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Description
Studies on bird song have provided some of avian biology's most seminal works on animal behavior, sexual selection, and speciation among many other topics. The conspicuous vocal displays of oscines are learned behaviors that are subject to cultural transmission between generations. In addition, songs are influenced by an impressive array of additional evolutionary pressures including sexual selection, morphological constraints, and differing acoustic environments. The potentially interacting effects of these evolutionary forces effect rapid evolutionary changes in bird song, which make vocal displays a difficult trait to study among species. Despite a rich history of intraspecific studies on various aspects of bird song, there persists a paucity of taxonomically driven, higher-level studies on avian vocal displays. Large-scale studies of monophyletic groups are important for uncovering macroevolutionary patterns among lineages. The onset of publicly available, online nature recording repositories and advanced computing software now facilitates the collection of large-scale, comparative data sets that include quantitative measurements of bird songs. In addition, the increased feasibility of gathering multilocus genetic data has facilitated the construction of large-scale, species-level phylogenies. In this study, I undertake the largest comparative study on bird song to date by exhaustively sampling audio recordings within a large radiation of Neotropical oscines known as tanagers. These data are analyzed in the context of a recently constructed phylogeny of the group. The first objective of this study is to examine the relationship between song and plumage elaboration. While avian vocal displays and the elaborate plumages of birds are two of the most celebrated examples of sexually selected traits, the relationship between these two prominent sexual signals remains contentious. Many prominent naturalists have alluded to a possible transference between sexual signals, referred to colloquially as the 'transfer hypothesis.' However, only one study currently exists that tests this prediction in songbirds, the lineage where these two sexual signals are most prominent. Furthermore, all assessments of the relationship between song and plumage elaboration have relied on human assessments of plumage complexity, which is known to be misleading because birds have superior color discrimination and the ability to see UV wavelengths. I found that song and plumage complexity are uncorrelated across all tanagers and also uncorrelated within eight of eleven subfamilies. Within the remaining three subfamilies, song and plumage elaboration are inversely correlated, providing the first documentation of transference within clades that have primarily melanin or structural based plumage. These findings suggest that instances of transference might be uncommon if not rare among lineages of songbirds.The second objective of this study is to test if differences in acoustic signaling environments impart selective pressures on the vocal displays of tanagers. All sounds experience the natural phenomena of degradation and attenuation as they travel through their signaling environment. However, higher frequency sounds experience greater levels of atmospheric attenuation and degradation from obstacles in the signaling path, such as vegetation. This body of theory together predicts that species in closed habitats, such as forests, should favor lower frequencies and have greater intervals between notes in comparisons to species of open habitats to minimize the effects of degradation and attenuation. These predictions are collectively coined the 'Acoustic Adaptation Hypothesis' and are tested here using a unique, model-fitting approach on ten temporal or frequency-related song measurements. I found that temporal and structural characters, such as frequency shift rates, average note bandwidths, and trill rates, demonstrate more evidence for the predictions of the Acoustic Adaptation Hypothesis than acoustic frequency characters, such as peak or minimum frequency. I also documented correlations between body mass and nine out of ten song measurements such that smaller birds sing songs with higher frequencies, faster paces, and more frequency shifts. Finally, I uncovered variation among eight subfamilies of tanagers with respect to the level of support for the directional predictions of the Acoustic Adaptation Hypothesis. This variation is correlated to song complexity such that those lineages with simple vocal displays are more likely to express acoustic adaptations in their songs than clades with complex songs.
