poultry use, would in theory, have very little benefit for Parrots, that have no caeca and hence a mainly Gram +ve gut flora.
The bacterial content of probiotics is limited by current laboratory culturing techniques, although many other species occur naturally. Having said this, many trials have taken place, again mainly with poultry, and Probiotics have been shown to have beneficial effects.
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