Both male and female blue-capped cordon-bleus exhibited quite rapid stepping behaviour during courtship displays, which varied among individuals but not across sexes. The evolution of this complex courtship display in both sexes is surprising because male ornaments tend to become exaggerated even in monogamous species because females are generally the choosier sex1. According to our pairing experiment, both male and female cordon-bleus chose to court with particular individuals, but high-motor performance individuals were not necessarily popular among the opposite sex (Supplementary Table S1b). Assortative mating did not tend to occur with respect to dance performances. The only sex difference in temporal pattern of the blue-capped cordon-bleus courtship display is the slightly shorter song produced by females17. Even if the small sample size prevented us from finding sex-related differences in dance performance, such sex differences would likely be small because the distribution of dance performances overlapped between males and females (Fig. 2a,b). Although other estrildid finches are known to show both dance and song courtship display14, the cordon-bleus displays (blue-capped and red-cheeked cordon-bleus; Supplementary Video S1, S3) are exceptional because as far as we know, theirs are the only dances that produce acoustic and vibratory signals and that are performed by both sexes.

Performing rapid stepping behaviour seemed to enable male and female cordon-bleus to communicate via multiple modalities. Our results suggest that both sexes produce multimodal (acoustic, visual and tactile) signals for intersexual communication that involves the coordination of several motor systems that control singing, bobbing, stepping and beak movements. Attention should be paid to where the birds dance in the wild because cordon-bleus reportedly dance also on the ground15. However, all dance displays were performed on perches in this captive study. Intensified stepping performance when partners were on the same perch likely enables performers to send vibration signals context-dependently. Considering that the limit of flicker fusion frequency is usually over 100 Hz in many birds18, stepping movements between 25 and 50 Hz could be visible and function as a visual signal for cordon-bleus. The bobbing movements seem to exaggerate the sexually dimorphic plumage of their heads and nesting materials that they hold. Intensified bobbing performance when partners stayed on the same perch could make them more visually appealing to highly motivated partners. Fewer steps and more frequent bobbing during singing are likely adaptations that help avoid interference between signals (i.e., stepping sound, song and body movements). Such coordination could be caused either by physical constraints associated with body movements (e.g., brown-headed cowbirds10), or by deliberately adjusting two independent signals. The amazing courtship display of cordon-bleus had features that are analogous to courtship displays in manakins, particularly with regard to multimodality and acrobatic movements19,20,21, but these species differ in vocal learning ability, behavioural mutuality and mating systems.

The most striking aspect of our findings is that they appear to produce two acoustic signals (non-vocal sounds and songs) simultaneously (Supplementary Video S1, S3, Supplementary Figure S1). Past research showed that some species spontaneously synchronize their movements with musical rhythms to which they are exposed22,23,24,25. However, little is known about how animals can temporally coordinate naturally produced multicomponent acoustic signals (i.e., vocal and non-vocal sounds26). Fewer steps during singing can be explained from the perspective of motor constraints because dance displays and singing are both physically demanding27. In contrast, increased bobbing tempo cannot be explained by trade-offs between singing and dancing signals. Sound pressure and the timing of steps seemed to be coordinated with the songs. A next step in the study of blue-capped cordon-bleus courtship display should be to investigate how singing, bobbing and stepping behaviours are coordinated within individuals and between partners. This would yield insights into how multimodal and multicomponent courtship display evolved.