Authors Clarke and Feo had previously shown that during courtship, male hummingbirds in one genus, Calypte (Calypte costae shown above; Calypte anna below) make sounds with their tailfeathers that are similar to sounds they make when singing. This paper tries to figure out how this particular signal evolved. One of the strengths of this paper is that they have four hypotheses about how the tail sound might have originated, three of which generate different predictions. The song and feather sound could:
- Be unrelated as far as the female is concerned; two entirely different cues. If so, there would be no reason for the sounds to be the same as each other in the same species, or to be the same in different species.
- Both be exploiting a female preference. If so, there is strong reason for the sounds to be the same as each other in both a single species, and between different species.
- Have evolved sequentially, with female preference for one sound shaping the other. If so, you would predict the sounds to be the same in one species, but to differ between the two species.
Strong inference all the way, baby!
This paper does several things. One, they nail down exactly which feathers are responsible for the sound by experimentally removing feathers, and artificially running blasts of air over them. In both species, the same feathers make the sound, suggesting that both birds inherited this feature from their common ancestor. Interestingly, while hummingbirds outside the genus dive the females, few sing, suggesting that the diving is more ancient than singing in this group.
They also show that the tailfeather sounds of each species most strongly resembles that of the song made the normal vocal way. Interestingly, a hybrid between the two species generates a sound intermediate to the other two.
That there is an “intermediate” tail sound made by a hybrid suggests where we’re going with the matching. The two species make rather different sounds with their feathers. The differences in song patterns are fairly complex, but as a for instance, the sounds made by C. costae are around the 7 kHz range, whereas the sounds made by C. anna tend to be lower-pitched.
All of which suggests that one of the two sounds the males make evolved as a sort of “enhancement” to the earlier evolved sound. Given that singing seems to be the more recent innovation in this particular group of hummingbirds, it looks like the males started to sing to enhance the sounds made by their tailfeathers, rather than the song having primacy, as we might intuitively expect.
Clark, C., & Feo, T. (2009). Why Do Hummingbirds “Sing” with Both Their Tail and Their Syrinx? An Apparent Example of Sexual Sensory Bias The American Naturalist DOI: 10.1086/648560
Picture of C. costae by users Lance and Erin on Flickr, used under a Creative Commons license.
Picture of C. anna by user wolfpix on Flickr, used under a Creative Commons license.