This post was contributed by Francisco Magdaleno, lead author (with co-authors Steve Rothstein and Adrian O’Loghlen) of a recent paper in The Wilson Journal of Ornithology about the audio-visual song display of the Brown-Headed Cowbird.
Why would male songbirds direct aggressive signals at females during courtship? This is an important unanswered question raised by our research on the audio-visual song display of the Brown-headed Cowbird.
We focused on a little-studied behavior, bill wiping, in the cowbird. While a graduate student studying this species’ behavior in aviaries at Andrew King and Meredith West’s Animal Behavior Farm, I noticed that cowbirds would often wipe their bills at the end of the elaborate wing-spread display that is done while singing. Although bill wiping is a common behavior in many birds, it has often been overlooked and assumed to not be related to communication. This is the case in the cowbird song display whose visual and auditory components have received a lot of attention from researchers over the years but not the bill wiping that accompanies the song displays even though early researchers noted its frequent occurrence.
Using data I collected as a graduate student and video footage collected by Steve Rothstein and Adrian O’Loghlen, we focused on bill wiping to determine its relationship to the cowbird song display. By looking at an extensive list of different behaviors during the breeding season and noting when bill wiping occurred, we could see that the evidence was clear—bill wiping is a frequent component of the audio-visual song display.
Next, we attempted to understand what kind of message or information is communicated by bill wiping. In order to do this, we looked at the social contexts in which both bill wiping and head-up displays occurred. In contrast to bill-wiping, head-up displays have been relatively well researched as a communicative display and appear to signal aggression in many species, including cowbirds. Although I did not collect data on head-up displays as a graduate student, Rothstein and O’Loghlen’s video data did capture head-ups produced shortly before or after song displays.
We found that bill wiping and head-up displays were both commonly produced when males were singing to other males. This provided evidence that, like head-up displays, bill-wiping is an aggressive signal. However, we did have data showing that males sometimes, albeit less commonly, direct bill wipes and head-up displays at females during courtship. This is surprising because past research shows that female cowbirds do not like aggressive signals such as high-intensity song displays being used during courtship. So then it is presumably not beneficial for a male to direct other aggressive displays such as bill wiping at a female during courtship either. Male cowbirds modulate their song displays to make them less aggressive when singing to females, and the importance of males properly modulating their displays during courtship has been documented in other species as well.
So why would male songbirds direct aggressive signals at females during courtship? Future research is needed to answer this question. Intersexual aggression does occur in various animal groups. It has also been noted in several songbird species, including Mountain Bluebirds and Reed Warblers, and we suspect that additional research will show that it occurs in others. Intersexual aggression also happens during the early dating phase in humans, which does not make sense but nevertheless does occur. We encourage more researchers to look at their model species for aggressive behavior between the sexes to better understand the development and evolution of female-directed aggression by males.