Various bird species regularly fast in connection with breeding, migration, or drastic climatic conditions. The metabolic response of penguins and domestic geese to fasting has been studied in detail. These large birds, in contrast to small species, do not become torpid when they are fasting. Nevertheless, they reduce their rate of energy expenditure by decreasing both resting metabolic rate and locomotor activity. From changes in the loss of body mass, the fast of penguins and geese has been divided into three phases: I, the loss of body mass decreases; II, it remains at a minimum level; and III, it increases. These phases reflect metabolic adjustments. Phase I is a rapid phase of adaptation, marked by a decrease in protein catabolism and mobilization of lipids. Phase II is a phase of economy, during which more than 90% of the energy expenditure derives from lipids, while protein catabolism remains at a minimum level. In phase III there still are lipid reserves and this phase is reversible; it is, however, critical because proteins are no longer spared. Data in the literature suggest that these three phases may also be used to describe how a wide variety of wild and domestic birds adapt to fasting.