Male Songbird Indicates Body Size with Low-Pitched Advertising Songs

Abstract

Body size is a key sexually selected trait in many animal species. If size imposes a physical limit on the production of loud low-frequency sounds, then low-pitched vocalisations could act as reliable signals of body size. However, the central prediction of this hypothesis – that the pitch of vocalisations decreases with size among competing individuals – has limited support in songbirds. One reason could be that only the lowest-frequency components of vocalisations are constrained, and this may go unnoticed when vocal ranges are large. Additionally, the constraint may only be apparent in contexts when individuals are indeed advertising their size. Here we explicitly consider signal diversity and performance limits to demonstrate that body size limits song frequency in an advertising context in a songbird. We show that in purple-crowned fairy-wrens, Malurus coronatus coronatus, larger males sing lower-pitched low-frequency advertising songs. The lower frequency bound of all advertising song types also has a significant negative relationship with body size. However, the average frequency of all their advertising songs is unrelated to body size. This comparison of different approaches to the analysis demonstrates how a negative relationship between body size and song frequency can be obscured by failing to consider signal design and the concept of performance limits. Since these considerations will be important in any complex communication system, our results imply that body size constraints on low-frequency vocalisations could be more widespread than is currently recognised.

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Variation in song system anatomy and androgen levels does not correspond to song characteristics in a tropical songbird

“…The differences we see between the sexes in pitch may instead be due to differences in body size. Scaling of pitch with body size is an honesty-enforcing constraint seen across taxa (Fletcher, 2004;Hall, Kingma, & Peters, 2013).…”

Maximum frequency of songs reflects body size among male dusky warblers Phylloscopus fuscatus (Passeriformes: Phylloscopidae)

“…We measured tarsus length (± 0.1 mm) 3 times at each capture, and used the average length as an indicator of body size. We used tarsus length as our measure of body size for the following three reasons: (1) it is observed to predict male quality and female choice in some birds (Kempenaers et al 1992;Møller & Ninni 1998;Hall et al 2013); (2) it has an observed relationship with call frequency (Price et al 2006;Brumm 2009;Geberzahn et al 2009;Linhart & Fuchs 2015); and (3) it is constant in adults.…”

“…Therefore, birdsong, as a sexually selected multidimensional signal, can reveal singer information to some extent. It is well known that advertising songs can indicate body size (Hall et al 2013;Price & Crawford 2013). In many studies, all song characteristics reveal signaler quality through traits such as repertoire size (Soma & Garamszegi 2011;Hesler et al 2012;Kagawa & Soma 2013), song timing (Poesel et al 2006), song amplitude (Brumm 2009;Schuchmann & Siemers 2010), song frequency, song bandwidth or song rate (Potvin 2013;Linhart & Fuchs 2015;Mason & Burns 2015).…”