Equivalent effects of bandwidth and trill rate: support for a performance constraint as a competitive signal

Sexual signals that are physically limited can be reliable indicators of quality or motivation in male-male competition. One such example of a motor constraint in birds is the production of repeated notes, which are limited in the frequency bandwidth and trill rate at which notes can be produced, such that it is difficult to produce wide-bandwidth notes at fast rates. How well birds maximize frequency bandwidth and trill rate is one measure of vocal performance, commonly referred to as 'vocal deviation'. In theory, fast songs with narrow bandwidths and slow songs with wide bandwidths should have similar values of vocal deviation. In many species, males respond to variation in vocal deviation, supporting the notion that it is a sexually selected signal. However, most studies test only one of these components, either trill rate or bandwidth, rather than both individually, when testing receiver response to vocal deviation. Therefore, a question remains as to whether songs with equivalent values of vocal deviation (e.g. fast songs with narrow bandwidths and slow songs with wide bandwidths) elicit similar levels of response from receivers. We tested whether receivers respond specifically to the trade-off between trill rate and bandwidth (i.e. vocal deviation) or only to variation in one of the component parts. Using territorial playback experiments with wild male white-crowned sparrows, Zonotrichia leucophrys, we found that males approached high-performance songs (fast trill, wide bandwidth) more closely than they did lower-performance songs (fast trill, narrow bandwidth; slow trill, wide bandwidth) and they did so regardless of whether performance varied because of differences in trill rate or bandwidth. Furthermore, we found that males gave similar responses to songs of similar vocal deviation. Our results empirically support the hypothesis that receivers respond specifically to the physical limitation on the production of repeated notes.