The natural history of many tropical bird species is poorly described, preventing more detailed studies of ecology, behavior, and evolution. For most sexually monochromatic tropical bird species, we lack field methodologies to categorize the sex of adults. In this study, we describe sex-based morphological differences of three monochromatic species in the genus Melozone: White-eared Ground-Sparrow, M. leucotis; Prevost's Ground-Sparrow, M. biarcuata; and Rusty-crowned Ground-Sparrow, M. kieneri. We collected six standard morphological measurements (tarsus length, tail length, wing chord length, culmen length, beak width, and beak depth) from live birds and museum specimens. We collected data from all recognized subspecies of the three Mesoamerican Melozone species. Morphological measurements capably distinguished males from females in all three species. In all cases, three or fewer morphological measurements were required to identify sex with accuracy levels that ranged from 75–100%, well beyond the 50% expected by chance. Comparisons involving all six measurements yielded accuracies that ranged from 58–93%. Our results provide the first field-based method for estimating the sex of individuals in this poorly studied genus of tropical birds. We recommend our findings be used to distinguish males from females in banding stations where Melozone ground-sparrow species occur.