We measured the field metabolic rate (FMR) and water influx rate (WIR) of the largest species of bird, the Ostrich (Struthio camelus), which lives in the Namib desert, one of the driest regions on earth. Along with doubly-labeled water (DLW) measurements of FMR and WIR, we examined the availability of plants in various habitats, the plants selected by Ostriches, and the daily activity patterns of these birds. During 6-8 d periods, adult Ostriches (88.25 kg) had an FMR of 18 040 kJ/d, while subadult birds (50.75 kg) metabolized energy at a rate of 15 428 kJ/d. Adult energy expenditure was 26% lower than predicted, but subadults had a FMR nearly equivalent to expectation, suggesting that adults may be more efficient at acquiring resources. Conclusions remain tentative because data for the Ostrich exceed previous data used for allometric equations by almost an order of magnitude. Gravel and stoney plains together accounted for 84% of the study area, whereas washes occupied @?1%. Vegetation cover was sparse in all habitats, varying between 7 and 19% in most areas, but comprising @?15% along drainage lines. Ostriches foraged on gravel plains nearly 65% of their daytime hours and 25.5% of their day in washes. They consumed a narrow range of green plants with Monechma arenicola, Schmidita kalahariensis, Blepharis spp., Trianthema triquetra, and Dicoma capensis representing the principal items in the diet. From a time-activity budget, Ostriches spent 7.5 h of their 24-h day walking, and roosted at night for 11.5 h. Transport between food patches accounted for 62.2% of FMR while nighttime rest was 19.0%. Preening and other maintenance behaviors were responsible for 1% of the Ostriches' energy budget. Ostriches had a frugal water economy when compared to other nonpasserine birds, with both subadults and adults having lower WIRs than predicted. Values for WIR were higher than allometric predictions based on four other desert birds previously studied, but because Ostriches are two orders of magnitude larger in body mass, firm conclusions await further data. Calculations of the water economy index (WEI), the ration of water influx to FMR (in millilitres of water kilojoule), showed that Ostriches conserved water like smaller desert birds. Four species of desert birds had WEI values averaging 0.16 mL/kJ; values of the Ostrich averaged 0.17 mL/kJ. An itemized water budget suggested that adults did not drink during the periods of observation, while water intake by subadults averaged 729 ml/d. This suggests that adults may have lower minimum water requirements than subadults. Combining available data for FMR and WIR with data for the Ostrich, we constructed new allometric equations that nearly span the entire range of bird size and include 62 species of birds.