Phlegopsis erythroptera (Formicariidae) follows army ants regularly for flushed arthropods between the Andes and the Negro/Madeira Rivers. Mainly a bird of terra firme forests, it is interspecifically aggressive. Low numbers at ant swarms are probably due to low productivity of arthropods flushed by ants on weathered terra firme soils, or to high species diversity of subordinate but active ant-following competitors in upper Amazonia. Sexual dimorphism of young and female erythroptera is attributed to low numbers over ants, so that dispersed individuals avoid attacks by bright-plumaged adult males rather than bluff them out at close range. Phlegopsis, Skutchia, Rhegmatorhina, and Gymnopithys are related to and perhaps congeneric with Pithys; all follow ants and seem a group derived from birds related to Hylophylax.