Birds do possess bone marrow, though its nature and distribution are highly distinct from what is found in mammals. The avian skeletal system balances the conflicting needs for strength, lightness for flight, and the production of blood cells. This specialization means that bone marrow is not universally present throughout every bone, but is concentrated in specific areas. The structural and functional differences in avian bone marrow are directly tied to the physiological demands of powered flight and the unique reproductive requirements of female birds.
Location and Primary Function of Avian Bone Marrow
The fundamental function of avian bone marrow mirrors that of mammals: it is the primary site for hematopoiesis, the process of creating new red blood cells, white blood cells, and platelets. In a newly hatched bird, active bone marrow is widely distributed throughout the skeleton, including the long bones of the wings and legs. As the bird matures, the distribution of this active marrow begins to change significantly.
In adult birds, the hemopoietic marrow often becomes restricted to specific bones, primarily the femur and tibiotarsus of the leg, along with the sternum and ribs in some species. The concentration of red marrow in these select bones ensures a continuous supply of blood cells. The marrow cavity contains a mixture of hematopoietic tissue and adipose tissue, but the ratio and location of active marrow are highly variable depending on the bird’s age and species.
Skeletal Adaptations for Flight: Pneumatic Bones
Many bones in a bird’s skeleton are partially or entirely hollow, an adaptation that significantly reduces body weight to facilitate flight. These structures are known as pneumatic bones because they are connected to the bird’s respiratory system via air sacs. The air sacs, which extend from the lungs, grow into and fill the marrow cavities of bones like the humerus and the femur as the bird develops.
This process of pneumatization is the main reason why many of the long bones, particularly in adult birds, contain little to no active bone marrow. The bones that are most extensively pneumatized, such as those in the wings and parts of the vertebral column, must be the lightest for efficient flight. The air-filled spaces within these bones also help to create a highly efficient, one-way airflow system for respiration, providing the high oxygen levels necessary for the energy demands of flight. This structural modification forces the hematopoietic tissue to be concentrated in the bones that remain non-pneumatized.
Medullary Bone: The Reproductive Calcium Reservoir
Female birds possess a unique type of bone tissue known as medullary bone, which is distinct from both the structural cortical bone and the blood-producing marrow. This specialized, temporary tissue forms inside the marrow cavities of certain long bones, such as the femur and tibiotarsus, just before and during the egg-laying cycle. Its purpose is to act as a rapidly mobilized calcium reserve for the formation of the eggshell.
The formation of medullary bone is tightly regulated by hormones, particularly estrogen, which surges as the ovarian follicles mature. This estrogen surge stimulates specialized bone cells to deposit a network of porous, interconnected bone spicules within the marrow cavity. The mineral content of this temporary bone tissue can be mobilized up to 15 times faster than the structural cortical bone. This allows the bird to meet the enormous daily calcium demand for eggshell production, which is often greater than the amount of calcium that can be absorbed directly from the diet.