To examine how hummingbirds that do not enter torpor at night store and utilize energy, open-circuit respirometry and a strain gauge were used to measure daily variation in O₂ consumption ($\dot{V}o_{2}$), CO₂ production, respiratory quotient (RQ), and body mass in Anna's hummingbird, Calypte anna, and Costa's hummingbird, Calypte costae. During the day, $\dot{V}o_{2}$ was highly variable primarily because of differences in activity among individuals. At night $\dot{V}o_{2}$ varied little between individuals, but mean $\dot{V}o_{2}$ was more than two times that predicted from body mass for resting postabsorptive birds. For C. anna, mean 24-hr energy expenditure was similar to that of free-living birds. Diel mass fluctuations were large, up to 16% in both species. However, much of the observed change in mass was probably due to factors other than changes in body fat content. The RQ was well above 1.0 throughout the day, suggesting continuous deposition of fat, and RQs remained high at night (> 0.85), indic...