The Paired Cervico‐Cephalic Air Sacs and their Role in the Vocalizations of Songbirds

The avian neck contains a coelomic cavity within which the esophagus and trachea are anchored by mesenteries (Cozic et al. 2014). In order to evaluate the role of the neck in the resonance (filtering) of sounds generated by the syrinx, we created a 3D animated model of the shoulder, neck, and head of a Northern Cardinal ( Cardinalis cardinalis ) based on x‐ray fluoroscopy and x‐ray CT data and supplemented with data from micro‐dissections. Our model revealed a space in the upper neck between the trachea and vertebral column, which increases in volume when the tongue is protracted during low‐frequency sounds and decreases in volume when the tongue is retracted during high‐frequency sounds. Our micro‐dissections revealed paired cervico‐cephalic air sacs within the upper cervical coelomic cavity of a Cardinal and a House Sparrow ( Passer domesticus ) where the fluctuating space is shown in the 3D model. These air sacs, which are not connected to the pulmonary system, were already described by Fanny Bignon (1889) in a variety of birds, but their functioning and role have been poorly understood. We infer that in songbirds these air sacs are passively inflated when the tongue and trachea are protracted and, thereby, fill the increasing space between the trachea and vertebral column, and are compressed and deflated when the tongue and trachea are retracted. Supported by NIH grant NINDS R01 NS029467 and LSU Foundation account “Functional Morphology of Birds”