We examined the relationship between daily energy expenditure (DEE) and basal metabolic rate (BMR) in birds and mammals. Two models of the relationship between DEE and BMR were distinguished: a "shared pathways" model in which DEE replaces BMR in the active organism and a "partitioned pathways" model in which DEE includes BMR-that is, BMR is separate from the metabolic pathways that result in activity metabolism (ACT), and DEE = ACT + BMR. The appropriate null hypotheses for the relationship between basal and active metabolism are r_{DEE· BMR} = 0 and r_{ACT· BMR} = 0, respectively. Correlations of the residuals (d and b) of the logarithms of DEE and BMR from their allometric regressions with the logarithm of body mass were tested against these null models. Using phylogenetically independent contrasts, we found no significant relationship between DEE and BMR in birds, but a strong relationship (r_db = 0.86) among mammals. Thus, the hypothesis that sustained working capacity is related to basal metabolism is supported for mammals but not for birds. Residuals of metabolic variables from allometric regressions on body mass were greater for mammals than for birds and suggest that mammals are more diversified in their energetic physiology. The idea that sustainable energy expenditure may be pushed to physiological limits in mammals but not in birds is not supported, however, because the ratio of DEE to BMR has a somewhat lower mean and greater variance in mammals compared to birds. The nature of the relationship between DEE and BMR in mammals and the apparent absence of such a relationship in birds remain major puzzles in animal physiology.