Sexual selection has long been proposed to accelerate speciation in animals. However, all previous studies have focused on the diversifying effects of sexual selection targeting male traits, whereas elaborate traits are often expressed in both sexes. Here, we test the relative importance of selection acting on male-only versus mutual ornaments in lineage diversification using birdsong, a classic sexual signal. We assess rates of speciation across phylogenies containing the majority of the world’s birds (N = 6601), finding that lineages with male-only song speciate at significantly faster rates than those in which both sexes sing. These findings provide robust support for the longstanding idea that song and sexual selection play a key role in avian speciation. They are also consistent with the idea that strong dimorphism (i.e., male-only song), is associated with contrasting sex roles and strong sexual selection, whereas monomorphism (i.e., mutual ornamentation, male-female song) is associated with balanced sex roles, lower sexual selection, and strong social selection in both sexes associated with defence of ecological resources. This study adds to the growing body of evidence that separate selective pressures can act on traits in males and females and of the key role of non-sexual social selection in shaping the evolutionary trajectories of animals.