We investigated whether Pekin ducklings (Anas platyrhyncos domesticus) exhibited any energy-saving mechanisms that could lessen the detrimental effects of reduced food intake during early development. Further, we evaluated the role of body compositional changes behind such potential mechanisms and the consequences on thermoregulatory capacity. The ducklings exhibited substantial energy-saving mechanisms as a response to diet restriction. After 5 d of diet restriction, the resting metabolic rate (RMR) of 10- and 20-d-old ducklings was 16.4% and 32.1% lower, respectively, than predicted from body mass compared with ad lib. fed ducklings (controls). These reductions in RMR could have been adaptive responses in anticipation of a lasting food shortage, or they could have been consequences of the restricted diet and the lack of essential nutrients. We argue that the responses were adaptive. The low RMRs were not a consequence of depleted fuel stores because the diet-restricted ducklings exhibited substantial amounts of stored lipids at the end of the diet-restriction periods. Hypothermia accounted for approximately 50% of the reduction in RMR in the 10-d-old diet-restricted ducklings, but hypothermia did not occur in the 20-d-old diet-restricted ducklings. Diet restriction resulted in a reduced liver and intestine size and an unchanged size of the leg muscles and heart, while the length of the skull increased (compared with controls of a given body mass). However, changes in body composition were only minor predictors of the observed changes in RMR. Peak metabolic rate (PMR) was approximately 10% lower in the diet-restricted ducklings compared with the controls. We have interpreted the lower PMR as a consequence of the reductions in RMR rather than as a consequence of a decreased function of the thermoregulatory effector mechanisms.