The annual cycle of the White-throated Sparrow, Zonotrichia albicollis, is reviewed with brief references to facets of nutritional and energetic importance (1) illustrating the life of a small bird in a varying environment, and (2) showing that certain annual events in field populations can be compared with similar manifestations in caged individuals. Data from captives are employed in discussions of energetic variations related to (1) food, the source of nutritive input, (2) fat, the major form of caloric storage in birds, and (3) caloric expenditure. Metabolizable energy is partitioned by phase of the annual cycle into existence energy, including the costs of thermoregulation, and productive energy, including expenditures for nocturnal activity (Zugunruhe) and molt. Costs of vernal migration in field birds are compared with costs of nocturnal activity in captives to show that energetic estimates in each situation are compatible. This conclusion is supported by a metabolic estimate made for field birds that is within 6% of the estimated metabolism of captives under similar conditions. Data and statistics from seven additional species of buntings are used to examine several bioenergetic principles for homoiotherms. (1) Minimal metabolism measured by energy balance methods is proportional to the 0.7 power of body weight but is higher than standard or resting metabolism measured by gaseous methods. (2) Metabolized energy is inversely related to ambient temperature below 25°C, the estimated ad libitum critical temperature. (3) Heat production and loss are proportionately higher in summer-acclimatized birds below the ad libitum critical temperature due to reduced insulation. Two summary plots, relating temperature, metabolizable energy, and body weight are given. Directions for future research in the study of avian nutrition are suggested.