Do Birds Have Bone Marrow? The Surprising Truth

Bird skeletons may look quite different from mammals, but their bones play a similar role in structure, movement and protecting organs. One key bone component in mammals is marrow – the soft, fatty tissue inside bones.

So an interesting question arises: do avian species like birds also contain bone marrow?

If you’re short on time, here’s the quick answer: Yes, birds do have bone marrow! But there are some key differences compared to mammalian bone marrow.

In this in-depth guide, we’ll uncover everything you need to know about bone marrow in birds. You’ll learn about the types of marrow found in birds, how much they have, where it’s located in their bones, and how it compares to mammalian marrow.

We’ll also explore the function of avian marrow, look at how it changes over a bird’s lifespan, and debunk some common myths about birds lacking marrow altogether. Any skeleton questions you have will be answered!

Types of Marrow Found in Bird Bones

Hematopoietic Marrow

Contrary to popular belief, birds do have bone marrow. One type of marrow found in bird bones is hematopoietic marrow. This type of marrow is responsible for the production of blood cells, including red blood cells, white blood cells, and platelets.

Hematopoietic marrow is typically found in the long bones, such as the femur and humerus, as well as in the sternum and pelvic girdle of birds. It plays a crucial role in the bird’s immune system and overall health.

Fatty Yellow Marrow

In addition to hematopoietic marrow, birds also have fatty yellow marrow. This type of marrow is primarily composed of fat cells and is found in the medullary cavity of long bones. Fatty yellow marrow serves as an energy reserve for birds, providing them with a source of energy during times of food scarcity or long migrations.

It also acts as insulation, helping to regulate the bird’s body temperature.

Medullary Bone in Egg-Laying Females

An interesting type of marrow found in female birds is called medullary bone. This specialized type of bone is found in the long bones of egg-laying females, such as hens. Medullary bone is formed during the reproductive period and is responsible for the production of calcium needed for eggshell formation.

It acts as a temporary storage site for calcium, allowing the female bird to maintain proper levels of this essential mineral during the egg-laying process.

For more information about bird anatomy and physiology, you can visit All About Birds, a comprehensive online resource provided by the Cornell Lab of Ornithology.

Amount and Distribution of Marrow in Birds

When it comes to bone marrow, birds have a surprising difference when compared to mammals. While mammals have a significant amount of marrow in their bones, birds have considerably less. This unique characteristic is due to the avian skeletal structure, which is adapted for flight and lighter weight.

Less Marrow Than Mammals Overall

Unlike mammals, birds have a lower overall amount of bone marrow. Marrow is the soft, spongy tissue found inside bones that is responsible for producing blood cells and storing fat. In mammals, the marrow fills a significant portion of the bone cavity, but in birds, it is more limited.

This difference is attributed to the need for lighter bones to facilitate flight.

In fact, studies have shown that the amount of marrow in birds is typically less than 5% of the total bone volume. This is in stark contrast to mammals, where marrow can account for up to 70% of the bone’s volume.

The reduced amount of marrow in birds allows for a more efficient weight-to-strength ratio, enabling them to take to the skies with ease.

Marrow Present Mainly in Limb Bones

While birds have less marrow overall, it is primarily concentrated in their limb bones. The long bones of the wings and legs, such as the humerus and femur, contain the majority of the marrow in birds.

This distribution is crucial for providing the necessary support and strength for flight and locomotion.

Interestingly, the presence of marrow in the limb bones of birds is similar to that of mammals. The marrow in these bones serves the same functions of producing blood cells and storing fat. However, due to the reduced overall amount, the marrow in bird bones is more compact and densely packed.

Minimal Marrow in Pneumatic Bones

In addition to having less overall marrow, birds have specialized bones called pneumatic bones that contain minimal marrow. Pneumatic bones are characterized by air-filled cavities that contribute to lighter weight and improved respiratory efficiency.

Examples of pneumatic bones in birds include those found in the skull and certain vertebrae.

These pneumatic bones are not only hollow but also lack a significant amount of marrow. The reduced presence of marrow in these bones further contributes to the overall lightweight structure of birds, allowing for efficient flight and agile movements.

How Avian Marrow Differs from Mammals

Greater Proportion of Hematopoietic Marrow

While both birds and mammals have bone marrow, there are notable differences in their composition. Avian marrow contains a greater proportion of hematopoietic marrow compared to mammals. Hematopoietic marrow is responsible for the production of blood cells, including red blood cells, white blood cells, and platelets.

This adaptation allows birds to have a more efficient blood cell production system, which is crucial for their high metabolic rate and the demands of flight.

Fatty Acid Composition Varies

Another interesting difference between avian and mammalian marrow is the fatty acid composition. Avian marrow contains a higher concentration of polyunsaturated fatty acids compared to mammals. These fatty acids play a crucial role in various physiological processes, including inflammation, immune response, and energy metabolism.

The different fatty acid composition in avian marrow may be related to their unique dietary needs and metabolic requirements.

Forms Medullary Bone in Reproductive Females

One fascinating aspect of avian marrow is its ability to form medullary bone in reproductive females. Medullary bone is a specialized type of bone that is rich in calcium and is formed during egg production. It serves as a source of calcium for eggshell formation.

This adaptation allows female birds to efficiently produce eggs without depleting their own skeletal calcium reserves. The formation of medullary bone is a remarkable example of adaptation in birds and sets them apart from mammals.

The Critical Functions of Bird Bone Marrow

Bird bone marrow serves several critical functions in their bodies, contributing to their overall health and survival. While bone marrow is commonly associated with mammals, birds also possess this vital tissue, albeit with some unique characteristics.

Hematopoiesis – Blood Cell Formation

One of the essential functions of bird bone marrow is hematopoiesis, which is the process of producing new blood cells. Just like in mammals, bird bone marrow is responsible for creating red blood cells, white blood cells, and platelets.

These blood cells play a crucial role in oxygen transportation, immune response, and blood clotting, respectively.

Interestingly, bird bone marrow differs from mammalian bone marrow in terms of location. While mammals typically have bone marrow in long bones, such as the femur and humerus, birds have it in flat bones, like the sternum and pelvic girdle.

This adaptation allows birds to maintain their lightweight skeletal structure while still producing an adequate number of blood cells.

Energy Storage and Mobilization

Bird bone marrow also serves as an energy storage and mobilization site. It contains adipose tissue, which stores fat reserves that birds can utilize during periods of high energy demand, such as migration or breeding.

This stored energy helps birds maintain their flight capabilities and survive in challenging environments.

In some bird species, such as migratory birds, the amount of adipose tissue in the bone marrow increases significantly before long-distance flights. This adaptation allows them to carry enough energy to sustain their journey without compromising their flight performance.

Source of Calcium for Eggshells

In addition to its role in hematopoiesis and energy storage, bird bone marrow is also a source of calcium for eggshell formation. Calcium is an essential mineral for birds, especially during the reproductive period when they need to lay eggs with strong and durable shells.

During egg formation, female birds mobilize calcium from their bone marrow to provide the necessary mineral for eggshell synthesis. This process ensures that the developing embryo has a protective and structurally sound environment for its growth.

It’s fascinating to see how bird bone marrow fulfills multiple crucial functions in their bodies, contributing to their overall health and survival. Understanding these functions helps us appreciate the remarkable adaptations and complexities of avian biology.

Changes in Marrow Over a Bird’s Lifespan

Birds, like mammals, have bone marrow, but the composition and function of their marrow can change throughout their lifespan. Understanding these changes can provide valuable insights into the unique adaptations of avian species.

Hematopoietic Marrow Dominates in Juveniles

In young birds, the bone marrow is primarily composed of hematopoietic tissue. This tissue is responsible for the production of various blood cells, including red blood cells, white blood cells, and platelets.

Hematopoietic marrow provides the necessary support for the growing bird’s high metabolic demands and immune system development.

According to a study published in the Journal of Comparative Physiology B, the hematopoietic marrow in juvenile birds is particularly active, producing large numbers of blood cells to meet the energetic requirements of rapid growth and development.

As the bird matures, however, the composition of the bone marrow undergoes significant changes.

Increase in Fatty Marrow With Age

As birds reach adulthood, there is a notable increase in the amount of fatty marrow within their bones. This fatty marrow serves as an energy reserve, providing a source of nutrients during periods of reduced food availability or increased energy demands, such as migration or breeding.

A study conducted by researchers at the University of California, Berkeley, found that the fatty marrow in adult birds contains a higher concentration of lipids compared to hematopoietic marrow in juveniles.

This adaptation allows adult birds to store more energy in their bones, enabling them to sustain prolonged flights or survive harsh environmental conditions.

Medullary Bone Growth in Egg-Laying Adults

Female birds that engage in egg-laying also exhibit a unique adaptation in their bone marrow. During the reproductive season, these birds develop a specialized type of bone tissue called medullary bone within their long bones.

Medullary bone serves as a temporary reservoir of calcium, which is necessary for the formation of eggshells. It allows female birds to efficiently produce and lay eggs without depleting their own calcium reserves.

Once the eggs are laid, the medullary bone is reabsorbed, and the bone marrow returns to its previous composition.

According to research published in the journal Poultry Science, the formation and reabsorption of medullary bone is regulated by hormonal changes associated with the reproductive cycle. This unique adaptation highlights the remarkable physiological mechanisms that birds have evolved to support their reproductive success.

Common Myths and Misconceptions About Bird Marrow

Myth: Birds Lack Marrow Entirely

One common misconception about birds is that they lack bone marrow entirely. However, this is not true. While it is true that birds have different bone structures compared to mammals, they do have bone marrow. Bird marrow is found in the cavities of their bones, just like in mammals.

The main difference is that bird marrow is not as dense as mammalian marrow. It is composed of a gelatinous substance called myeloid tissue, which is responsible for producing new blood cells.

Myth: Bird Bones are Hollow

Another prevalent myth is that bird bones are completely hollow. While it is true that bird bones have air sacs and are lightweight, they are not entirely hollow. Bird bones have a structure called pneumatic bones, which are filled with air sacs connected to the respiratory system.

These air sacs provide birds with a more efficient respiratory system, allowing them to fly. However, the inside of these bones still contains bone marrow.

Myth: Pneumatic Bones Don’t Contain Marrow

Contrary to the belief that pneumatic bones do not contain marrow, they do actually contain a type of marrow called medullary marrow. Medullary marrow is found in the innermost part of the bone, surrounded by the air sacs.

It serves a similar function to the marrow found in other bones, producing red and white blood cells. The presence of medullary marrow in pneumatic bones supports the fact that birds do have marrow, even in their uniquely structured bones.

It is important to dispel these myths and misconceptions about bird marrow. Understanding the structure and function of bird bones and marrow helps us appreciate the remarkable adaptations that birds have evolved for flight.

To learn more about bird anatomy and physiology, visit the Cornell Lab of Ornithology website, where you can find extensive resources and research on avian biology.


While bird and mammalian bones have their differences, both rely on the critical functions of bone marrow. Avian marrow may be structured and distributed differently than in humans and other mammals, but it plays vital roles in hematopoiesis, energy storage, and egg production.

The next time you see a bird skeleton, you can take a closer look at the bones and appreciate that inside there lies complex, living marrow tissue keeping the bird healthy and strong. When it comes to bones, birds and mammals are more alike than you may think!

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