Many species of passerines are sexually monochromatic and thus sex cannot be determined based on plumage characteristics. Northern Waterthrushes (Parkesia noveboracensis) and Swainson’s Thrushes (Catharus ustulatus) are two such species. The objective of this study is to examine morphological differences between males and females of both species and determine whether this information may be used to determine sex. With genetically sexed birds, I indicate wing chord values can be used to determine sex with 95% confidence. This information would allow field researchers to determine sex of 58% of Northern Waterthrushes and 33% of Swainson’s Thrushes with 95% confidence of correct assignment. If age is taken into account, the proportion of individuals for which sex can be determined increases to 62% and 38%, respectively. This information may be used by avian ecologists in future studies of behavioral ecology, conservation biology, or evolutionary biology these species.