Nutritional Observations, Hand-Feeding Formulas, and Digestion in Exotic Birds
Proper dietary nutrition and food quality are important in fast-growing exotic birds. The right balance of protein (amino acids), energy (fat), solids (water), and other essential nutrients can be achieved with different types of hand- rearing formulas, including homemade recipes, monkey-food- or dog-food- based formulas, and specific commercial formulas. However, some diets are easier to prepare and have a smaller chance of being nutritionally inadequate because of improper mixing and contamination. Digestion in birds is a rapid and efficient process because of the specificity of their digestive organs, which functions to swallow, store, and chemically and physically digest the food.
Hand-feeding baby birds is a vital part of any successful aviculture operation. Babies from artificially incubated eggs, those removed from neglectful parents, and those pulled at a young age to simulate reclutching of parents all must be hand-fed for 3 to 5 months. Most exotic birds kept in captivity, such as psittacines and the many types of softbills, are altricial. That is, their young are hatched blind, are helpless in food gathering and are unable to thermoregulate. Unfortunately, only limited scientific work on the nutritional requirements of exotic birds has been performed; however, aviculturists, by trial-and-error feeding, have found formulas that successfully raise hatchlings to adult size.
Secretions of the upper gastrointestinal (GI) tract present in regurgitated parental food appear to enhance the growth of parent-fed psittacine chicks over that of incubator-hatched chicks hand-fed the same diet (author’s observations). The nature of those secretions and their effect on the food fed to chicks is of continued interest.
Requirements for Growth
Because poultry are precocial, little information on their relatively easy care is directly applicable to parrots, yet their well-researched and published nutritional requirements may act as guide.1.2 Poultry formulas are designed to maximize growth rate, ie, meat production, which is not as critical in exotics as is the proper development of the baby and its long-term health. The following is a discussion of a few nutritional concepts important to the feeding of exotic birds.
Growth curves have been published for only some of the hundreds of different species of psittacines and for very few other exotics.3.4 Individual and sex-related variation in size limit the use of average weight grains to monitor the health of individual babies on a particular diet. Measures of body weight taken each morning before the first feeding are less influenced by residual food from the previous feeding still in the gut of the bird if babies are allowed to eliminate residual food overnight. The time between feedings should be slowly increased as the baby gets older, but any sudden slowdown in digestion is an indication of illness.
Protein and Amino Acids
A whole-carcass amino acid composition comparison of the chicken with a small parrot, the budgerigar (Table 1), and the parallel structures of the digestive tract indicate similarity of diet between most psittacines and poultry.5 Studies conducted at the University of California, Davis, showed that cockatiels, grow best when hand-fed a diet containing 20% to 25% dry-matter protein mixed as 7% solids for the first 4 days, followed by 20% to 30% solids, and that their requirement is 1% of the diet on a dry-matter basis.6.7 These percentages are very similar to those in the National Research Council (NRC) growth requirements for broiler chickens,1 supporting use of the NRC requirements as estimates for psittacine growth requirements.
The use of animal protein sources in diets instead of supplementing with limiting amino acids to improve the biological value of plant proteins is controversial. Similar amino acid profiles and protein levels can be achieved with both methods. However, if the animal or fish products are not processed properly, there is greater potential for bacterial contamination. The proximate analysis of the oilseed kernels of sunflower, safflower, and peanut, common food for captive parrots, indicates that they contain very high fat and moderate protein levels when taking into consideration their high caloric densities (Table 2). When comparing the essential amino acid profile for growth requirements in poultry with the levels found in oilseed kernel protein, it appears that lysine and methionine are limiting in all three seeds and threonine is also limiting in safflower and peanut protein (Table 1).
Estimated essential Amino Acid Requirements of Parrots and Composition of Three Common Seed Kernel/ Nut Meats
|Essential Amino Acids*||Estimated Requirements**||Sunflower Kernel 8.9||Safflower Kernel 10.11||Peanut Meat 11.12||Budgie Carcass 13||Chicken Carcass 2|
|Glycine & serine||5.0||9.3||9.3||10.4||10.8||—|
|Lysine||5||3.9 (78%)+||3.2 (64%)+||3.2 (64%)+||7.1||9.9|
|Methionine & cystine||3.6||3.7||3.2+||2.5+||—||4.3|
|Methionine||1.9||1.8 (95%)+||1.5 (79%)+||1.2 (63%)+||2.2||1.9|
|Phenylalanine & tyrosine||5.9||7.4||7.2||8.9||6.8||6.7|
|Threonine||3.7||3.8||3.2 (86%)+||3.1 (84%)+||4.0||3.4|
|No. of limiting amino acids||2||3||4|
Note. Measurements are grams of amino acid per 16g of nitrogen.
- * Essential for growing poultry.1
- ** Based on NRC requirement 1 for broiler chickens receiving a diet of 20% protein and 3.200 kcal ME/kg.
- + Limiting amino acids and percentage of requirement supplied when compared with estimated requirements.
Although serine, cystine, and tyrosine are not essential amino acids, they are included with essential amino acids because they may spare a portion of the requirement for glycine, methionine, and phenylalanine, respectively. However, under certain conditions the requirement for cystine may not be met by synthesis from methionine, especially if the diet is low in both of these amino acids. Feather protein contains a higher amount of cystine than do other body proteins13; thus, during maximum feather growth in a nestling or molting in an adult, the requirement for cystine may increase. Cystine also serves as the precursor of taurine, which in chickens, and probably parrots as well, is conjugated with cholic acid during the formation of bile.2 Bile formation is induced by fat content, which is high in oilseed kernels and approximately twice as high in most psittacine hand-feeding formulas than in standard poultry starter diets.
Proximate Analysis of Seed Kernels or Nut Meat Expressed as a Percent and Their Gross Energy Value
|Kernel or Nut Meat||Moisture||Crude Protein||Crude Fat||Crude Fiber||Ash||NFEµ||Gross Energy
(kcal / kg)¥
Note. Percentage derived from mean values of two samples sent to separate laboratories.
- µ NFE (nitrogen-free extract) is the difference between 100% and the sum of the percentages of moisture, protein, fat, fiber, and ash. It is considered to represent the carbohydrate other than fiber.
- ¥ Mean values based on four bomb calorimeterics for each feed.
- ¤ Hulled confectionery sunflower as used in the ME trail.
- § Hulled by hand from seeds found in Topper Bird Ranch Diet (Lexington, NC).
The dietary energy the bird can use is referred to as metabolizable energy (ME).14 The ME of formulas can very significantly; the NRC for broiler chickens is based on a 3,200 kcal ME/kg diet, whereas oilseed diets are probably around 5,750 to 6,250 kcal ME/kg (based on gross energies of 6,800 to 7,400 kcal/ kg [Table 2] and approximately 85% use). Knowing the daily energy requirements and the ME value of a diet is helpful when formulating vitamin, mineral and protein levels to ensure optimum intake of these nutrients. Birds flying free in the wild have much higher daily caloric needs than sedentary cage birds. Flights to and from the nest to feed offspring further increase caloric needs. Thus, the different caloric needs of captive birds may limit feeding “natural” diets similar to those found in the wild.
Amino acid and the nutrient requirements are often listed in percentages of the diet; however this method of expressing them makes it difficult to compare the amino acid profiles of diets with different energy and protein levels. Expressing amino acid levels as percentages of the protein and in isocaloric portions makes comparisons more meaningful. the protein levels of formulas are often compared without taking into account differences in ME. The first diet scientifically tested on a psittacine had half the fat level now found in most formulas, which results in higher energy values.5 Thus, the levels of vitamins, minerals, and protein have to be increased over estimated requirements, which are based on lower MEs.
Some say that because the minimum nutrient requirements for most exotic species are not known, companies should not state that they have a complete diet. However, if the diet has successfully and regularly raised a significant number of young of a certain species, then it would be difficult to say that formula has not met the requirements for growth. Other criteria in evaluating a diet should include the health of the babies, the ease of formula preparation and storage, cost, and availability.
Microbial contamination of formula is a serious concern, whether it is fed to babies by the parents or by the aviculturist. There are three types of food-borne disease bacteria: those that multiply in the intestinal tract or other sites in the body and cause disease by infection of the host (eg. salmonellae); those that grow and elaborate toxins in foods before they are eaten (eg. botulinal and staphylococcal food poisoning); and those that apparently act through a dual mechanism (eg. Clostridium perfringens, Escherichia coli). In the last-named case, illness occurs after ingestion of large numbers of the living organism, which in turn release toxins into the intestinal tract. Other microorganisms may themselves not be harmful, but many indicate the possible presence of pathogenic organisms or unsanitary processing and handling and unnecessary wastage.
An obvious way to reduce the chance of feed being contaminated with pathogenic microorganisms is to not use ingredients that may contain them, such as animal by-products, poultry, or fish. Food samples can be sent out to laboratories to check for pathogenic bacteria. Using more than one laboratory reduces the chance of misinterpretation due to laboratory error. Most laboratories require at least a 100-g sample of feed to run basic proximate analysis as well as some microbiology tests (Table 3). Digestive function may be upset by bacterial infections or foreign- matter blockage of the crop opening, such as with bedding substrate, perhaps the result of chicks ingesting bedding material because of understanding. Food that remains in the crop too long may begin to ferment, leading to a “sour crop.” Dietary causes of a slowdown in the digestive tract include food that is too cold, food with inadequate moisture content or ability to hold water (gelatin), food too high in fat or protein, and food with incorrect fiber levels. Hard lumps may form in the crops of some babies if the solid matter of the hand-feeding formula separates from water. Treatment consists of feeding a little warm water and massaging the lump in the crop until it dissolves.
Secondary disease problems, such as yeast infection in the crop, often are associated with the slowing of gut transit time and continual refilling of the crop, which thus never completely empties. The primary problem with the diet, environment, digestive tract, or other cause needs to be addressed. For instance, a case of an abnormally high incidence of Candida infections in palm cockatoos may have been the result of excessive levels of simple sugars in the sweetened applesauce and sugar ingredients (honey) of the diet being used.15
Water and Dry Matter Levels
A clean source water and its proper use in formulas is a critical part of hand-feeding. Water systems can be significant sources of pathogens and should be checked by an independent laboratory (Table 3). In areas with questionable water quality, boiled or distilled water should be used to prepare formulas. Old building pipes may also harbor pathogenic bacteria; their effect can be minimized by running the tap for 1 or 2 minutes before use.
|A&L Mid West Laboratories, Inc
13611 “B” Street
|Hazleton Laboratories America, Inc.
3301 Kinsman Boulevard
|Industrial Laboratories of Canada, Inc.
95 Townline Road
|Woodson-Tenent Laboratories, Inc.
345 Adams Avenue
Neonates (chicks less than 3 to 5 days old) fed traditional formulas require the formulas to be more diluted with water, with about 5% to 10% solids, or their crops may become impacted. Even when this is done there still may be a lag of about 1 week in growth; however. This growth lag is no longer apparent at the end of the raising period.4 There have been reports that babies will develop faster during the first week on a formula with significantly lower protein. Than regular formula, about equal fat content, and containing mostly digestible carbohydrates, such as found in rice and corn syrup.16 Analysis indicates that this diet is not as diluted as regular formula. It could be that the increased level of water needed in traditional diets is to assist the neonate in using the nutrients. Feeding less complex diets for the first few days may allow the chick to develop it digestive system and remain fully hydrated without having to deal with nutrients it cannot efficiently use. Parent- fed chicks do not appear to have this problem with traditional formulated diets, perhaps because they receive secretions from the parent that assist in digestion.
Older babies (>1 week old) should receive formula 20% to 30% solid content, which usually results in a consistency a little thinner than applesauce. Such a texture may not represent the correct nutrient density because thickeners, such as cornstarch powders, can make a formula with low dry matter levels appear much denser. Feeding frequency or gut transit time is dependent on the percent solids of the formula, its digestibility, and caloric density. Formulas that are watered down too much will require more feedings and may result in inadequate weight grains.
Many different recipes have been found to satisfactorily raise baby birds, but probably an equal number have resulted in stunted or diseased birds. For instance, baby birds may have protein/energy malnutrition and/or calcium/vitamin D deficiency, which lead to bent bones and severely deformed bodies. Homemade recipes are often complicated and contain many ingredients; a successful diet used by several large breeders in California includes 11 different ingredients. However, breeders are hesitant to change from a diet that works well because of a lack of confidence in other diets even if they are easier to make and cheaper.17 Vitamin and mineral supplementation are important with these diets, but commercial products used to supplement these recipes may not be designed to correct deficiencies in, for example, fat, amino acids, calcium, or vitamins. Likewise, excess of other nutrients may result. One of the largest and most professionally run psittacine breeding farms had a problem with hypervitaminosis D in baby macaws linked to excessive use of vitamin and mineral supplements in their hand-feeding diet.18 Because of species differences in tolerance of excessive levels of nutrients, other psittacine species fed on the same diet did not develop problems.
Monkey Chow/Dog Food Base
Peanut butter, baby foods, apple sauce, and calcium are often added to monkey-food or dog-food based formulas to increase the fat content or improve the texture or nutritional value.4.16 When feeding cockatoo chicks, some breeders add ground sunflower kernels to the monkey chow base, which increases the fat and protein levels, with a resulting increase in weight gain.19 Some crush the monkey chow and simply add water, but most breeders soak the biscuits in water and then fully cook them in a microwave.4 Cooking destroys most of the bacteria often found in these foods. To save on preparation time, some aviculturists cook a large batch of monkey chow and then freeze small portions, which are reheated when required. However, this processing may reduce the vitamin content of the formula, as several breeders using these preparation methods reported increased growth rates when a vitamin supplement was added to the cooked batches.16 Although diets meant for dogs or monkeys may appear to satisfactorily raise birds, the companies that market these diets do not recommend them for birds nor is nutritional research on birds being supported by the sale of these products.
Specific Commercial Formulas
Commercial diets attempt to provide optimum levels of nutrients for as wide a range of species as possible without dangerous excesses. For a few rare species in aviculture, some formulas may need modifications, such as fat supplementation, to achieve maximum growth rates. Some breeders too quickly blame commercial diets for poor results, without examining all aspects of husbandry and all possible sources of disease. Most commercial formulas are simple to prepare, needing only to be mixed with hot water. This encourages the use of fresh, rather than old stored food at each feeding, reducing the chance of contaminating the babies with high levels of bacteria. Mixing these products with significant quantities of human baby foods or other ingredients may dilute some essential nutrients to the point that they become deficient and subject to the problems associated with other recipes.
Diets based on extruded ingredients that are ground up into a fine powder are becoming more common than diets of powdered ingredients mixed without any further processing. The high-temperature cooking used during extrusion has some benefits, including significantly lowering bacteria and fungi levels and gelatinizing starches, which improves retention of water. The percentage of chicks reaching the fledging stage significantly increased in parent-fed psittacines given an extruded formulated diet versus those who were fed seeds, fruits, and vegetables.20
Enzyme and Bacterial Additives
The “normal” aerobic alimentary tract flora for baby psittacines may include Lactobacillus sp, Staphylococcus epidermidis, Stretococcus sp, Corynebactrium sp, and Bacillus sp.21 Food regurgitated by the parents to feed chicks may contain some digestive enzymes that aid in digestion of food by the chick. Amylolytic activity has been reported the saliva of turkeys, geese, domestic fowl, and pigeons.22 However, the lingual apparatus of psittacines is unique among birds and may contain salivary glands that secrete significantly more enzymes and/or mucus than birds previously studied.23
A large California facility, now closed, used crop washes from healthy adult birds to feed to hacthlings not exposed to parental feeding as a means of chicks obtaining normal flora.24 There is, however a chance of spreading disease from subclinical carriers and no conclusive evidence exists to support the benefit of this transfer or flora.4 Likewise, the supplementation of “friendly” bacteria, such as the common Lactobacillus strains used in agriculture, has not yet shown any significant benefits in psittacine hand-feedings diets; however, adult birds colonized with species-specific Lactobacillus do show resistance to colonization by pathogenic Enterobacteriaceae.25,26 The positive effects of a microbial product in growing animals may only be discernible in situations where the management, sanitation, and diet are lacking.25 There are as yet no controlled studies that show that species- specific bacterial additives will improve growth, health, or digestibility of the hand- feeding diet, and further studies of possible benefits are needed.
The Upper GI Tract and Digestion
The avian GI tract is one of the most efficient GI tubes of the vertebrates, the entire length being far shorter per unit of body weight or surface area that is found in nonavian vertebrates. Each section of the upper GI tract, that is, the esophagus, crop, proventriculus, and gizzard, performs a separate function in most birds, including psittacines; these functions collectively occur in the single stomach of primates. Gastric digestion in birds differs in that gastric juice is produced in one organ, the proventriculus, and gastric proteolysis usually occurs largely, or entirely, in a second organ, the gizzard.
The process of digestion involves all of the mechanical and chemical changes that ingested food must undergo before it can be absorbed in the intestines. Mechanical changes include swallowing, maceration, and grinding of food in the muscular stomach. Chemical digestion consists of secretion of enzymes from the mouth, stomach, intestines, and pancreas, of bile from the liver, of hydrochloric acid from the stomach, and of bacterial action. The efficiency of digestion and absorption are greatly affected by digestive secretory activity, the physical nature of the food, and the length of time available for absorption.
Salivary glands are distributed in the oral and pharyngeal cavities of most birds. They may be poorly developed, as in many fish eaters, or well developed, as in granivorous species on a dry diet. Salivary glands consist of masses of secretory cells that are arranged around a duct system into which part of the content of the secretory cells is released to form saliva.23 Because mastication does not occur in birds, the main requirement is for lubrication to assist the swallowing process, which is achieved by the mucinous nature of the saliva. Peristaltic contractions carry food along the esophagus either to the proventriculus directly or into the crop.27
The salivary glands of psittacines form compact glandular bodies. In the African Grey Parrot (Psittacus erithacus) the duct systems of all lingual salivary glands open on the lingual surface, each through a single orifice.23 Glandular secretions in the mediacaudal section of the esophagus, together with external water, contribute to the softening and swelling of food within the crop.28
Three amylolytic Lactobacillus strains isolated from the chicken crop had amylase activities similar to those of other starch-hydrolyzing bacteria isolated from the gastrointestinal tract.22 These enzymes, together with those of the salivary glands, may provide some hydrolysis of complex carbohydrates.
The crop is a membranous storage pouch on the ventral surface of the esophagus and is deeply folded to facilitate distension.29 It normally serves only as a storage site for ingested food before it is digested by the rest of the GI tract.30 The crop in parrots is oriented transversely across the neck.31 The crop also serves as a storage chamber for food for the young; in feeding the young, the food is regurgitated by a special reverse peristalsis.29 It is possible that some minor digestion occurs in the crop as a result of both enzymic and bacterial action.32
The proventriculus is an ovoid structure located between the lower oesophagus and the gizzard and is lined with a glandular mucous membrane that contains the gastric secretory glands.29 Following the intake of food, most of this mucus is discharged and is subsequently replaced with secretion from the basal region of the mucosal cell. The main function of the proventriculus is the production of gastric juice (mucus, hydrochloric acid, and pepsinogen) and the propulsion of juice and food into the gizzard.27 Passage of food to the proventriculus is dependent on the motor activity of the crop and the lower esophagus, which are, in turn, regulated by the activity of the gizzard.
The flow of gastric juice, which has a pH approaching 0.2 to 0.5 is approximately 8 to 9 mL/kg/h for the chicken, a rate that far exceeds that of man, dog, rat, and monkey.32 Even the basal (unstimulated) secretory rate is higher than other species and indicates the high rate of digestive activity in birds.
The gizzard has two highly specialized morphological features: massive muscle development and a thick, hard covering over the mucous membrane: these features relate to the gizzard’s functions as a food-grinding chamber and as a site of peptic proteolysis.27 The main body of the gizzard is composed of two thick, opposed, lateral muscles, the ends of which are attached to a central aponeurosis, and two thin anterior and posterior intermediary muscles.27 Among the granivorous birds, the most poorly developed gizzard musculature occurs in the parrots, who not only remove the shells of seeds, but also fragment the seeds mechanically with their bills.33 The inner layer of the gizzard consists of a thin submucosa, a glandular mucous membrane, and a thick, abrasion-resistant lining mainly composed of the hardened secretion of the gizzard glands and showing longitudinal ridges and grooves. The secretion is a keratinlike substance that has been termed koilin. Attrition of this lining by the grinding action of the powerful muscle contractions, especially in the presence of grit and hard, large food particles, and the corrosive effect of the acid-enzyme mixture that flows into the gizzard from the proventriculus is countered by slow secretory activity of the gizzard glands, which renews the koilin lining.34 The koilin lining of the muscular stomach is especially well developed and distinctly structured in granivorous species, such as parrots.33
The muscular gizzard favors mechanical demolition of food particles, but also contributes to the efficiency of the system by refluxing (thick muscle action) the semiliquid digesta forward to the level of the proventriculus and aborally (thin muscle action) into the duodenum, where absorption begins.35
- National Research Council: Nutrient Requirement of Poultry
(ed 8), Washington, DC, National Academy Press, 1984.
- Scott ML, Nesheim MC, Young RJ: Nutrition of the Chicken
(ed 3), Ithaca, NY, M.L. Scott, 1982.
- Hanson JT: Hand-raising large parrots, methodology and expected weight gains, Zoo Biol 6:139-160, 1987.
- Clubb SL. Clubb KJ, Schubot R: Psittacine Aviculture. Loxahatchee, FL, Avicultural Breeding and Research Center, 1992.
- Massey DM, Sellwood EHB, Waterhouse CE: The amino acid composition of budgerigar diet, tissue and carcase. Vet Rec 72:283-287, 1960.
- Grau CR, Roudybush TE: Lysine requirement of cockatiel chicks. Am Fed Aviculture Watchbird 12(6):12-14, 1986.
- Roudybush TE, Grau CR: Food and eater interrelations and the protein requirement for growth of an altricial bird, the cockatiel,. J Nutr 1 116:552-559, 1986.
- Robinson RG: Amino acid and elemental composition of sunflower and pumpkin seeds. Agron J 67:541-544, 1975.
- Sastry MCS, Murray DR: The tryptophan content of extractable seed proteins from cultivated legumes, sunflower and Acacia. J Sci Food Agri 37:535-538, 1986.
- VanEtten CH et al: Amino acid composition of safflower kernels, kernel protein, and hulls and solubility of kernel nitrogen. Agri Food Chem 11(2):137-139, 1963.
- Food and Agriculture Organization: Amino acid contents of foods and biological data on proteins. Rome, FAO Nutr Stud No. 24 FAO, 1970.
- Pancholv SK, Deshpande AS, Krall S: Amino acids, oil and protein content of some selected peanut cultivars, Proc Am Peanut Res Educ Assoc 10:30-37, 1978.
- Deschutter A. Leeson S: Feather growth and development. World Poultry Sci 42:259-267, 1986.
- Miller MR. Reinecke KJ: Proper expression of metabolize energy in avian energetics. Condor 86:396-400, 1984.
- Sheppard C, Turner W: Handrearing palm cockatoos. AAZPA Annual Processing, Portland OR, September 1987, pp 270-278.
- Voren H, Jordan R: Parrots: Hand-Feeding and Nursery Management, Pickering, Ontario, Canada, Silvio Mattcchione, 1992.
- Worth G: Personal communication, August 1990.
- Takeshita DL, Graham L, Silverman S: Hypervitminosis D in baby macaws. Proceedings of the Association of Avian Veterinarians, 1986, pp341-346.
- Silva T: Amonograph of endangered parrots. Pickering Ontario, Canada, Silvio Mattacchione, 1989.
- Ullrey DE, Allen ME, Baer DJ: Formulated diets versus seed for psittacines, J Nutr 121:S193-S205, 1991 (suppl).
- Drewes L, Flammer K: Preliminary data on aerobic microflora of baby psittacine birds. Proceedings of the Jean Delacour/IFCB Symposium on Breeding Birds in Captivity, Hollywood, CA, 1983, pp 73-81.
- Jerrett S.A. Goodge WR: Evidence for amylase in avian salivary glands. J Morph 139:27-46 1978.
- Homberger DG: The lingual Apparatus of the african grey parrot. Psittacus Erithacus dinne (Aves: Psittacidae): Description and theoretical analysis. Ornithological Monographs 39, Washington, DC, American Ornithologists’ Union, 1986.
- Flammer K: Pediatric medicine, in Harrison GJ, Harrison LR (eds): Clinical Avian Medicine Surgery, Philadelphia, PA, Saundes, 1986, pp 634-650.
- Joyner KL: The use of a lactobacillus product in a psittacine hand-feeding diet; its effect on normal aerobic microflora early weight gain, and health. Proceedings of the Association of Avian Veterinarians, 1988, pp 127-137.
- Gerlach H: Repulsion of Enterobaceriaceae by using Lactobacilli. Proceedings of the Second International Parrot Convention, Loro Parque, 1990, pp E1-E17.
- Hill KJ: Thephysiology and Biochemistry of the Domestic Fowl, new York, NY, Academic Press, 1971.
- Griminger P: Digestive system and nutrition, in Abs M(ed): Physiology and Behaviour of the pigeon. New York, NY, Academic Press, 1983, pp 19-40.
- Duke GE: Alimentary canal: Anatomy, regulation of feeding, and motility, in Sturkie PD (ed): Avian Physiology, New York, NY, Springer-Verlag, 1986, pp 269-287.
- Hill KJ: The structure of the alimentary tract, in Bell DJ, Freeman BM (eds): Physiology and Biochemistry of the Domestic Fowl, New York, NY, Academic Press, 1971.
- Evans HE: Anatomy of the budgerigar, in Petrak MI, (ed) Diseases of Cage and Aviary Birds (ed 2). Philadelphia, PA, Lea & Febiger, 1982. pp 111-187.
- Duke GE: Alimentary canal: Secretion and digestion, special digestive functions, and absorption, in Sturkie PD (ed): Avian Physiology, New York NY, Springer- Verlag, 1986, pp 289-302.
- Ziswiler V, Farner DS: Digestion and the digestive system, in Farner DS. King JR (eds): Avian Biology, vol2. New York, NY Academic Press, 1972, pp 343- 430.
- Hill KJ: Physiology of the digestive tract, in Freeman VM (ed): Physiology and Biochemistry of the Domestic Fowl, vol 4, New York, NY, Academic Press, 1983, pp 31-49.
- McLelland J: Digestive System, in King AS, McLelland J (eds): Form and Function in Birds, vol.1. New York, NY, Academic Press, 1979, pp 69-181.
By Mark Hagen, M.Ag.
Director of Research
From the University of Guelph, Ontario, Canada.
Address reprint requests to Mark Hagen, MAg,
Copyright © 1992 by W.B. Saunders Company