Although birds are generally known for their vocally produced songs and calls, some species have evolved alternate means of acoustic communication that do not require the syrinx. While many of these mechanical sounds are used in a courtship context, the importance of among- and within-individual variation in these sounds is almost entirely unknown. We investigated feather-produced sounds in male Greater Sage-Grouse (Centrocercus urophasianus), which congregate on leks during the spring breeding season and perform elaborate displays to attract females. Despite decades of research on the vocal components of the display, the frequency-modulated and mechanically generated “swish” sounds remain poorly studied. We used 2 years of acoustic data to evaluate the relationship between the time and frequency characteristics of the swish display and male mating success. Although characteristics of the swish sounds showed individual-specific patterns of variation, neither univariate nor multivariate analyses revealed direct effects of the acoustic qualities of these mechanical sounds on number of copulations. However, we did find that the frequency range of individual notes was correlated with note duration, and that males who successfully copulated showed a larger frequency range for a given duration than unsuccessful males. Furthermore, successful males increased this frequency change more strongly with the approach of a female than did unsuccessful males. These results parallel previous findings that successful and unsuccessful males show different patterns of adjustment with changing courtship conditions. Our results emphasize the importance of considering the interaction among multiple components of displays in analyses of mate choice, and help to broaden our understanding of the function of mechanical sounds in this and other species of birds.
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