Female signal jamming in a socially monogamous brood parasite.

Acoustic signalling is vital to courtship in many animals, yet the role of female vocalizations is understudied. Here, we combine observational and experimental methods to assess the courtship function of the female chatter call in brown-headed cowbirds, Molothrus ater. While the chatter call is likely multifunctional, it is frequently used in social interactions and overlapping duets with males during the breeding season. Based on a combination of focal- and scan-sampling data from large naturalistic aviaries, we did not find support for the hypothesis that the chatter call elicits male attention or encourages continued courtship. However, we did find evidence that the chatter call plays a role in pair bond formation, as females preferentially chattered in response to songs from pair-bond males in the 2 weeks leading up to the median date of first copulation. Females were less selective in male-directed chatter use after copulations began. We also found support for the hypothesis that chatter is used to signal-jam male songs. Frame-by-frame video analysis revealed that the majority of female chatter calls were tightly time-locked to song, occurring less than 500 ms after male vocal onset. To test the effect of signal jamming on male song potency, we designed a laboratory experiment in which male song playbacks were jammed by various recorded stimuli. Natural chatter calls more effectively reduced female copulatory responses to song than high-pass filtered chatter calls, suggesting that the low frequencies in natural chatter (2-4 kHz) are important for interfering with male song and reducing its potency. Our results suggest that sexual conflict is operating in cowbird courtship, with signal jamming serving as a mechanism by which females guard, resist or select their mates. We also discuss ways in which cowbird vocal interactions may function cooperatively to coordinate reproduction or transition females into breeding condition.