We tested the potential for social selection to act as a mechanism driving rapid plumage divergence in two sympatric monarch flycatchers, the White-capped Monarch (Monarcha richardsii) and the Kolombangara Monarch (M. browni), endemic to a single island group in the Solomon Archipelago. Solomon Island monarchs are famous for dramatic patterns of morphological divergence across very narrow water gaps, little parallel ecological variation, and minimal genetic differentiation among sister taxa inhabiting adjacent island groups. Social selection theory predicts that plumage traits evolved in allopatry may transmit important social information and that responses of dimorphic and monomorphic taxa to territorial intrusions will differ. For the dimorphic M. richardsii, we presented mounted specimens of subadult or female—and adult male—plumaged individuals to territorial birds and quantified their responses to these simulated intrusions. Territorial male M. richardsii generally responded alone, reacting most aggressively to adult male mounts. This response suggests that the bright white cap and occipital and nape patches on male M. richardsii function as social signals. In contrast, in the monomorphic M. browni, both sexes responded aggressively to intrusions of an adult-plumaged conspecific mount. Furthermore, in a variety of Melanesian forest passerines, individuals of dimorphic species generally responded singly to simulated territorial intrusions, whereas individuals of monomorphic taxa usually responded in pairs. Together, these data suggest social selection may be an important mechanism of population divergence driving some of the most extreme patterns of geographic variation among birds.