What Makes Vocalisation Frequency an Unreliable Signal of Body Size in Birds? A Study on Black Swans

Abstract

In many animal species, the frequency (pitch) of vocalisations correlates negatively with body size and may thus signal competitive ability. However, this relationship is absent in other species. Understanding why this difference exists across species may help to explain some of the diversity of vocal communication systems. We assessed whether vocalisation frequency signals body size in black swans (Cygnus atratus), and how this is affected by (i) variation in frequency within individuals and (ii) size variation across individuals. Frequency was correlated with body size and mass, with slopes close to the allometry expected if the birds were maximising sound radiation, but the explained variation in frequency was low. Within‐individual variation in vocalisation frequency was greater in male than female swans, and the reliability of frequency as a signal of size in males was correspondingly lower. A review of the literature on the relationship between the frequency of avian vocalisations and body size also showed smaller effect sizes for more variable vocalisations (birdsongs), than for simpler vocalisations. Vocalisation frequency was more reliably correlated with body size when the sexes were pooled (creating a larger range of variation in size) than when the relationship was examined for either sex separately, although male and female data followed the same allometric line. These results show that variation in frequency within individuals and low variation in size across individuals reduce the reliability of vocalisation frequency as a signal of body size, which helps to understand differences among species in the signal value of vocalisation frequency.

Male Songbird Indicates Body Size with Low-Pitched Advertising Songs

“…However, fundamental frequency rarely correlates with size in mammals [10], instead formant dispersion more commonly indicates size [11], [12]. For the complex vocalisations of songbirds, empirical support is rare (reviewed in [13], [14]) despite detailed study [15], and limited to a single example: larger male barn swallows sing the rattle syllable of their songs at lower frequencies [16]. Other studies have demonstrated frequency variation associated with size differences between populations [17], [18], likely due to population differences in habitat or selection pressures [18].…”

“…Since frequency ranges are usually larger than those expected due to within-sex variation in size, this vocal complexity could obscure a relationship between body size and frequency [13], [14]. However, theory predicts that body size limits the production of loud, low-frequency sounds [3], so it is only the lower limit of the full frequency range that should be constrained by size.…”

Body size and vocalization in primates and carnivores

“…In both of these studies, the inclusion of species exhibiting a wide range of body sizes (~5–1400 g in10 and 14–155 mm in11) appears to have been critical. Studies examining narrower ranges of body sizes, e.g., within a single species, have produced mixed results, finding evidence for negative allometry in some cases121314 but not others15161718.…”

Five mechanisms of sound symbolic association

“…It is important to note that while many studies have found a relationship between fundamental frequency and body size in several species (e.g., Bowling et al, 2017;Charlton & Reby, 2016;Gingras, Boeckle, Herbst, & Fitch, 2013;Hauser, 1993;Wallschläger, 1980), others have not (e.g., Patel, Mulder, & Cardoso, 2010;Rendall, Kollias, Ney, & Lloyd, 2005;Sullivan, 1984). As noted by Bowling et al (2017), a relevant factor seems to be the range in body sizes 16 The authors theorize that two separate, but potentially related, processes may be at work.…”