Captive Costa's hummingbirds (Calypte costae) were fed liquid purified diets containing 0%, 0.75%, 1.5%, or 3.0% protein (dry-matter basis). The birds were weighed and excreta samples were collected at days 5 and 10 of 10-d experimental periods. Body-mass maintenance data revealed that the hummingbirds require about 4.5 mg N/d, or 1.5% protein in their diets. Nitrogen balance analysis suggested that these birds should be in balance when ingesting only 1.11 mg N/d (0.4% protein), but this level did not maintain body mass. Similar positive N balances without growth have been documented in other avian and mammalian species, but the fate of this unaccounted N has not been explained. The bodymass data indicated that adult, nonreproducing hummingbirds have a low protein requirement. These results are compared with published time-budget studies of free-living hummingbirds, which have shown that the birds spend most of their feeding time foraging for nectar to meet their high energy needs.