Birds possess one of the most efficient respiratory systems in the animal kingdom, but deep-diving species face unique mechanical challenges when descending into deep water. To prevent collapse under severe hydrostatic pressure while sustaining high metabolic oxygen demands, aquatic birds rely on uncinate processes—small, hook-like bony projections that extend posteriorly from the vertical rib segments to overlap adjacent ribs. Inherited from non-avian theropod ancestors like Maniraptora and early archosaurs, these uncinate processes act as mechanical levers that radically transform avian ventilation.
In diving birds such as the tufted duck and stiff-tailed duck, uncinate processes are significantly longer than those found in soaring or terrestrial birds. When these birds inhale, specialized muscles attach to the uncinate processes, using them as high-torque levers to push the sternum forward and expand the thoracic cavity with minimal energy expenditure. During deep dives, when water pressure compresses the chest wall, these elongated levers dramatically increase the mechanical advantage of respiratory muscles, allowing deep-diving ducks to pull air into their air sacs rapidly between dives.
This anatomical innovation facilitates hyper-ventilation at the surface and precise buoyant control underwater, allowing them to forage efficiently in deep environments where other air-breathing vertebrates would struggle to expand their lungs.