Uncinate processes in birds: Morphology, physiology and function

The avian respiratory system is remarkable in terms of its complexity and efficiency. The evolution of this system with its unique lung morphology and physiology has contributed to birds being one of the most successful vertebrate lineages. Despite holding the attention of the scientific community for a long time, much remains to be discovered about the complexities of this system. Recent advances have highlighted the important role that accessory breathing structures, the uncinate processes, play in understanding not only how this system functions but how it evolved. Almost all species of extant bird have uncinate processes extending from the midpoint of the vertebral ribs. These processes are integral to the mechanics of ventilation in birds, being active in both inspiration and expiration but also playing some role during locomotion. The morphological variation in the uncinate processes suggests that the constraints placed on the body by adaptations to different forms of locomotion are key to understanding differences in how birds breathe. These processes also occur in the theropod dinosaurs, providing further evidence that they are the ancestors of modern birds but also highlighting the intrinsic flexibility in the ventilatory systems of these animals.