Nucleotypic theory suggests that genome size play indirect roles in determining organismal fitness. Among endotherms this theory has been demonstrated by an inverse correlation between basal metabolic rate (BMR) and genome size. Nonetheless, accumulation of variables, especially for some key groups of endotherms, involved in C-value enigma (e.g., cell size) will fortify this theory. In this sense, hummingbird species are of particular interest because they are an energetic extreme in avian and endotherm evolution. Knowing that cell size is proportional to C-value, in this study we tested for a relationship between mean corpuscular volume of red blood cells and BMR in four species of hummingbirds ranging from 4 to 20 g. In comparison with other birds, our hummingbird data show higher BMR and the smallest mean corpuscular volumes, thereby providing further support for the nucleotypic theory