Observations of exaggerated traits and ornamentation in female animals are foundational to evolutionary theory, yet they have historically been cast aside as anomalous or byproducts of sexual selection on males. More recently, many cases of ornamentation in females have been found to be adaptive, and often for different reasons as those in males (i.e., sexual selection). Hypotheses for the function of female ornamentation would ideally be studied through discrete within-species variation in female ornaments. However, this type of variation, female-limited polychromatism (FLP), is understudied despite its unique ability to address questions of ornamentation and sexual dimorphism. In this dissertation, I developed and tested hypotheses to explain FLP in a tropical hummingbird, the white-necked jacobin (Florisuga mellivora). In Chapter 1 I show that coloration in white-necked jacobins is polymorphic. All males express an ornamented plumage type (the “androchromic” type). However, 30% of females are androchromic like males, while the rest express a drab plumage type (the “heterochromic” type) distinct from androchromes. I found a remarkable developmental molting pattern in which both male and female juveniles were ornamented in androchromic plumage like adult males. Males continue in this plumage after molting, while most females shift into the heterochromic type as adults. This unique ontogeny excludes competition for mates as an explanation for ornamentation in females. Instead, my observations are consistent with the idea that social harassment underlies this female-limited polymorphism. While Chapter 1 shows that social, not sexual, selection benefits androchrome females by allowing them to escape aggression, Chapter 2 explores why this benefit has evolved. Androchomic plumage is either an indicator of resource holding potential (RHP) in both males and females, or androchromic females avoid aggression from other hummingbirds through mimicry of males. I tested critical predictions of these hypotheses by observing behavior and comparing measures of RHP between males and females. I found that males had higher RHP than females, but RHP did not differ between female types. Observations of birds implanted with radio-frequency identification (RFID) tags utilizing a network of logger-equipped feeders revealed that androchrome females accessed high quality feeders for longer durations than did heterochrome females. Androchrome females therefore have higher access to resources through mimicry of males rather than through direct advertisement of competitive ability. Female-limited polymorphism, such as in white-necked jacobins, is less studied than male-limited polymorphism, and no unifying theory exists for its evolution. Alternative hypotheses are typically only developed within taxonomic groups or specific cases, and the relationship between hypotheses across taxa and sexes are typically not considered. In Chapter 3 I show that hypotheses for most polymorphisms can be described two major axes: (1) the type of balancing selection, and (2) the type of competition involved. I demonstrate how female-limited polymorphism, in hummingbirds and other taxa, has evolved repeatedly through non-sexual social selection and typically persists through negative frequency dependent fitness.