Earlier studies have shown that the negatively allometric brain-body weight association in mature mammals changes to an isometric association when body weights are adjusted for their rates of oxygen consumption. Birds are endogenous homeotherms, and so their brain weights were analyzed according to their body weights and metabolism (estimated energy supply). As expected, the brain and body weights of the 83 species of neognathid birds have a negatively allometric association. The same species, however, have a brain weight-to-estimated energy supply which cannot be separated from isometry. While passerines have bigger brains for their body weights than altricial nonpasserines, the relative brain sizes of the two avian groups cannot be separated once the metabolic rate is used to adjust the body weights. Ratites or paleognathid birds may have a different brain-to-metabolism association. Consideration of bioenergetics helps clarify brain and body weight associations.