Evolution of the syrinx of Apodiformes, including the vocal-learning Trochilidae (Aves: Strisores)

The vocal organ of birds, the syrinx, represents a key innovation in the evolutionary history of vertebrate communication. Three major avian clades: passerines, parrots, and hummingbirds, independently acquired both specialized syringeal structures and vocal-production learning, between which a functional relationship has been proposed but remains poorly understood. In hummingbirds, the syrinx has never been studied comparatively alongside non-learning relatives in the parent clade Strisores. Here we describe the anatomy of the syrinx in three swift species using enhanced-contrast computed tomography, which reveals structures previously unreported in the clade. We also tested for correlations between syringeal and acoustic traits in a sample of hummingbirds and swifts using phylogenetically informed regressions. The swift syrinx presents lateral labia located on the first pair of bronchial half-rings, which are present in hummingbirds and may be ancestral to Strisores. The further enlarged lateral labia of hummingbirds are found to be significantly correlated to the reduction in length of their trachea and m. tracheolateralis. Acquisition of intrinsic muscles and loss of the sternotrachealis muscle co-occur with these shifts. We recover a significant negative correlation between tracheal elongation and maximum vocalization frequency, suggesting that tracheal shortening in hummingbirds facilitated the acquisition of high-frequency vocalizing.