Many birds commonly kept in captivity are considered granivorous, such as most passerines, psittacines and backyard poultry. For these species, a diet consisting of one or a few types of seeds may produce deficiencies in vitamin A, calcium, aminoacids and other nutrients. However, it should be considered that adult birds that do not reproduce have very low requirements for many nutrients, and therefore those deficiencies are most commonly seen in growing animals. Adult birds in poor diets more frequently show bad feathering due to deficiencies on aminoacids. A diverse diet consisting on a mixture of different seeds, vegetables and some fruits is adequate provided that the bird does not select. Growing animals may need a supplement of calcium, vitamins and protein. Pelleted food can be provided as supplement of nutrients, but a sole diet of pellets is not recommended as they are high in most nutrients. If a pelleted diet is provided, it should not account for more than 50% of the total diet. Abrupt diet changes in sick birds should be avoided. Fresh water should always be provided.

Birds that feed on frozen fish (mainly aquatic zoo birds) need a supplement of thiamine (vitamin B1) and vitamin E. Raptors eating whole prey may not need any supplement. Frugivorous and nectarivorous birds have very low requirements of protein and vitamins; these birds produce large quantities of urine. 

Hypovitaminosis A 

Vitamin A deficiency is widely regarded as the most common vitamin deficiency suffered by captive parrots being fed a seed based diet. However, the prevalence of hypovitaminosis A has been exaggerated for decades, as most adult parrots don’t require large amounts of vitamin A. Hypovitaminosis A typically presents as squamous metaplasia of epithelial membranes leading to changes in functions such as vision, reproduction, immunity, cellular differentiation, growth and embryonic development. Birds with hypovitaminosis A may present with white plaques in the mouth, rhinolits, respiratory infections and atrophy of the choanal papilla, although these lesions can also be seen with other conditions. Vitamin A can be stored in the liver for years, and a bird with good reserves can survive a diet deficient in vitamin A for a long time. Vitamin A is plentiful in mangos, carrots, spinach, red pepper and sweet potato, and is commonly lower in seeds and nuts. 

Iodine deficiency 

Most commonly seen in budgerigars on seed diets. It produces hypothyroidism (goitre) with thyroid hyperplasia. Enlarged thyroids compress oesophagus and trachea and clinical signs may include regurgitation, change of voice and inspiratory noises. It responds well to iodine supplementation. 

Hypocalcaemia 

Growing chicks and laying females have high calcium requirements. When calcium or vitamin D (or both) are not available, growing birds exhibit bone deformities and laying females may produce soft-shelled eggs and suffer from dystocia. Grey parrots are particularly susceptible to this condition, and even adult animals may exhibit twitching and convulsions due to lack of calcium and/or UV light. A hypocalcaemic bird can be presented as an emergency. When investigating hypocalcaemia in birds it is more useful to measure serum ionized calcium levels rather than total calcium levels. Treatment consists of the provision of appropriate levels of calcium and UV light. Cases that do not respond to this supplementation should be investigated for other problems. Nuts, beans and spinachs are a good source of calcium. 

Energy
Many diets for captive birds are very high in energy. Obesity is common in budgerigars and Amazons, and atherosclerosis is common in medium to large parrots. Dermal lipomas (discrete accumulation of fat) and xanthomas (discrete accumulation of cholesterol crystals) are also common, particularly in budgerigars. All these conditions are nearly always associated with reduced activity. Prevention should include the provision of exercise and the reduction of the energy content of the diet. Most lipomas and xanthomas require surgical resection if they pose a problem. Diets high in saturated fats (palm or coconut oil) may predispose to atherosclerosis. Obesity can also be common in other species of birds, such as raptors. 

Calcium

In an attempt to prevent hypocalcaemia and other deficiencies, some breeders and keepers over-supplement the diets of their birds with minerals and vitamins. Some of those supplements are a powder containing very high concentration of nutrients and are used with no control. Several excesses can occur in those situations, particularly calcium and vitamin D, which cause renal failure and gout. Raptors eat whole prey and they do not generally need any supplement, which make them a lot less likely to develop problems due to excess of nutrients. 

Iron

Haemosiderosis and haemochromatosis are a cause of disease in birds, particularly frugivorous and nectarivorous species such as starlings, toucans and lorikeets. Clinical signs of severe iron storage disease include hepatomegaly and problems secondary to liver disease. Haemosiderosis is increasingly being diagnosed in granivorous parrots, and may be associated with hypervitaminosis A. 

Protein

Excess of protein can produce kidney disease in adult birds and growing problems in chicks. “Angelwing” is a condition commonly seen in growing ducks and geese on high- protein diets; the distal carpometacarpal bone rotates laterally, giving an irreversible twisted appearance to the distal wing. The condition is easily prevented by reducing the protein content in the diet.