Many traits influence birdsong diversity. Patterns observed in the acoustic parameters can be a result of morphological constraint and can also be explained by phylogenetic relationships. Understanding morphologic mechanisms that can act on song structure might account how they can catalyze speciation and how they evolve in lineages sort. We analyzed the evolution of beak volume and song constraints in “finch-like” species of Neotropical seedeaters. We tested if beak volume limits the song structure of territorial songs, based on differences in the beaks of 19 species from the genus Sporophila (Thraupidae, tanagers). We also tested (1) if body size constrained song structure, and (2) if beak volume and body size were related to each other. The relationship of song parameters (e.g. maximum and minimum frequencies, frequency bandwidth and note rate) to these two morphological variables was evaluated through an analysis which phylogenetic relations were controlled (PGLS), testing a null and Brownian model. To perform a faithful analysis between morphologic and acoustic parameters, our data was based on measurements of the beak and territorial song for each individual that we analyzed. None of the analyzed parameters was related to beak volume or body mass, and beak volume was not associated with body mass. Beak volume, note rate, and minimum frequency showed a phylogenetic signal. These results do not support the theoretically motivated prediction that beak size acts as a limit on song structure in oscine birds. The shape and variations of song in Sporophila tanagers (Seedeaters) may be a consequence of the species’ phylogenetic history, since the seedeaters showed wide plasticity in many acoustic parameters, unrelated to their beak volume and body mass. Song structure was better explained by the evolutionary relationships among the species than by morphological constraints.