Data on energy expenditure by 553 individuals of 28 species of small bird (10–150 g) are presented. All estimates of energy expenditure were obtained using the doubly-labelled water technique. Intraspecies variation in daily energy expenditure was found to be positively correlated with brood provisioning rates, percentage of time flying and the frequency of non-resting activity. Correlations were also shown with body-mass, body-size and several environmental factors. Published data on basal metabolic rates (BMR) sometimes differed substantially from estimates either made specifically as part of the studies considered here or calculated from allometric equations. For the purpose of interspecific comparisons, specific estimates of BMR are to be preferred. When expressed as a function of BMR, energy expenditures of free-living birds ranged from 1 + to 7 + times BMR with a mode at 3 +. Values of daily energy expenditure exceeding 4 times BMR were found in up to 48% of species and 30% of individuals, so that, contrary to earlier suggestions, 4 times BMR is not a universal upper limit to the sustained work rate of small birds. Observed upper limits tended to be higher in species with energy-expensive foraging habits. Energy expended by breeding birds is likely to involve a balance between the benefits a greater expenditure has for offspring production and any fitness penalty associated with the high level of energy expediture which nest provisioning involves.