The evolution of the eufalconimorph syrinx informed by phylogenetic comparative methods and diceCT imaging of...

In birds, the ability to produce vocalizations learned from conspecifics (vocal production learning, VPL) is well-characterized from an acoustic and neurological perspective, but the evolution of sound source morphology in clades with vocal learners remains poorly known. The syrinx, the vocal organ of birds, shows increased complexity in the three known avian clades with this trait-passerines, parrots, and hummingbirds-but evolutionary patterns preceding, and associated with, inferred acquisitions of VPL have never been studied in a phylogenetic context. Here we redescribe the syrinx of eufalconimorphs-passerines, parrots, and the non-learning falconiforms-and investigate evolutionary shifts in its morphology using phylogenetic comparative methods. Syringeal muscle insertions on, or closely adjacent to, sound sources are estimated to be ancestral to eufalconimorphs, while complex intrinsic syringeal muscles in passerines and parrots are recovered as two independent acquisitions. Passerines have several unique syringeal traits, including enlarged labia, optimized to occur prior to oscine VPL acquisition. We hypothesize that these traits may have facilitated later independent acquisitions of precise motor control of sound sources associated with complex vocalizations. We discuss other traits that may be associated with vocal production differences among clades, including variation in upper vocal tract anatomy and life-history characters.