Variation in female songbird state determines signal state needed to evoke copulation
Female preferences for male signals largely determine courtship success. In most songbirds, females control reproduction via the copulation solicitation display (CSD), an innate, stereotyped posture produced in direct response to male displays. In some species, CSD can be elicited under controlled experimental conditions, allowing CSD production to be used as a bioassay for the underlying mechanics of courtship, however historically our ability to measure variability in CSD was limited. Using computer vision approaches to precisely quantify the trajectory of CSD in female brown-headed cowbirds (Molothrus ater), we show that song potency (how often a song elicits CSD) and female responsivity (how likely a female is to produce CSD) each predict whether a female produces CSD in response to a given presentation. Within elicited CSD, potency and responsivity also predict the duration and onset latency of the displays. By modeling and testing two alternative hypotheses of preference formation, we show that changes in response rates are consistent with a shifting receiver response threshold, suggesting that the song quality required to evoke the copulatory posture in a particular female will vary over time. These results establish a novel framework for exploring CSD and emphasize the importance of the internal state of receivers in determining behavioral responses and suggest that female responsivity acts in conjunction with male signal strength to determine the efficacy of male courtship signaling.