Why do Calypte hummingbirds "sing" with both their tail and their syrinx? An apparent example of sexual sensory bias.

Courtship displays frequently include complex signals that females use to pick a mate. Male Costa's hummingbirds (Calypte costae) generate two acoustic signals during courtship: a vocal song produced close to a female and a dive-sound produced during a courtship dive. The song and dive-sound sound similar, and both were assumed to be produced vocally by the syrinx. Here, we show that they are not; whereas the song is produced by the syrinx, the dive-sound is produced by high-frequency fluttering of the outermost tail feathers. The Anna's hummingbird (Calypte anna), sister to the Costa's, also sings a vocal song and produces a dive-sound with the wings and outermost tail feathers that sounds similar to a portion of the song. The interspecific match in signal form between the two species is not as strong as the intraspecific match. Phylogenetic reconstruction indicates that the dive-sounds may have evolved first, suggesting that the song may have evolved to mimic the dive-sound. We propose the "sexual sensory bias" hypothesis as an explanation for the match in form between the song and the dive-sound within each species, in which we suggest that new sexual signals can arise in response to preexisting female preferences for older sexual signals.