The bird sternum, located in the center of a bird’s chest, is a central component enabling avian flight. Its unique structure and placement are directly linked to the mechanics of powered flight. Understanding the sternum provides insight into how birds generate the necessary force and control to navigate the skies.
Anatomy and Primary Function
The bird sternum is a large, flattened bone that forms the floor of the bird’s chest cavity. Its design provides a substantial surface area for the attachment of the powerful muscles responsible for wing movement.
The Pectoralis muscle is the primary muscle for the downstroke of the wing. This muscle originates from the sternum and inserts onto the humerus, the upper arm bone of the wing.
Beneath the Pectoralis lies the Supracoracoideus muscle, which is responsible for the upstroke of the wing. This smaller muscle, about one-fifth the size of the Pectoralis, also originates from the sternum. Its tendon loops over the shoulder joint, acting like a pulley to elevate the wing. The sternum’s robust structure and broad surface allow these muscles to generate the leverage and power required for sustained flight.
The Significance of the Keel
A prominent feature of the sternum in most flying birds is the keel, also known as the carina. This blade-shaped bony extension projects perpendicularly from the midline of the sternum. The keel’s primary purpose is to dramatically increase the surface area for the attachment of large and powerful flight muscles.
Without a keel, the sternum would be a flat bone, lacking sufficient surface area to anchor the muscle mass needed for powered flight. The larger surface area provided by the keel allows for the necessary muscle volume and efficient transfer of force for both the downstroke and upstroke. This anatomical adaptation directly correlates with a bird’s ability to achieve and maintain flight.
Sternum in Flightless Birds
The sternum of flightless birds differs significantly from flying birds, reflecting their adaptation to terrestrial locomotion. Many flightless birds, such as ostriches, emus, and cassowaries, belong to a group called ratites, characterized by a sternum that either completely lacks a keel or has a greatly reduced one. This absence or reduction of the keel means there is no large anchoring point for the powerful flight muscles necessary for aerial movement.
For example, the ostrich possesses a flat breastbone, consistent with its inability to fly and its reliance on powerful legs for running. Similarly, emus and cassowaries have reduced keels. This adaptation allows them to develop strong leg muscles for ground-based movement instead of flight.
Variations and Other Roles
Beyond the presence or absence of a keel, the sternum exhibits variations in shape and size across different bird species, influenced by their modes of locomotion. Birds that engage in rapid, sustained flapping, like hummingbirds, have a robust caudal border, supporting fast wing movements. Conversely, birds that employ slower, more powerful flaps, such as albatrosses, possess a prominent sternum structure. Narrower sterna, for instance, are associated with running abilities in some species.
The sternum also serves additional functions beyond muscle attachment for flight. It forms a ventral enclosure, providing protection for vital internal organs like the heart and lungs within the thoracic cavity. The sternum also plays a role in avian respiration. During inhalation, the sternum swings forward and downward, expanding the chest and abdominal cavity, which helps draw air into the bird’s extensive air sac system. This movement contributes to the unique unidirectional airflow through a bird’s respiratory system, which is highly efficient for gas exchange.