Bronchial morphology shapes vocal frequency in suboscine birds

Abstract

The syrinx is the unique vocal organ found in birds, enabling them to produce diverse vocalizations, such as songs and calls. Among the Passeriformes (songbirds), the suborder Tyranni (suboscines) exhibits a less structurally complex syrinx compared to the suborder Passeri (oscines). Unlike oscines, suboscines typically have innate stereotyped songs that develop without requiring external acoustic stimuli, and they lack the syrinx’s muscular complexity and flexibility that oscines rely on for song learning. Several studies suggest that in suboscines—particularly tracheophones—sound production involves vibrations of the tympaniform membranes within the syrinx, whose interaction with airflow contributes to shaping the acoustic properties of vocalizations. Despite the well-documented vocal diversity of suboscines and the existing basic understanding of their syrinx anatomy, the specific ways in which syrinx morphology shapes acoustic traits remain poorly understood in a phylogenetic framework. We measured the syrinx morphology of 112 fluid-preserved specimens from 38 tracheophone species, alongside their songs. Specifically, we aimed to determine how syrinx, beak size, and body weight influence vocal frequency in this group. Notably, our findings reveal that narrower bronchial diameters, largely independent of body mass, are associated with a higher peak frequency (i.e. the frequency of maximum energy within a selected section) of the fundamental harmonic, a pattern that is consistent with how syringeal morphological constrictions may influence airflow dynamics during song production. This novel discovery may have significant implications for understanding how morphological traits shape the mechanisms that determine the spectro-temporal components of song in suboscines and, more broadly, across passerines.