Plumage colour can be used as an honest signal to convey health and status, which has traditionally been examined in the sexual selection context of choosy females and elaborate males. We use a model avian system to study the role of plumage coloration in a social context such as inter- and intrasexual competition over food resources. The diamond firetail (Stagonopleura guttata) is an endemic Australian finch: females have more white flank spots than males, and white spot number was correlated with cell-mediated immune response in females. We use two experimental designs to test the role of white flank spots for feeding dominance and dominance discrimination in a group-living bird. The results from two-choice trials and from single-arena trials showed that female ornamentation was consistently important in social food contests, and males consistently responded to female spot number. Females with higher spot number fed first, in trials with males and/or females. Also, females preferred to feed next to test birds with low spot number, but males showed no preference for feeding next to birds with few or many spots. Finally, latency to feed was predicted by spot number: both males and females had longer latency to feed if test birds had more spots than the focal birds. We conclude that female, but not male, ornamentation was important for inter- and intrasexual food competition. This is one of the very few studies to show that the same plumage ornament can have a different function between the sexes as a signal of social status. Moreover, this study shows that white plumage can be a signal of dominance.