Identifying ecological drivers of interspecific variation in song complexity in songbirds (Passeriformes, Passeri)

Abstract

A diversity of selective pressures and stochastic processes have likely created substantial variation in song structure, creating difficulties in quantifying the influence of specific ecological factors. This problem is further compounded by differences in study taxa and methods of data analysis between studies. Large comparative studies offer the potential to mitigate some of these methodological difficulties by maximizing the power of statistical analyses and minimizing the probability of misidentifying the magnitude and direction of relationships between independent and dependent variables. In this study, we quantified song complexity for 367 species of globally distributed songbirds (Passeriformes, Passeri). We quantified eight individual acoustic variables that have previously been linked to audio complexity which we analyzed independently, and after applying multivariate statistics to the variables. We used Bayesian linear mixed effect models to test multiple hypotheses regarding song complexity: that it should be greater in open habitats, in migratory species, for sexually monomorphic species, at higher latitudes and altitudes, and that it should co‐vary with clutch size characteristics. Our results challenge perceptions of the effect of habitat structure on song complexity; for instance, counter to expectation, we found songs in closed environments to have reduced syllable diversity. Additionally, our results suggest song complexity may not be ubiquitously a means of communicating male quality, with no significant difference between recordings from monomorphic and dimorphic species. By estimating song complexity in multiple ways, and quantifying these over large taxonomic and spatial scales, we are able to gain a more nuanced understanding of how song complexity is potentially affected by a range of biotic and abiotic factors. Our results also suggest that caution is required when making generalized statements about the relative influence of different factors on song complexity; more densely‐sampled, group‐specific studies are necessary complements to this taxonomically broad analysis.

Contingency and determinism in the evolution of bird song sound frequency

“…Here we used song duration as a proxy to evaluate the strength of sexual selection on song because duration is an aspect of song motor www.nature.com/scientificreports/ performance (longer songs can place motor and ventilation challenges 53 ), and song motor performance can be sexually selected 54 . In addition, song duration correlates with syllable diversity and repertoire sizes across species (e.g., 34,35 ), and sexual selection has often been found to favor long song 55 , or diverse repertoires 56 . Also, songs are generally much longer than calls (Fig.…”

“…To ask if species experiencing stronger sexual selection on song tend to evolve sound frequency in one particular direction or in unpredictable directions, we also measured song duration as a proxy for song elaboration and, therefore, for the strength of sexual selection specifically on song. We used song duration because it is an aspect of song motor performance, and because song duration correlates with syllable diversity and repertoire size across species (e.g., 34,35 ). We did not use more general indexes for the strength of sexual selection, such as color ornamentation or sexual dichromatism, because species differences in ornamentation are generally not associated with song elaboration (reviewed in 36,37 ) and, therefore, these indexes may bear little relation to the strength of sexual selection acting specifically on song.…”

Species‐level repertoire size predicts a correlation between individual song elaboration and reproductive success

“…We return to the parallel with sexual selection for tail length in birds (Figure ), where species‐level exaggerated tail length could suggest that sexual selection is operating on this heritable trait within a species, predicting an individual‐level correlation between tail length and reproductive success. There are little data on the cultural and genetic heritability of repertoire size, but existing analyses indicate that repertoire size has strong phylogenetic signal (Crouch & Mason‐Gamer, ; Snyder & Creanza, ; Tietze et al, ). Sexual selection theory predicts that if females consistently choose males with more elaborate repertoires or if males with larger repertoires are more fecund, the distribution of repertoire sizes in a species may shift toward larger values over time.…”

Structure and function of the vocal repertoire of the Rifleman ( Acanthisitta chloris ), a member of the earliest diverging passerine suborder, Acanthisitti

“…Calls are also often assumed to be innate due to their short duration and thus have less variation than songs (Nieder and Mooney 2020, but see Vicario et al 2002, Vicario 2004, Sewall 2011, Walløe et al 2015. Vocal complexity is influenced by ecological, morphological, and social factors (Brumm et al 2009, Soma and Garamszegi 2011, Krams et al 2012, Fischer et al 2017, Crouch and Mason-Gamer 2019, Yambem et al 2020. Studying vocal complexity is important for understanding the evolutionary and ecological drivers of vocal elements, their functions, and importance to fitness (Fischer et al 2017, Lipkind et al 2017.…”