Certain vocal signals can be difficult or challenging to produce due to constraints on vocal mechanics. In the case of birdsong, both body size and beak morphology have been shown to constrain spectral and temporal song parameters. Therefore, variation within a clade in both these morphological traits could exert an important effect in vocal evolution and diversity. However, given the allometric relationship between body size and beak size, it is not always easy to separate the effects they may have. Additionally, different habitats impose different sources of sound attenuation and degradation, and birdsong seems to be adapted to minimize habitat-specific sound degradation. Here, we analyze vocal variation in relation to both body size and beak morphology in 13 of 14 species in the “blue” clade of the Cardinalidae, controlling for habitat characteristics. On one hand, we found the predicted negative correlation of body size with song frequencies, but no effect of body size on temporal variables. On the other hand, we found no relationship between different beak dimensions (corrected by body size) and song frequencies, but we found that beak length was correlated negatively with note rate and positively with the duration of the interval between notes. Similar results have been found previously, but mostly in species with trilled songs, in which notes are equal and produced at a constant rate. Our results show that different morphological variables, even if strongly correlated, can exert differential constraints in a complex behavioral trait such as song. A better understanding of the relevance of these proximate factors in shaping vocal evolution is fundamental to understanding the causes of avian vocal diversity.
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