Clinical Perspectives on the Principles of Avian Nutrition

Key Points

  • The types of foodstuffs consumed in the wild are often used to classify the nutritional requirements for groups of animals.
  • The physiology of a particular species determines its nutrient requirements. Requirements are determined for basal, maintenance, and total physiologic states.
  • Requirements for companion birds are not well established. Few nutrient requirements in birds have been evaluated scientifically, instead requirements are often based on a best guess derived from galliforms.
  • Understanding the energy requirements of the bird in its life stage is important for determining the amount of diet that can be fed and/or consumed.
  • While birds eat to meet their daily metabolic requirement, they may continue to eat out of boredom in captive environments or they may become obese when their foods are high in energy but are unbalanced from an amino acid perspective.
  • Molt results in increased protein needs to grow new feathers, as well as increased energy expenditures to stay warm and acquire protein sources (food).
  • Vitamin A deficiency leads to signs of hyperkeratinization of mucous membranes (appearing as immature basophilic cells on gram stains), anorexia, poor conditioning, and increased susceptibility to infection. Parrots also exhibit squamous metaplasia of the salivary glands.
  • Vitamin D-deficient hens lay eggs with thin shells and develop osteomalacia with pathologic fractures, while birds with excess vitamin D have increased mobilization of calcium with soft tissue mineralization. Vitamin D toxicity may lead to renal failure in chicks.
  • Deficiencies of vitamin E result in a number of symptoms that relate to cell membrane dysfunctions.
  • Calcium is necessary for bone mineralization and eggshell calcification; in its ionic form, it is needed for nerve conduction and for myofibril contraction. Egg binding in cockatiels is associated with a lack of calcium and the active form of vitamin D.
  • A formulated diet is merely a starting point for providing balanced nutrition for the needs of the diverse group of birds classified as psittacines. Nutritional requirements may vary between species of birds and different life stages.

Introduction

The types of foodstuffs consumed in the wild are often used to classify the nutritional requirements for groups of animals. Usually birds within the Order Psittaciformes are considered to consume plant-based foodstuffs and are classified as florivores. Subdivisions within this category include granivores (budgies and cockatiels), frugivores (many of the macaws, e.g. green winged macaw), and nectarivores (lorikeets and lories). Yet these artificial lines are sometimes too simplistic, as many psittacine birds cross over a category to consume a larger variety of foodstuffs. An example is the scarlet macaw (Ara macao), which is classified as a frugivorous-granivorous psittacine.

Psittacine birds generally consume plant-based foodstuffs and are classified as florivores. Photo credits:  Florida Center for Instructional Technology (upper left), Steve Bittinger via Flickr Creative Commons (upper right), Peter via FCC (lower left), Derrick Coatzee (lower right)

 

The availability of certain foods, the sex of the bird, and its age also influence what it eats. However, some species have a very limited diet, such as the glossy backed cockatoo (Calyptorhynchus lathami), which feeds almost exclusively on seeds from a single species of tree in their native environment.

 

Table 1. Feeding strategies and common diet ingredient of wild psittacine birds.
(Table adapted from Koutsos, et al. Nutrition of birds in the order Psittaciformes: a review. J Avian Med Surg. 2001;15:257–275.)
Species name Feeding strategy Common diet ingredients Time spent feeding*
Blue and gold macaw
(Ara araraunda)
Florivore Seeds, fruits, nuts NR
Military macaw
(Ara militaris)
Florivore Seeds, nuts, berries, fruits NR
Green-winged macaw
(Ara chloroptera)
Frugivore Fruits (hymenaea), palm nuts, seeds NR
Orange-winged Amazon      (Amazona amazonica) Frugivore Fruit (85% from palm fruit) NR
Scarlet macaw
(Ara macao)
Frugivore-granivore Fruits, nuts, bark, leaves, shoots NR
Budgerigar parakeet
(Melopsittacus undulatus)
Granivore Seeds NR
Cockatiel
(Nymphicus hollandicus)
Granivore Seeds (prefers soft, young overmature, hard seeds 3 h/d
Hyacinth macaw
(Anodorhynchus hyacinthinus)
Granivore Palm nuts (50% lipid content) NR
Sulphur-crested cockatoo
(Cacatua galerita)
Omnivore Seeds (primarily sunflower), grubs, rhizomes NR

Nutrient requirements

Nutrients from foods provide the energy to maintain life and the building blocks (precursors) needed to synthesize the structural and functional macromolecules. The macromolecules provide the majority of the diet and include lipids, proteins, carbohydrates, and water. Micronutrients from the diet include vitamins and minerals. Optimal health requires essential nutrients. The quantity of nutrient needed is described as the requirement for that particular nutrient. Some nutrients may not be synthesized in sufficient amounts to meet metabolic demands.

The qualitative and quantitative components of the nutrient requirements are well known in some of the domestic galliforms. Essential nutrients are required in similar proportions with these species and for this reason have been used as a model for psittacines. A variety of nutrients can be oxidized during metabolism of foodstuffs to produce energy.

The physiology of a particular species determines its nutrient requirements. Requirements are determined for three physiologic states: basal, maintenance, and total. The basal requirements are those needed to maintain basic life functions. The maintenance requirement is the amount of nutrients needed for basal functions, plus that needed to find and consume food, interact with other animals, and maintain body temperature. The total energy requirement is the combination of all requirements for life and its stages including growth, reproduction, and molt.

Few nutrient requirements have been evaluated scientifically in the various psittacine birds, so nutrient requirements are often based on the “best guess” from those derived from galliforms. Two methods have been used to determine requirements: 1) empirical calculations and 2) calculations based on factorial summation of specific needs. With empirically based recommendations, experimental diets with graded nutrient levels are fed to a particular species. The minimal level that optimizes the birds’ health and performance is considered the requirement for that nutrient.

One example of an empirical calculation involved feeding groups of galliform chicks increasing levels of methionine. With this nutrient, their growth rate increased linearly until a point was reached where the growth rate did not increase with increasing levels of methionine; this point was empirically determined to be the requirement level of this nutrient. However, the line between levels that are deficient and those needed is not often sharp and instead follows the law of diminishing returns.

Factorial calculations involve adding together various requirements to determine the needs for a particular situation. For example, the requirement of methionine during the egg laying stage has been determined by adding the maintenance requirement of the egg (as determined from the amounts of this nutrient found in the egg by chemical analysis) with the amount needed by the reproductive tract during the egg laying period. This later method is often used for calculating energy requirements, amino acid requirements, and necessary calcium levels. This technique can be very accurate as long as the information that it is based on is also accurate.

One problem with companion birds is that these requirements are often not well established. Another problem is that the efficiency of absorption of nutrients, especially for life stages, may not be accurate. Varying absorption efficiencies may thus throw off the calculations. Both empirical methods and calculations based on factorial summation of specific needs have been used to formulate the requirements for galliforms and ducks  published in the US National Academy of Sciences report. These values represent the nutrient requirements under optimal conditions for these species. Particularly in companion birds, a margin of safety is added to these values in galliforms and provides the “best educated guess” for adequate nutrition. The Association of Avian Veterinarians worked with a panel of experts in diverse areas of avian nutrition to provide general recommendations for psittacines and passerines. These values were the best collective guess to start the discussion on nutrient requirements in these groups of birds.

As avian clinicians, we realize that individual species have differing needs based on our clinical impression over time and the natural behaviors of their wild cousins. Additional concerns about feeding whole food diets in mammals and providing foods that allow our companion birds to exhibit their natural foraging behaviors is adding to a better understanding of their nutritional needs.

Birds in the wild do not necessarily select adequate diets nutritionally, although they appear to be able to balance their energy needs, amino acids, and calcium, but not their needs for other requirements. Birds in captivity do not appear to select appropriately either. A recent dietary approach has been to provide a pelleted or extruded mixture that provides all of the required nutrients above the estimated requirements. This approach has greatly improved the health of companion birds over an all-seed diet but has some shortfalls. The major criticisms are that seed-eating companion birds are difficult to switch to pelleted diets; there is concern about the lack of variety of food stuffs compared with their experience in the wild; various species may have nutritional needs not met with a pelleted diet and these diets offer no opportunity for birds to display their innate foraging behavior.

As we learn more about ways to provide foraging opportunities, changes will emerge in how companion birds are fed in captivity. Presently, the addition of vegetables and fruits to the diet for variety and enrichment suggests that nutrient dilution may occur. Theoretically, that would require that the diet be reformulated. However, this only becomes more of an issue when the bird eats a smaller portion of the formulated diet than the table foods offered. The reason a nutritional deficiency may not occur is that most of the domestic fruits and vegetables are predominately water.

 

Energy

Birds eat to meet their daily metabolic requirement. For example, a bird confronted with food items that have reduced energy will adjust the amount consumed to meet its energy needs. However, birds may continue to eat out of boredom in captive environments or may become obese when the foods that make up the diet are high in energy but are unbalanced from an amino acid perspective.

Daily energy requirements have only been documented in budgerigars, so calculations for other psittacine species are based on extrapolation, using body weight and a multiplication factor based on the housing condition. The amount of food required to meet those requirements is dependent on the density of the food items consumed, the digestibility of the items, and the ability of the digestive tract of that species to digest and absorb nutrients. Granivorous and omnivorous species are comparatively more efficient than nectivorous species in their ability to digest and absorb nutrients.

The maintenance energy requirement is the amount of dietary metabolizable energy (ME) needed to support basal metabolism. It can be expressed as the sum of the basal metabolic rate (BMR) plus the energy required for supporting maintenance activities and thermoregulation.

Understanding the energy requirements of the bird in its life stage is important for determining the amount of diet that can be fed and/or consumed. It appears that the BMR of psittacine birds depends on the thermal climate of the species of origin and is unrelated to food habits and water availability. Tropical psittacine species have BMRs similar to other tropical nonpasserines and can be expressed as BMR (kcal/d) = 73.6 X body weight (kg)0.73. Species from temperate climates have a BMR that is 21% higher than that found in tropical species. However, the BMR actually reflects the energy expended while sleeping, because the act of alertly and actively perching in budgerigars requires a two-fold increase in energy. The energy required in free-living Port Lincoln parrots (Barnardius zonardius) and galahs is about 3.0&3.23 times the BMR respectively because of their increased expenditures through foraging, thermoregulation, and defense.

 

Protein and amino acids

The essential amino acids for birds are arginine, isoleucine, leucine, lysine, methionine, phenylalanine, valine, tryptophan, and threonine. Research with chickens suggests that glycine, histidine, and proline are also considered essential as their rate of synthesis is less than their metabolic needs. Research suggests that glycine is also an essential amino acid for budgerigars. Protein levels consumed must meet the nitrogen requirements for that species in its housing condition and life stage. Growing chicks require greater amounts of protein, especially hatchlings. Hens laying large clutches of eggs also require larger amounts of protein than those at maintenance. Birds within a dietary strategy (e.g., granivorous) that have an increased body size also have higher protein requirements than smaller species.

Feathers compose the largest portion of the protein mass of birds. In budgerigars, they represent 5.7% of the protein mass, which is 28% of the total body protein. Molt results in increased protein needs. Feathers are enriched with cysteine and many of the nonessential amino acids. These amino acids are incorporated into the feather during its formation on a continual basis, while uptake from the GI tract occurs only after consumption of a meal, requiring that they be manufactured from tissue protein sources. Molt is also expensive energetically as the bird loses insulation during that period, consequently requiring increased energy. There are also increased expenditures to acquire more protein to synthesize feather proteins.

Protein digestibility is assumed to be similar in the granivorous psittacines, when compared to chickens. This assumption is based on the fact that assimilation efficiencies of complete diets are similar in psittacines, when compared with chickens. The quality of the protein in the diet reflects the digestibility of the components and the amino acid balance. Rate-limiting amino acids require that more food be consumed to meet that need, which may lead to obesity. If more food is not consumed, a clinically observed deficiency may result. For example, methionine deficiency during chick growth results in dark stress bars on feathers.

High protein diets have been assumed to lead to gout in birds, but this supposition has not been supported by research. Adult male cockatiels, when fed diets with up to 70% crude protein for 11 months, maintained their body weight and general body condition without evidence of renal dysfunction. There was evidence of liver damage because birds developed sinusoidal and periportal lipogranulomas at the 70% level, but not at the 20% or 35% level. It has been suggested that sudden changes to high protein diets might lead to hyperuricacidemia with possible nephritis or gout. However, there is no research at present that indicates that this hypothesis is valid. However, sudden changes are not recommended.

 

Vitamins

Vitamins play an important role in avian nutrition. Vitamins A, D, E, and K are fat-soluble vitamins and their excretion is more problematic than the water-soluble ones. Vitamin A is the vitamin that is more likely to be deficient in both captive and wild birds as the amount consumed in foodstuffs can be quite variable. Vitamin A represents all noncarotenoid derivatives that have a biological activity similar to all trans-retinol. Most appear as retinol and retinol esters that are absorbed from the gastrointestinal tract and transported by portomicrons for storage in the liver. There are two basic functions of vitamin A in cells&the hormone-like regulatory actions of retinoic acid and the photoreceptor actions of retinal. The hormone-like action involves binding to nuclear and cytoplasmic receptors to induce the regulation of cellular replication, differentiation, and preprogrammed cell death.

The exact requirement of vitamin A is unknown in psittacine birds. Feeding trials in female cockatiels suggest that the range may be between 2000 and 10,000 IU/kg in this species as there were no clinical signs that the birds experienced signs of a deficiency or toxicity. However, less grain-dependent species may have greater requirements than the low levels required of cockatiels. Birds deficient in vitamin A exhibit signs of hyperkeratinization of their mucous membranes (appearing as immature basophilic cells on gram stains), anorexia, poor conditioning, and increased susceptibility to infection. Parrots also exhibit squamous metaplasia of the salivary glands, including their excretory ducts and glandular epithelium. Vocalizations in cockatiels may be affected by vitamin A status, as cockatiels fed a diet high in vitamin A (100,000 IU vitamin A/kg diet) had increased numbers of vocalizations compared with birds fed 2000 IU/kg. Cockatiels fed a diet entirely deficient in vitamin A had reduced peak amplitude and total power of vocalization.

Vitamin D activity is found in a group of related sterols including cholecalciferol (vitamin D3), ergosterol (vitamin D2), and other metabolites. Birds are able to synthesize cholecalciferol from cholesterol but require an adequate amount of sunlight to do so. Because most companion birds do not have sufficient UV exposure for endogenous conversion, they need to have a dietary source of vitamin D. Vitamin D has hormonal actions as it regulates calcium and phosphorus metabolism including bone mineralization and eggshell formation. Vitamin D-deficient hens lay eggs with thin shells and develop osteomalacia with pathologic fractures. Vitamin D toxicity is associated with increased mobilization of calcium with soft tissue mineralization. Recent unpublished data suggest that increased levels of vitamin D can result in renal failure in chicks (D. Phalen, unpublished data, October 2003).

Vitamin E actually consists of 2 groups of compounds with antioxidant activity: the alpha-tocopherols and the alpha-tocotrienols. The most biologically active form of vitamin E is the D-α tocopherol, which is used as the dietary standard. In birds, it appears that only the alpha-tocopherols are incorporated into tissues. Vitamin E is transported by plasma lipoproteins and incorporated into the lipid bilayer of the cell’s membranes. It acts to stabilize these membranes and to quench reactions of oxygen intermediates and polyunsaturated fatty acids. These alpha-tocopherols can compete for free radicals faster than polyunsaturated fatty acids. They can also work in concert with a number of enzymes including superoxide dismutase, glutathione peroxidase, and catalase to protect cell membranes. These enzymes require some of the trace minerals including zinc, manganese, selenium, and iron as cofactors for proper function. When deficient, the absolute requirement of vitamin E increases, demonstrating that many of the nutrients are linked.

Vitamin E has other physiologic effects primarily through modulation of eicosanoid metabolism. Conversion of arachidonic acid into prostaglandins and thromboxanes can be modulated by vitamin E levels. This would result in alteration of the immune responses to a challenge.

The levels of vitamin E required in the diet varies, depending on a number of factors including the levels of polyunsaturated fatty acids, levels of vitamin A and/or beta carotene, the presence and quantity of other dietary antioxidants, rancidity of the fats in the diet, and the content of selenium. A selenium deficiency results in an impairment of the glutathione peroxidase antioxidant system, increasing the need for vitamin E.

Deficiencies of vitamin E result in a number of symptoms that relate to cell membrane dysfunctions. These commonly observed symptoms include encephalomalacia, exudative diathesis, muscular dystrophy, myopathy of the ventriculus, and increased fragility of red blood cells. Encephalomalacia in chicks results in crazy chick disease characterized by torticollis, crying out, and excessive wing flapping while maintaining abnormal postures. Higher than normal levels of vitamin E can result in the symptoms associated with deficiencies of the other fat soluble vitamins. The bioactivity of vitamin E is drastically reduced during processing due to heat and moisture, along with increased levels of unsaturated fatty acids and trace minerals.

 

Minerals

Calcium is necessary for bone mineralization and eggshell calcification; in its ionic form, it is needed for nerve conduction and for myofibril contraction. The calcium requirement(s) for psittacine species have not been determined but the maintenance requirement for chickens is 0.1% of the diet. Many of the seeds consumed by companion birds are less than 0.03% of the diet, suggesting that the requirement is larger than 0.05%.  Grey parrots are particularly prone to hypocalcemic seizures. The pathophysiology is unknown but may result from an inability to mobilize bone acutely. This problem seems less apparent clinically when birds eat at least half of their diet as pellets because vitamin D3 is provided. African greys are one species that have a uropygial gland and, consequently, they may have a higher requirement or sensitivity for cholecalciferol when not exposed to sunlight. Sunlight is needed to convert the inactive form of vitamin D to the active form or it needs to be supplied in some food on a regular basis.

The requirement for growing psittacine chicks in unknown, but they would be expected to require a similar ratio of calcium to phosphorous ratio (between 1.4:1 and 4:1), assuming the vitamin D levels are adequate. The calcium requirement for altricial species of birds is less than precocial species because the number and size per egg is greater in the latter birds. The calcium requirement in egg-laying chicken hens is 3.3% of the diet, while budgerigars and cockatiels require as little as 0.85% and 0.35%, respectively, for normal calcification. Calcium deficiency occurs when there is too little calcium or the active form of vitamin D or too much phosphorus in the diet. The lack of calcium and the active form of vitamin D in all-seed diets is the underlying problem with egg binding in cockatiels.

 

Conclusion

Avian nutrition has multiple components and dietary requirements are just a starting point for determining the needs of a diverse group of birds classified as psittacines. Many aviculturists know, through trial and error, that some species are not successful in captivity without certain food items. These nuances need to be added to the body of knowledge to provide improved health and well being for these magnificent creatures.

Disclaimer: Dr. Orosz consults with the Lafeber Company.

 

References