Chickens Have Two Kinds of Bone

Chickens, like many other birds, have two kinds of bone. One contains pockets to enhance air circulation. The other stores calcium for egg production. Here’s a closer look at the two kinds of bone making up a chicken’s skeletal system and why this information is important for chicken keepers to know.

Pneumatic Bone

One type of bone in a chicken’s skeleton is pneumatic bone. The word pneumatic derives from the Latin word pneumaticus, meaning “belonging to the air.”

Pneumatic bones contain blind tubes, or diverticula, filled with circulating air. These partially hollow bones increase the flow of oxygen needed for activities involving exertion, such as flight. In general, bones used for wing flapping are pneumatized, while bones that are weight-bearing are less so or not at all.

Birds that fly long distances have more pneumatized bone, while ground-dwelling birds have less. Gallinaceous birds— like chickens, guineas, and turkeys — have a moderate amount.

The pneumatic bones directly connect a chicken’s skeletal system to its respiratory system via air sacs. These thin-wall bubble-like pouches circulate fresh air throughout the chicken’s body.

The extensive system of air sacs wraps around the internal organs and fills nearly all of the body cavity that’s not occupied by other organs. Air sacs also extend into the pneumatic bones.

When a chicken needs extra oxygen, it flaps its wings to improve airflow. Wing flapping expands the air sacs, drawing more air into the pneumatic bones. A rooster, for instance, flaps its wings prior to crowing so it can belt out a mighty “Cock-a-doodle-do!”

To compensate for having less mass, a bird’s pneumatic bones are denser than mammal bones of similar size. They are therefore stronger and break less easily, but when they do break they shatter. A chicken with a fractured pneumatic bone may have trouble breathing and is susceptible to air sac infection.

Medullary Bone

The other type of bone in a chicken’s skeleton contains marrow, similar to marrow in mammal bones, needed for the production of blood cells. But in birds these bones also contain a fluctuating, porous substance called medullary bone. The word medullary derives from the Latin word medulla (from medius, meaning “middle” or “central”).

Medullary bone lies inside the structural bone. It both lines the structural bone and extends as numerous tiny needle-like projections into the marrow. This feature is unique to female birds. The purpose of medullary bone is to store readily accessible calcium.

A pullet approaches sexual maturity, and estrogen kicks in, about 2 weeks before she lays her first egg. At that time her system switches from developing structural bone to packing some of the structural bones with medullary bone.

From that point on, as long as the hen continues laying, specialized cells within her bones resorb medullary bone to provide calcium for developing eggshells. To some extent they also resorb structural bone.

Bone Loss

A good laying hen that produces an egg nearly every day can’t absorb calcium from her diet fast enough. To put a sturdy shell around each egg, she uses up a considerable amount of skeletal calcium.

Therefore, throughout a hen’s productive life, she gradually loses structural bone. This condition is characteristic of osteoporosis, in which the bones become brittle and fragile and fracture more easily.

Along with osteoporosis, highly productive industrial hens housed in cages can develop cage fatigue. This form of paralysis leads to the hen’s inability to reach food and water. Cage fatigue can therefore end in death. Interestingly, if the cage is fitted with a solid bottom, or the hen is removed from the cage, she may recover in a matter of days.

In addition to genetics and environment, a hen’s bone loss is influenced by nutrition. If a hen’s diet is deficient in calcium, she will more rapidly deplete her body of calcium. As a result, her eggs will have thinner shells and she will gradually lay fewer eggs until she stops laying altogether.

When an otherwise healthy hen stops laying, her body reduces the production of estrogen. She then resumes the formation of structural bone, restoring the strength of her skeletal structure.

Understanding the two kinds of bone in chickens explains a few things about why they behave the way they do. And that can help you become better a steward of your backyard flock.