Part Four: The Skeletal System
Biology of the Chicken, Part 4
Our bird models, Hank and Henrietta, reunite in this article on the skeletal system of the chicken. Both the male and female chickens are highly adapted for flight. This evolutionary development precipitates in some very unique features in their bones. In this article, we will discuss some special features of the avian skeleton, including how Hank and Henrietta differ slightly to accommodate egg production. I hope to give you a greater appreciation of the design and strength of the chicken skeleton that we too often consider only soup stock after the Sunday roast chicken dinner is over.
The skeletal system in all vertebrates, including us, provides support and protection for the rest of our systems, organs, and tissues. In general, the chicken skeleton is much like other mammals. The important differences come into play to accommodate a bird’s need to be light enough to fly. Even the chicken’s skeletal structure must be able to withstand the shock of taking off and landing.
Pneumatic bones, more commonly known to us as the skull, humerus (upper wing), clavicle (collar bone), keel (breast bone), pelvic girdle (hip bones), and lumbar/sacral vertebrae (backbones) are somewhat hollow. The prefix “pneu” implies to contain air as these bones aid in the high respiratory demand of a flying bird. A crisscrossing of struts or trusses makes for a strong but light framework on which to attach the muscles of flight. It is important to note that it is ill-advised to feed your dog chicken bones. The pneumatic bones tend to splinter when compressed by Rover’s strong jaws and can be damaging to his system.
Another flight adaptation inherent in the chicken skeleton is the fusion of some vertebral (backbone) sections. The tail is a good example of ossification or growing together of bone of the final caudal vertebrae. Called the pygostyle (pahy-guh-stahyl), this structure will be the site for the attachment of musculature for the tail feathers.
Strength is important here due to the stress of basic bird navigation, especially slowing down to land. My father would call this, “the part that went over the fence last,” and the muscle/meat of the tail was my mother’s favorite part of the chicken.
Compared to other animals the chicken has a significantly smaller skull. A large head would of course make flying very difficult. The neck that supports the skull is inherently longer compared to other animals. There are several advantages associated with this physiology. The long neck tends to act as a shock absorber to the stress of landing after flight. With a ridged body, which is necessary for flight, a longer neck affords flexibility in reaching food located on the ground. Have you ever watched a chicken walking with that head-bobbing strut? The longer neck also serves to adjust the center of gravity when changing from the horizontal position of flight to a more vertical position of walking or roosting.
I would say that the sternum (breastbone or keel) is by far the most noticeable feature of the avian or chicken skeleton. The large surface area of the sternum affords the attachment of the main flight muscles.
It can be helpful to know that the keel or posterior portion of the sternum is cartilaginous in young poultry and ossifies to harder bone in older birds. I will cover the chicken’s muscular system in the next part of “The Biology of the Chicken” series.
There are seven pairs of ribs that originated from the thoracic vertebrae forming the rib cage. All but the first and second pairs attach to the sternum in a unique fashion. Again adaptation for flight mandates a strong rib cage called the uncinate process. This process involves hooked flaps that overlay and connect adjacent ribs by a ligament to avoid collapse of the thoracic cavity (ribcage) during flight.
No discussion of the bones of the chicken would be complete without including the sought-after wishbone. Many a furcula (wishbone) I would anxiously hunt for in the carcass of the Sunday roast chicken. I would impatiently wait for it to dry on the window ledge anticipating my wish coming true should I get the bigger half of the break. Structurally, the furcula (wishbone) is the part of the chest forming the pectoral girdle with the coracoid (collarbone) and the forementioned sternum.
Chicken bones do have some similar names and like functions to our human bones. Both the human leg and the chicken leg have a femur (thigh bone), and a fibula and tibia (lower leg bones). The bones of the chickens wing and the human arm are very similar in structure but are very different in function. Both humans and chickens have a humerus (“funny bone”/upper arm/wing), radius, and ulna (lower arm/wing). The shoulder for both is the joint between the scapula and humerus. The elbow is the joint between the humerus and the radius/ulna. Correspondingly, the wrist for humans and chickens is the joint between the radius/ulna and the metacarpus, what we call our hand.
One last unique feature of Henrietta’s skeletal system are her medullary bones. We have mentioned these individual bones previously as universal to both sexes. However, in the female of the chicken, this group of bones serves for more than just support. Medullary bones include the tibia, femur, pubic bone, ribs, ulna, toe bones, and the scapula. The eggshell is primarily calcium and hens take almost half of that demand from their own structure. Medullary bones provide a source of calcium to assist in a strong eggshell. Hens in high production cannot get enough dietary calcium and are easily depleted of this critical element causing weakness in bone structure. As mentioned in my reproduction article, calcium in a well-prepared feed and free choice oyster shell is very important to your layers.
In this article, we discussed how the chicken skeletal system is different from mammals with its adaptation for flight. I mentioned that though designed for flight, the chicken skeleton has similarities to our own human skeleton. A special group of bones was identified as important to our friend Henrietta’s ability to produce many eggs. I hope you can now look at that holiday turkey or Sunday roast chicken with a better awareness of what holds it together and why. And remember, when it comes to breaking the furcula (wishbone), don’t say that wish out loud or it won’t come true.