Dr. Sudhanya Nath1* and Dr. Vivekanand2
1PhD Scholar, Department of Animal Nutrition, West Bengal University of Animal & Fishery Sciences, Kolkata, West Bengal – 700037
2Consultant- National Animal Disease Control Programme, Ministry of Fisheries, Animal Husbandry & Dairying, Department of Animal Husbandry & Dairying, Cattle & Dairy Division, Krishi Bhawan, Govt. of India, New Delhi – 110001
* Correspondence: sudhanyanath@yahoo.com
INTRODUCTION
Psittacines, commonly known as parrots, are birds of the roughly 393 species in 92 genera comprising the order Psittaciformes, found mostly in tropical and subtropical regions. The order is subdivided into three superfamilies: the Psittacoidea (“true” parrots), the Cacatuoidea (cockatoos), and the Strigopoidea (New Zealand parrots). These species are found primarily in many tropical and subtropical environments around the world, as well as in temperate climates in New Zealand and southern Australia. Many of these species are kept in captivity as companions and display specimens and for conservation purposes. Proper diet and nutrition are essential for maintaining strong health and achieving normal life expectancy in all living animals but are of particular relevance to psittacines kept as companion animals, for which malnutrition continues to be a concern. Malnutrition can increase susceptibility to disease via immunomodulation and cause a numerous disorders and behavioural problems. One major contributor to this issue is lack of knowledge on energy and nutrient requirements for psittacine species. Evolution of nutrition for psittacines has followed 3 stages. Early diets were based upon food habits of wild birds. Next, nutrient requirements that were scientifically determined for gallinaceous birds, a distantly related group (poultry) were adopted, at least in part, as standards for diets of captive psittacines. We are now in a third phase in which research populations of easily propagated species, such as budgerigars and cockatiels, are being used to investigate dietary preferences, nutrient requirements, and metabolic capabilities. Despite this progress, many commonly fed captive bird diets are deficient in at least one or more fundamental nutrients. Although psittacines are generally well adapted to a plant-based diet (seeds, fruits, nectar), captive ones may not feed frequently enough, because of lower energetic demands in captivity, to obtain the necessary minerals and vitamins from a seed-exclusive diet. Non-supplemented seed diets can be deficient in other essential nutritional components, such as amino acids, calcium, and vitamins A, D, K, and B12. Pellet-based diets are potentially a better option because each pellet is designed to contain the full spectrum of vitamins and minerals required for complete nutrition. However, some psittacines take considerable coaxing to switch from whole seed to pellet-based diets and may exhibit a strong preference for whole seeds because they are frequently raised on seed-only diets that can be high in fat.
NUTRITIONAL ECOLOGY AND DIETS OF PSITTACINES
Most birds in the order Psittaciformes consume plant-based diets, and they are classified as florivores. On the basis of consumption of primary types of parts of plants, further subclassifications can be made. Common psittacine subclassifications include granivory (grain or seed-based diet, including budgerigars and cockatiels), frugivory (fruit-based diet, including many macaws), and nectarivory (nectar-based diet, including lorikeets and lories). Within the category of granivorous birds, smaller birds tend to select grass seeds and larger birds tend to select increased proportions of seeds from shrubs, which contain higher levels of protein. In contrast to birds that specialize on one specific feed type, many psittacines consume more diverse diets, including foodstuffs from two or more categories. Frugivorous-granivorous psittacines include the red-fronted macaw, regent’s parrot, and scarlet macaw. Species commonly considered nectarivores, such as rainbow lorikeet and Stephen’s lory, feed on fruits, seeds, and insects. Many psittacines eat a large variety of ingredients that include animal matter and are best categorized as omnivorous (cockatoos and parakeet species). Selection of foodstuffs by these birds depend upon nutrient availability, sex of the bird, age, their ability to adapt to and exploit introduced, nonnative, or domesticated sources of food. Budgerigar, cockatiel, rose-ringed parakeet, redfronted macaw, regent’s parrot, golden parakeet, kakapo and hooded parrot have been reported to eat nonnative plant matter, most commonly from grain crops and introduced ornamentals and fruit trees. Glossy black-cockatoo feeds almost entirely on seeds of a single species of native tree (Allocasuarina verticillata), the forest red-tailed black cockatoo eats the seeds of only two native trees (Corymbia calophylla and Eucalyptus marginata), and the vulturine parrot is a specialized frugivore that consumes the fruit of only 1 or 2 of the 38 extant species of figs (Ficus species) in its native New Guinea. Finally, the amount of time spent foraging and/or feeding per day varies depending on the species being considered. In general, parrots actively feed in 2 sessions – morning and evening. Forest red-tailed black cockatoo eats not in 2 distinct bouts but in one long daily feeding period. In many species, including the kaka, pacific parakeet, and ground parrot, foraging and feeding represent the majority (50%) of daily activities.
HAND FEEDING OF PSITTACINE CHICKS
Hand rearing is a common practice for the propagation of psittacines, both for the pet market and for conservation aviculture. Accurate information on the nutritional requirements of growing animals is essential for the formulation of captive rearing diets. However, research on the nutrition of growing psittacines is limited and the nutritional requirements for the growth of psittacine chicks are not well understood. As a result, nutritional imbalances resulting in problems such as stunted development, rickets, and vitamin deficiencies have been common. Generally hand fed parrots grow slower than parent fed and present a delayed grow rate. In the absence of further research, or comprehensive data on the nutrient composition of the diets of wild psittacines, nutritional prescriptions for their maintenance and growth are generally extrapolated from dietary recommendations for poultry and modified based on experience rather than on scientific study. However, psittacines are not closely related to poultry which have been artificially selected for multiple generations and differ both developmentally and ecologically. Hand feeding diets for psittacines were traditionally home-made recipes which required extensive preparation but now there is a wide array of commercially available products that require minimal preparation. These products are intended to be used without supplementation and fulfil the nutritional requirements of most species. Parrots feed their chicks a regurgitated coarse mix of foods. Feeding whole grain to young chickens has been associated with a more muscular gizzard and less occurrence of proventricular hypertrophy. Greater development of the gastrointestinal tract suggests that feed may be retained in the upper digestive tract for a longer period allowing for increased enzymatic digestion and digestive efficiency. Captive psittacines are usually hand fed diets of very small particle size (finely ground). When attempting to hand feed a coarse texture similar to the regurgitate fed by their parent, the mortality of newly hatched chicks increased and created problems with the food passage time in older chicks. The capacity of a formula to maintain the solids in suspension is another important factor because separation of ingredients at the mixing dish will result in nutritional inconsistencies of the formula. If this situation occurs in the chick’s crop, the solids will settle while the liquid is absorbed, making it more difficult to pass, and leading to dehydration and crop stasis.
NUTRITIONAL REQUIREMENTS
Water
Often forgotten, but the most critical nutrient for most species, water is essential for maintenance of cellular homeostasis, epithelial integrity, food digestion, waste excretion, hygiene, and numerous metabolic reactions. The exact quantity required each day is dependent upon body size, diet, and environmental temperature. Generally, birds conserve water very effectively because feathers minimize evaporative losses and excretory losses are low because of the effectiveness of the renal– cloacal complex. In budgerigars at thermoneutral temperatures, about 35% of daily water loss occurs via excretion and 65% via evaporation. Very small granivorous birds can survive without any drinking water because they produce sufficient water metabolically through oxidation of carbohydrate and fats to replace water losses. The budgerigar (27 g) and Bourke’s parrot (39 g) can live without drinking water at cool temperatures (10 – 20ºC) but require drinking water at higher temperatures. Larger species require water at all environmental temperatures. MacMillen and Baudinette determined the water requirement of adult parrots (ranging from 48 to 295 g) to be approximately 2.4% of body weight. As environmental temperatures rise, water requirements increase. Physiologically, this change in requirements is due to increased water evaporation from the skin and the lungs as the bird cools itself by panting. For example, monk parakeets housed at 45ºC have 12 times the rate of evaporative water loss compared with birds housed at 30ºC. Though some evaporative water loss is replaced by metabolic water, most must come from drinking water, and the amount of drinking water consumed would be expected to increase by a factor of about 10-fold in very hot weather. When water is freely available, most birds drink considerably more water than the minimum amounts described above. For example, budgerigars, which have no requirement for drinking water, choose to consume an average of 4 ml water/day. There is also considerable individual variation in water consumption, and polydipsia can be psychogenic. Birds consuming fresh fruits and vegetables, which are often in excess of 85% water, would be expected to consume much lower than expected levels of drinking water.
Energy
The maintenance energy requirement is the amount of dietary metabolizable energy (ME) needed to support basal metabolism, as expressed by basal metabolic rate (BMR), plus additional energy to fuel activity and thermoregulation. Growing birds also need energy to support the accretion of new tissues, reproducing birds need additional energy for accretion of gametes, and molting birds need energy to support feather growth. Obviously, the total energy requirements vary depending upon the environment, stage of life cycle, and genetics of the individual. Knowledge of energy requirements is very important because birds usually eat the quantity of food needed to satisfy their energy needs. The amount of food required depends upon the density of ME in the diet and its digestibility. Thus, when provided low energy density diets (e.g., high-fiber), animals increase the amount consumed but decrease total intake when given high energy diets (e.g., high-fat). Growing birds need energy for basal requirements, thermoregulation, physical activity, and growth. Kamphues and Wolf measured the rate of protein and lipid gain in growing budgerigars (177 mg/d and 160 mg/d, respectively) and lovebirds (153 mg/d and 153 mg/d, respectively). Correcting these rates for the cost of deposition (52 kJ/g) gives the additional energy needed for growth at 17.5 kJ/g for budgerigars and 15.9 kJ/g for lovebirds. The relative amount of energy needed for growth is based upon the fractional growth rate. In altricial nestlings, the proportion of energy requirement partitioned to growth changes with age, being proportionally highest at the beginning when percent weight gain is the highest and thermoregulation and activity are minimal. Birds in the order Psittaciformes are among the slowest growing of altricial species but they also develop endothermy at an early age. Thus, their energy requirements are likely to be more similar to precocial species than to highly altricial species like Passeriformes, which grow faster and thermoregulate later. Optimal growth of chickens 0 to 12 weeks of age is achieved when offered a diet with 11.9 MJ ME/kg. However, according to Wolf and Kamphues the energy density of hand rearing formulas for parrots varies widely (13.0-16.8 MJ ME/kg) and consistently exceeds estimated needs. For budgerigars, a diet containing 13 MJ ME/kg results in maintenance of body weight, but diets of 14 MJ/kg or above resulted in obesity.
Protein and amino acids
Avian species are unable to synthesize the ‘‘essential’’ amino acids arginine, isoleucine, leucine, lysine, methionine, phenylalanine, valine, tryptophan, and threonine. Glycine, histidine, and proline are often considered essential on the basis of research in chickens that demonstrated that rates of synthesis of these amino acids cannot meet metabolic demands. As demonstrated in chickens, a requirement for glycine has been observed for the budgerigar, suggesting that psittacines are unable to synthesize enough glycine to meet metabolic demands. Furthermore, an essential level of protein must be included in the diet to meet nitrogen requirements of the birds. The quantitative requirement for amino acids is dependent upon the physiological state of the bird, being lowest in adults at maintenance and highest in hatchlings and females laying large clutches of eggs. In the wild, some psittacines time their breeding to the seasonal availability of higher protein foods, indicating that amino acid nutrition is a major determinant of reproductive output. In captivity, cockatiels increased egg lay after protein content of the diet increased. However, because of the higher fractional growth rate of psittacines (due to their altricial mode of development), an increase in the total amino acid requirements might be expected. The amino acid composition of the tissues of budgerigars is very similar to that of chickens, so the balance of AA required may be similar among a broad range of avian taxa. Differences in the concentration and balance of AA among species would be driven by the different developmental patterns and fractional growth rates. Experiments with captive cockatiels have shown that optimum growth is achieved feeding a diet with 20% DM crude protein (1.0% methionine + cysteine, 1.5% lysine, 14.6 MJ ME/kg). A diet with 13.2% protein (0.65% lysine, 0.78% methionine + cysteine, 13.4 MJ ME/kg) reported maximal growth in growing budgerigar chicks. The protein requirement for growth of cockatiel chicks is 20% CP (1.0% methionine, 1% cysteine, 1.5% lysine, 14.64 MJ ME/kg) for maximal growth and survivability. Budgerigars maintain breeding performance with a 13.2% protein diet (13.39 MJ ME/kg, 0.65% lysine, 0.78% methionine 1% cysteine) as indicated by number of eggs laid and number of chicks hatched.
Feathers are a predominant part of the protein mass of birds. In budgerigars and lovebirds, they compose 5.7% and 3.5%, respectively, of the body weight, which is 28% and 22% of total body protein. Most adult birds molt several times a year. This periodic event is associated with increased amino acid needs for the synthesis of replacement feathers and, to a lesser degree, for the synthesis of new feather follicles, feather sheaths, and epidermal blood vessels. Feathers are enriched in cysteine and many of the nonessential amino acids, whereas budgerigar feathers contain only 18% as much lysine and 32% as much methionine as the carcass. Thus, the primary need for feather growth is cysteine and amino nitrogen. Whereas feathers grow continuously throughout the day and night, dietary amino acids are supplied only after meals. In the post-absorptive state, most of the amino acids needed for keratin synthesis are mobilized from tissue proteins, although tissue (especially liver) glutathione may supply some of the required cysteine. This diurnal deposition and mobilization of large amounts of tissue proteins markedly decreases the efficiency of amino acid and energy use for molt. Molt is energetically expensive because of the loss of feather insulation, the cost of synthesizing feather protein, and increased body protein synthesis and degradation. The percentage increase in the energy expenditure associated with molt often exceeds the percentage increase in protein needed for the molt. Consequently, molt may not be associated with a change in the dietary amino acid requirement when expressed as percentage of the diet or as mg/kJ. This is because increased food consumption to meet energy requirements results in sufficiently increased amino acid intake to meet requirements for molting.
Protein or amino acid deficiency is manifested in reduced growth rates and skeletal muscle deposition. An AA imbalance may cause anorexia in addition to aforementioned symptoms. Deficiencies of specific amino acids can produce distinctive pathology. For example, a methionine deficiency during chick growth or molting results in dark, horizontal ‘‘stress marks’’ on feathers.
Fat and fatty acids
Dietary lipids supply energy, essential fatty acids (FA), vitamin transportation, and pigments. Fatty acids have remarkably varied roles in animal physiology. Linoleic acid and α-linolenic acid are considered essential nutrients and birds lack the desaturases needed to produce them. Arachidonic and docosahexaonic acid cannot be synthesized by carnivorous mammals or fish from linoleic and linolenic precursors, and it is unknown if birds have the capacity to synthesize them. Linoleic acid is the only essential FA for which dietary requirements have been demonstrated in poultry (1% DM [11.93 MJ ME/kg] for 0-12 weeks leghorn chickens). The diet of the Hyacinth macaw and the Lear’s macaw in Brazil contains predominantly SFA.
Minerals
Calcium is a vital component of bone and body fluid, and is important for chick growth. The requirement of calcium for growth has been determined empirically for poultry; it decreases from 1.0% to 0.8% between 0 and 8 weeks showing that the requirement is higher in early life when the growth rate is highest, and decreases in the adult bird. Previous research on scarlet macaws chicks crop content found 1.4% Ca DM. Phosphorus is an important constituent of bone, proteins, carbohydrates and lipid complexes. Evaluation of the Ca:P ratio in the diet is important as excess P can inhibit the uptake of calcium and result in bone growth abnormalities especially in growing animals. To a lesser extent surplus Ca reduces P uptake. In leghorn chickens, the Ca to non-phytate P ratio increases from 2.2 to 2.7 between 0 and 8 weeks, while in the scarlet macaw crops was found to be 2.9. The rate of skeletal growth of altricial hatchings is considerably faster than that of precocial birds, but the requirement for Ca has not been investigated. Presumably, the combination of a faster growth rate and lower calcification of the skeleton at hatching cause altricial species to have greater requirements than precocial species.
A calcium deficiency will occur when diets contain too little calcium or vitamin D (essential for calcium homeostasis) or too much phosphorus. Deficiency is manifested in decreased bone mineralization and skeletal abnormalities, particularly in growing birds. For example, in budgerigars, a calcium deficiency causes a demineralization and narrowing of the cortex of the femur. In chickens, calcium toxicity is less common but results in hypercalcemia and precipitation of calcium urates leading to kidney nephrosis. Also in chickens, signs of phosphorus deficiency and toxicity are similar to those of calcium-deficient birds.
Vitamins
Vitamin A is critical for vision, cellular differentiation, immune function, and numerous other parameters. Vitamin A, like vitamins D, E, and K, is a fat-soluble vitamin, and excretion of this nutrient is much more difficult than for water-soluble vitamins. On this basis, vitamin A deficiency & toxicity may be quite prevalent in companion bird species, and this incidence is reportedly quite high in captive birds. Vitamin A requirements for psittacines are not precisely known, but recent research provides an indication of appropriate dietary levels. In adult female cockatiels, diets containing 2000 or 10,000 IU vitamin A/kg were sufficient for maintenance, without signs of deficiency or toxicity. In addition, cockatiels could be maintained on a diet completely devoid of vitamin A (0 IU vitamin A/ kg) for 8 months, although immunocompetence was impaired, on the basis of reduction in secondary antibody titers. Symptoms of vitamin A deficiency include keratinization of mucous membranes, anorexia, ruffled plumage, increased susceptibility to infection, and poor conditioning. In parrots, focal metaplasia of the excretory duct or the glandular epithelium of the salivary glands was identified when liver vitamin A levels reached extremely low levels (50 IU/g liver). Liver vitamin A levels below 2 IU/g liver were associated with extensive metaplasia of the salivary glands. Behavioural changes, specifically changes in vocalization patterns, were observed when cockatiels were fed either deficient or excessive levels of vitamin A. Cockatiels fed 100,000 IU vitamin A/kg diet had increased numbers of vocalizations compared with birds fed 2000 IU/kg (considered to be an adequate level for maintenance), and the peak frequency of vocalizations was reduced. Cockatiels fed 10,000 IU vitamin A/ kg or 0 IU vitamin A/kg diet had reduced numbers of vocalizations, and 0 IU resulted in reduced peak amplitude and total power. Although vocalizations may be affected by vitamin A status, it remains to be determined if other nutrients cause a similar change. However, these data suggest that vocalization patterns may be indicative of nutrient status of cockatiels. Sources of vitamin A in the diet include plant and animal matter. Carotenoids from plants serve as vitamin A precursors for chickens. Capacity of psittacine species to convert carotenoids into vitamin A has not been studied, although on the basis of foods consumed in the wild (plant based), it appears that psittacine species should be quite cable of carotenoid conversion. In foods of animal origin, vitamin A is abundant in the form of retinyl esters, which are considered to be highly available. Both vitamin A and D toxicosis have been reported in macaws as a result of misuse of liquid vitamin supplementation.
Conclusion
Formulation of appropriate diets for captive birds in the order Psittaciformes requires knowledge of the birds’ wild feeding strategy, digestive anatomy, and physiology and specific knowledge of nutrient requirements in that species or a related species. Results to date indicate that the energy, protein, and calcium requirements are lower in psittacines than in poultry during all stages of the life cycle. At this point, little or no research has been conducted on the trace nutrient requirements of psittacines, and those established for poultry remain, by default, as the standard. However, there is no evidence that levels of vitamins and trace minerals recommended by the National Research Council for poultry cause either deficiencies or toxicities in psittacines. Experimental and clinical evidence demonstrates that diets based on unsupplemented domestic food items are nutritionally incomplete and must be fortified with a variety of amino acids, vitamins and minerals. The use of pellets formulated to be nutritionally complete as the primary dietary component has proven to be optimal for growth and reproduction of many captive psittacines.
