Nutrition in Psittacines
The pet bird population consists largely of psittacine species, primarily passerines and other genera such as those that include toucans. Psittacines eat mainly a plant-based diet and can be classified overall as florivores. The content of the diet—fruit, nectar, seeds, or a combination—varies by species. Some species ingest a percentage of insects or carrion. Although requirements and dietary sensitivities vary among psittacine species, the pelleted and extruded diets that have been produced for parrots have tremendously improved the nutritional intake and subsequent health and quality of life of these birds. However, pelleted diets differ in content and quality and must be evaluated individually. Extruded pellets in different shapes and sizes are available for maintenance and breeding purposes. Many pellets contain omega-3 fatty acids as well as probiotics.
Psittacine nutrition has been the focus of research throughout the past several decades. Several myths of psittacine nutrition have been debunked during this period. Grit, while probably necessary for some passerines and Columbiformes to aid in mechanical digestion, is not needed by psittacines. If seeds are consumed by psittacines, they are hulled before ingestion. Monkey chow biscuits are nutritionally incomplete, and some brands tend to harbor bacteria or promote excessive gram-negative bacterial growth when included in bird diets. Strictly seed diets, regardless of supplementation, are suboptimal for psittacine species. Deficiencies of vitamin A, protein (the amino acids lysine and methionine, in particular), calcium, and other nutrients are seen in most psittacine species on seed-based diets. Conversely, excessive vitamins, such as vitamin A, are added to some pelleted diets, which can have equally detrimental effects.
Protein (amino acid) requirements of psittacines have not been well established. The amino acid deficiencies most consistently noted in psittacine birds on seed-based and table-food diets are lysine and methionine. Fiber content must always be considered when determining dietary protein requirements, because increased fiber causes increased fecal protein “loss.” Birds with low-fiber, more readily digestible diets (such as nectar-feeding lories and lorikeets) may do well on diets with easily digestible protein levels as low as 3%–5%. Adult maintenance levels of protein for budgerigars and cockatiels (7%–12%) are lower than those for African Grey parrots (10%–15%). Protein requirements for growth and for egg-laying hens are higher than maintenance levels in all birds. Periods of heavy molt also greatly increase protein requirements, particularly the need for the sulfur-containing amino acid cysteine, because feathers average 25% of the total body protein content of birds.
Excessively high dietary protein has the potential to cause renal insufficiency and gout in birds with preexisting renal impairment or a genetic predisposition to gout. Cockatiels with no preexisting renal disease have been shown to tolerate extremely high dietary protein levels (up to 70%) with no renal impairment. A genetic predisposition to renal disease/gout has been documented in some strains of poultry and may be seen in other avian species.
Sudden, dramatic increases in dietary protein may overload the kidneys, producing hyperuricemia and visceral gout. When increased dietary protein is indicated, it should be increased gradually to avoid renal damage.
Dietary fat provides essential fatty acids, energy, and hormone precursors. It also contributes to egg yolk formation and aids in absorption of fat-soluble vitamins. Diets should contain 5%–12% fat, depending on the species, the general condition of the psittacines, and the physiologic stage and brood condition. At least 1% of the dry diet should consist of PUFA (eg, linoleic acid). Excessive dietary fat leads to obesity, metabolic diseases, cardiac disease, and atherosclerosis (also see Pet Birds). The fat requirements of psittacines for reproduction are generally lower than those of poultry, because the psittacines’ altricial young do not require the same quantity of fatty acids as do the precocial young of chickens. However, diets that are borderline deficient often manifest as problems in either the psittacine hen or the chicks during reproduction.
Vitamin A is necessary for vision, reproduction, immunologic integrity, and growth, and for the maintenance of epithelial cells in respiratory, GI, and renal tissues. Vitamin A deficiency has historically been noted in psittacines on all-seed diets, so supplementation is commonly recommended. However, indiscriminate supplementation leads to vitamin A toxicosis, as well as to decreased absorption of other fat-soluble vitamins and carotenoids. In nature, psittacines do not consume vitamin A but obtain vitamin A precursors such as carotenoids from various plants. Pelleted diets should contain vitamin A at levels of 5,000–8,000 IU/kg of feed. Higher amounts should be avoided. The source of vitamin A added to bird feed is not regulated, and significant quality control issues have been documented. Ideally, a quality pelleted diet for psittacines will contain multiple carotenoids and other vitamin A precursors, with a minimum level of vitamin A.
Some carotenoids in birds are precursors for the body’s formation of vitamin A. Carotenoids also act as antioxidants and are necessary in some species (such as canaries and flamingos) for feather pigmentation.
The primary function of vitamin D is to increase absorption of calcium and phosphorus. Vitamin D can be obtained either directly from the diet or from UVB (285–315 nm) light exposure. Vitamin D deficiency is probably rare in nature because of exposure to sunlight. Birds living in polar conditions get their vitamin D during the winter by consuming diets with high amounts of vitamin D (eg, fish, plankton). In the absence of natural sunlight, the minimum oral vitamin D requirement for African Greys is likely to be 500–1,000 IU/kg.
Vitamin D deficiency may be caused by dietary deficiency or lack of exposure to UVB radiation. Limited studies have shown that species variation in psittacines for UVB light requirements exist. Unfortunately, many birds are housed totally indoors, and owners often mistakenly assume either that the birds do not need direct sunlight or that the sunlight the birds receive through glass will supply UVB radiation. Owners of pet birds should be encouraged to expose their bird to direct sunlight (with appropriate cautions regarding excessive heat) or to purchase and properly use UVB bulbs. However, pet bird owners must consider that the further away from the equator, the less UVB is generated, especially in late autumn, winter, and early spring. Some research indicates that UVA and/or UVB can affect vitamin D synthesis in some bird species, the finding of food, well-being, and feather colors. However, more research is needed to determine how much UVA and/or UVB is needed for each bird species.
Vitamin D toxicity is caused by excessive supplementation. Some psittacine species, notably macaws, are sensitive to excessive dietary vitamin D and may develop soft-tissue calcification and renal failure. Toxic levels for psittacines have not been established, but levels that may be toxic for poultry begin at 2,800 IU/kg of feed.
For discussion of vitamin E, see Nutrition in Piscivorous Birds.