Little is known about the energy requirements of pet birds. Because animals generally eat to meet energy requirements, the nutrient content of a diet must be balanced with the energy content. To formulate balanced diets for a range of bird species, both the energy needs of the relevant bird species and the energy content of the diet must be calculated. The most practical way of understanding the energy requirements of pet birds is by studying their daily energy expenditure. Factors affecting energy requirements are body size, activity patterns, environmental temperature, plumage cover, age, and physiologic state. Energy costs of maintenance are directly related to the lean body mass and the relative size of the different organs, because these are the tissues that actively use oxygen. Activity patterns also have a considerable impact; the difference between sitting and standing can affect the daily energy expenditure by as much as 42%. There are a variety of published allometric equations for determining avian energy requirements, but those derived from fasted species are inappropriate. For companion birds, equations derived for the particular body-weight range of the avian species in question are the most applicable. Two equations are recommended: one for calculating the energy requirements of avian species less than 100 g, and the second for avian species whose body weights are in the range 100–1500 g.