<p>We document median-crown stripe variation in two dichromatic neotropical cardinalid species, the red-crowned ant-tanager <i>Habia rubica</i> and the red-throated ant-tanager <i>Driophlox fuscicauda</i>. While definitive (adult) males always have a red median-crown stripe and juveniles lack stripes, definitive females and formative birds (those after molting the juvenile plumage) of both sexes exhibit up to three variants: no stripe, intermediate dull-yellow stripe, and full bright-yellow stripe. Among these age/sex groups, occurrence of full stripe was highest in definitive female red-crowned ant-tanagers, followed by formative males of both species. By contrast, the median-crown stripe occurs at low frequencies in definitive female and formative red-throated ant-tanagers. We propose that this phenotypic plasticity in degree of the median-crown stripe may have evolved as an adaptation to the complex social systems of these two species, likely facilitating the formation of heterogeneous territorial groups and increasing winter survival and extra-pair fertilizations.</p>