Syrinx anatomy and vocalizations in New World vultures (Aves: Cathartidae): Implications for avian vocal evolution

New World vultures (Cathartidae) are the only birds proposed to lack a syrinx, the avian vocal organ. The prior literature is inconsistent regarding the presence or lack of key syringeal elements in Cathartidae, such as syringeal muscles, membranes, and/or modified tracheal or bronchial rings. Additionally, New World vultures have largely been described as incapable of producing vocalizations, perhaps due to their described lack of a functional syrinx; despite this, there is a diverse repertoire of sounds made by species within Cathartidae available in online sound libraries. Here, we investigate the cathartid syrinx, airway, and larynx through dissections, diffusible iodine-based contrast enhanced computed tomography (diceCT), and histology. We further assess available recordings and explore the potential identity of these as larynx- or syrinx-based vocalizations. New World vultures indeed lack labia, bronchial half-rings, and marked differentiation of the tracheobronchial juncture, challenging simple definitions of a syrinx. However, they retain extrinsic syringeal muscles, have modified terminal bronchial rings, extended bronchial membranes, and muscles from the thoracic wall extending to these bronchial membranes unknown in any other birds, all of which may be involved in sound production. This evidence, in conjunction with the variety of vocalization types produced, indicates that cathartid vultures may in fact have a novel form of the syrinx. These findings have implications for broadening our understanding of the necessary components of a syrinx and provide insights into the loss and addition of syringeal elements in Aves. Assessing syrinx diversity is essential to understanding form-function relationships in Aves and the evolution of vocal behavior.