Morphological Adaptation Influences the Evolution of a Mating Signal

Abstract

Theory predicts that forces of natural selection can reduce the intensity of sexually selected traits. In this study, I investigate how morphological adaptation to feeding ecology influences a mating signal. In birds, changes in feeding ecology can cause rapid divergence in bill morphology. Because bills are also important for song production, feeding ecology may influence song divergence. During song, birds can rapidly change vocal tract resonance using bill movement, yet are constrained in rate and magnitude of bill movements resulting in a trade‐off between trill rate and frequency bandwidth. Male swamp sparrows vary in their ability to produce rapid, broad‐band trills and females prefer more physically demanding songs. Populations of swamp sparrows adapted to the feeding ecology of tidal marshes have larger bills than inland populations. Larger bills should increase the constraints of producing rapid, broad‐band trills allowing for a test of how changes in feeding ecology affect a feature of song used in mate choice. I found significant differences in acoustic features of song consistent with the hypothesis that coastal males are less able to meet the physical demands of song production because of the constraints of having larger bills. As possible compensation for decreases in song performance, coastal populations exhibit an increase in song complexity. These changes support the current model of how motor constraints influence song production and suggest a mechanism by which feeding ecology can influence signal evolution.

The ability to perform physically challenging songs predicts age and size in male swamp sparrows, Melospiza georgiana

“…The vocal tract itself is also involved in dampening harmonic overtones by rapid changes in the oropharyngeal cavity (Riede et al 2006), and bill movements may be important only in the production of frequencies produced above 4 kHz (Nelson et al 2005;Riede et al 2006). In the population of swamp sparrows that I studied, the average high frequency produced is about 7.7 kHz (Ballentine 2006a); thus, bill movements at least partly constrain song production in swamp sparrows (Westneat et al 1993;Hoese et al 2000). While the specific model for how constraints on the production of rapid, broadband trills presented here may be incomplete, the production of rapid, broadband trills is nevertheless physically limited, and vocal performance provides a framework for how song can be an accurate indicator of male quality.…”

“…The method for quantifying vocal performance is described in detail elsewhere (Podos 2001;Ballentine et al 2004;Ballentine 2006a). Briefly, I plotted trill rate (X axis) and frequency bandwidth (Y axis) of songs, then calculated an upper boundary regression, which estimates the performance limit on trill production (Podos 2001).…”

Macrogeographic Vocal Variation in Subspecies of Swamp Sparrow

“…Some distinctions among the three subspecies' songs have already been noted (Mowbray 1997), and Ballentine (2006) has compared song traits between M. g. nigrescens and georgiana. Despite these reports, there has been no study coupling a description of song with an experimental test of vocal discrimination in these two subspecies.…”

“…These two characteristics are inversely related to each other due to the physical constraints that prevent maximal vocal performance in both traits (Podos 1997, 2001, Podos and Nowicki 2004, Ballentine et al 2004, Ballentine 2006. They are also ideal characteristics to examine because they measure concrete quantitative traits of vocal performance with which different song types can be compared.…”

Bill Morphology Does Not Influence Vocal Performance in Darwin's Small Tree Finch on Floreana Island

“…A growing body of research highlights the links between morphological variation and song variation, suggesting that divergence in morphological traits that have ecological significance (e.g. bill morphology) may also influence song divergence (Palacios & Tubaro, 2000;Podos, 2001;Ballentine, 2006;Huber & Podos, 2006). While bird song is produced when air passes over the syrinx causing vibration of the tympaniform membranes; the size, shape, and movements of the vocal tract play a significant role in filtering harmonic overtones resulting in the characteristic pure tone sound of birdsong (Nowicki, 1987;Westneat et al, 1993;Beckers et al, 2003).…”