Intraspecific and interspecific vocal variation in three Neotropical cardinalids (Passeriformes : Fringillidae) and its relationship with body mass

Several studies, of a large number of bird species, have shown that song frequencies have a strong negative relationship with body size. However, mixed results were obtained at the intraspecific level or when comparing closely related species. Here, we compared the vocalisations of the monotypic Glaucous-blue Grosbeak (Cyanoloxia glaucocaerulea), three of five subspecies of the Ultramarine Grosbeak (Cyanocompsa brissonii) and all four subspecies of the Blueblack Grosbeak (Cyanocompsa cyanoides). These taxa of Neotropical cardinalids differ significantly in body size, so we compared them to examine the allometric relationship of body size with song frequencies, using body mass as a proxy of body size. The maximum frequency and the emphasised frequency (i.e. the frequency of the song of highest amplitude) showed the expected negative correlation with body mass, supporting the idea that the aforementioned relationship stands when comparing closely related lineages in Cyanoloxia and Cyanocompsa. We also found that the duration of notes, rate of production of notes and duration of inter-note interval correlated with body mass: heavier species had longer notes produced at a lower rate and separated by longer intervals. The correlation of temporal variables with body mass suggests that body size could also influence the temporal structure of song. Our findings highlight once again the importance of considering the role of avian morphology when studying the evolution of song, even if differences in frequency and temporal variables of songs are small.