DERRYBERRY, Elizabeth P; BALLENTINE, Barbara E; Duke University; Duke University: Historical changes in vocal performance: implications for mate choice.

Recent studies on female mate choice suggest that female songbirds assess males’ relative abilities to perform the same song type. The production of physically challenging songs requires rapid and precise coordination of vocal tract movements. In trilled songs this results in a trade-off between trill rate and frequency bandwidth. This trade-off defines a constraint on song production observed as a triangular distribution in the acoustic space of trill rate by frequency bandwidth, with an upper boundary that represents a performance limit. In several species, females have been shown to prefer males that produce songs closer to this limit. It is unknown, however, if females show equal preference for high performance songs that exhibit different acoustic features. That is, are all songs that are equidistant from the performance limit functionally equivalent? If high performance songs are functionally equivalent, then we expect that over time there will be a significant change in mean trill rate and mean frequency bandwidth for a population but the mean distance from the performance limit will be constant or decrease. We compare song data collected from four populations of mountain white-crowned sparrows (Zonotrichia leucophrys oriantha) in the 1970’s and 2000’s. During this time interval trill rate and frequency bandwidth have changed significantly in these populations, presumably as the result of cultural drift. Finding that the mean distance from the performance limit has not changed or has decreased in thirty years but that frequency bandwidth and trill rates have changed is consistent with our hypothesis that songs that are equally well performed are functionally equivalent.