To understand how a budgie takes a particle of food and converts it into some form of energy, we must first understand the how and why of digestion, and subsequently absorption and then assimilation.
Unlike humans, that have a fixed plate of bone for the upper part of their mouth called a "maxilla", the budgerigar has two movable appendages called "rnandibles". If you removed the upper mandible, and look at the under surface, you would find a small vertical ridge running from the back forward. Branching out perpendicularly, there are smaller ridges running laterally towards the outside of the mandibular undersurface. The budgie uses these surfaces to more or less grip or trap the seed, while the lower mandible crushes the husk off the seed. When the husk is discarded, the budgie then proceeds to literally grate the soft fibrous inside of the seeds into small pieces before swallowing them. This is why the inside surface of the budgies mouth is totally dry. For if it were not, the above explanation would shortly become an extremely sticky mess. Actually, the saliva is produced in the crop (or forestomach) of the budgie. It is in the crop that the foods are stored, and the initial mixing starts to take place. Bear in mind that absolutely no digestive process occurs in the crop. Whether or not birds have an enzyme in their crops called ''ptyalin'', which starts the digestive process of simple starches, I can not tell you, as I can not find any reference to its presence, or lack thereof! I would suspect that ptyalin is present due to the fact that crop milk is produced in the crop, and most authorities say it is a fat and protein mixture which is produced in the rubbery type cellular lining in this organ. Anyone having observed a cock feeding a hen, or a hen feeding chick will quickly tell you that the regurgitated mixture is quite fluid. We know from cellular study (called histology) that when birds are feeding chicks, the cellular growth rate of these cells increases to approximately 600% of their normal or nonmated status. We know that crop milk is for the most part extremely rich in fat, protein, vitamin A and the B complex vitamins.
Realizing that I have digressed from passing the food down the alimentary tract, we will now procede to the budgies stomach, or proventriculus. Like most seed eating animals, this glandular stomach has very little to do with digestion,
save possibly to steadily push the already moistened mixture into the grinder called a gizzard. The gizzard is really a remarkable and highly sophisticated organ. It is able to contract several times each minute and pass out a mixture ready to be absorbed and assimilated by the intestinal tract; however, it retains the grit (or teeth) inside itself provided the grit is large enough and/ or rough enough to continue to function as it's teeth. When this grit becomes inefficient, the gizzard passes it on with the rest of the mixture until it is eliminated with the rest of the waste. This is why it is necessary to constantly keep a fresh supply of
clean grit before your birds. Probably one of the greatest errors a breeder can do, is it to not change the grit periodically. Give your bird the option of what size and shape grit it wants. Just because the bowl looks adequate, does not make it so; therefore, change all of it periodically, and more than likely you will immediately see your budgie start picking up from the fresh bowl of grit.
When sufficient grinding and mixing has been performed the food passes out of the gizzard and into the first one third of the small intestine called the duodenum, where fat emulsifier called bile is secreted. Bile is produced by the liver and stored in the gallbladder until it is needed for emulsifying fats. At the same time enzymes from the pancrease (lipase and amylase) are released through a common duct into the duodenum and these break down the starches, which are the primary source of energy. If you just take into account how fast a budgie breathes or what it's normal body temperature is, you can equate how much it has to take in just to sit on the perch and eat, let alone fly around.
The liver by the way only has 39 other functions other than producing bile. It stores vitamins (refer to hypervitaminosis later in this article), stores quick energy sugar in the form of glycogen, converts the by products of their metabolic processes into particles for nutrition and waste such as urea (which is placed in circulation so it can be filtered out by the kidneys), acts as a bacterical and possibly viral filter, collects old parts from destroyed red blood cells and sends them on to the long bones to be made into new red blood cells, and this is only to mention a few - remarkable organ in anybody's book.
The food passes out of the doudenum
and into the jejunum where it is mixed further and starts being absorbed. Then into the ileum for further absorption, after which it passes into the larger intestine for final absorption. At this point most of the fluid content is totally absorbed. The waste is then deposited in the cloaca where it is mixed with the waste products from the kidney and finally expelled through the vent. The green portion of a birds exereta is from the alimentary digestive tract; and the white portion is primarily uric acid percipitate eliminated by the kidneys. And here all along you thought it was just seeds, greens, grit, cuttlefish and water!
Cage birds, in contrast to wild or free flying birds that forage for their foodstuffs, are entirely dependent on what types of foodstuffs we supply them. Many are kept on sub-optimal diets because of a lack of knowledge of nutrition on the part of their keepers, or very rarely because of a lack of interest!