Poultry Bones

Avian skeletons are unique and complex structures.

Poultry Bones

Reading Time: 5 minutes

Avian skeletons, including those found in songbirds, gallinaceous birds such as chickens and turkeys, and most waterfowl are unique and complex structures. Made up of several types of boney material, a bird’s skeleton serves many purposes. It provides a framework for the rest of the muscles and other tissue to anchor, allowing the bird to stand, walk, or fly. Specialized hollow bones, known as pneumatic bones, dramatically lessen the overall weight, making flight easier. In some flying birds, the hollow bones are also part of the respiratory system, increasing oxygen storage and processing capability for flight. Other specialized bones in female birds, known as medullary bones, act as calcium reservoirs and actually supply calcium for eggshell formation. The skeleton also acts as an integral part of the mineral and pH-balancing system (mineral homeostasis) for both birds and mammals. 

Types of Bone Structure Found in Poultry and Other Birds: 

Cortical Bone:  This is the hard, solid bone found on the exterior of the long, round bones, such as the femur and humerus (yes birds also have a “funny bone”!), as well as “flat bones” like the skull, hips, and pelvis, and outside of the sternum. 

Trabecular Bone: Sometimes known as “spongy bone,” this is the spongy-looking material inside of the bones. While it may look like a poorly designed honeycomb or air-filled tissue mass, it is actually a highly designed structure that is made of rods and plates called trabeculae. Trabecula (plural trabeculae) actually translates to “small beam” in Latin. Seen under magnification, they are actually arranged in an architectural fashion to give strength to the bone. They are positioned at the ends of the long bones, such as the femur in the leg, supplying structural strength. In birds with pneumatic bone structures, the individual trabeculae are spaced more widely apart, creating air pockets. Spaces between the individual trabecula contain bone marrow. This is where the red blood cells, as well as many of the white blood cells, are made. 

Pneumatic Bone: Unique to birds, pneumatic bones are hollow or semi-hollow air-filled structures that reduce weight for flight. They also act as an extended portion of the respiratory system in many avian species. While hollow, they are by no means weak. The solid portion of these bones is much denser than bones found in mammals. These bones include the ilium, or hip bones, the humerus in the wing, the keel or breast bone, the clavicle, some lumbar and thoracic vertebras, and the occipital bones of the skull. Interestingly enough, not all birds have pneumatic bones. Ostriches, which do not fly, have just a few. Penguins and puffins, both flightless, diving, ocean birds, have only solid bones. Loons, which look similar to ducks in many ways, are both migratory/flying as well as diving birds. Even though loons fly and migrate, their bones are entirely solid, which aids them in diving for food. Loons can dive to depths of 250 feet and stay underwater for as long as five minutes. 

Unique to birds, pneumatic bones are hollow or semi-hollow air-filled structures that reduce weight for flight. They also act as an extended portion of the respiratory system in many avian species.

Medullary Bone: This is a highly-specialized bone structure found in female birds as well as some female reptiles. In birds and reptiles that lay eggs with calcified, hard shells, medullary bones act as a calcium reservoir to supply the female with a ready source of calcium for the eggshells that she produces. Medullary bones in laying hens, female ducks and other poultry include the femurs and tibias in the legs, the ulnas in the wings, the scapulae or shoulder blades, the pubic bones (which sit at the back end of the ilium or hip bones), ten of the upper ribs and the phalanges or toe bones. 

Medullary bone structure is a woven network of calciferous tissue inside the hollow spaces of the long bones, as well as the other medullary bones mentioned above. The interior areas of these bones are highly vascularized, which aids in the rebuilding and resorption of calcium within the bone. While it resembles a woven network, it is not thought to be a strength-adding structure like a trabecular bone. The medullary bone structure in a female bird is built up, resorbed back into the body to aid in eggshell formation, rebuilt, and then resorbed again, time after time. This process can happen many times in the bird’s life. In a modern laying hen producing many eggs, it is easy to see how she can rapidly decrease bone calcium to dangerous levels. The structure and physiology of medullary bone have only received complex, scientific study over the last five decades or so. There is still much that we are learning.  

It was once thought that the medullary bone was only resorbed back into the female’s body when dietary calcium was in too short supply for eggshell production. Continuing research, however, found that this is not the case. According to information from the University of Kentucky’s Cooperative Extension Service, only 53% of eggshell calcium is supplied by a hen’s current dietary intake. Approximately 47% comes from the calcium buildup in the medullary bones. These figures are for hens receiving prescribed levels of dietary calcium. High-production hens, as well as egg-laying female ducks, can have serious skeletal depletions of calcium in very short order if adequate dietary calcium is not supplied. It was once postulated throughout the egg industry that 2.5% calcium was sufficient for most laying hens. Today, however, we realize that level is far too low. Current industry standards put necessary calcium levels in laying hen feed at a minimum of 3.5% to 4.8%. Many manufactured laying feeds now meet this level. One recent bulletin I read from one of the commercial laying-hen producers upped the maximum level to a full 5%. 

When medullary bone is resorbed into the body, the bone shape or exterior bone mass does not change. The medullary calcium structure is inside of the hollow sections of the bones, so externally the bones still look the same. In hens with serious calcium depletion, calcium may be pulled from the cortical or hard bone, leaving the hen vulnerable to broken bones and osteoporosis. Interestingly enough, some of this calcium that is pulled will go back into rebuilding medullary bone, in a feeble attempt by the hen’s body to bring itself back to proper levels. It is easy to see why adequate dietary calcium in laying hens and other egg-producing poultry is so important. 

How much calcium do growing pullets need? 

While egg-producing hens may need 3.5% or more dietary calcium, such levels can be injurious to growing pullets. Excess calcium can cause renal or kidney damage to young chickens and other poultry. Currently prescribed safe levels of dietary calcium for young, growing poultry (including future laying birds) is currently set at 1% to 1.4%. Growing birds also need higher protein levels than egg-producing hens (18% versus 16% average for laying feed). Consequently, for maximum growth and lifelong health of your birds, raising them on growth rations, when they are young, instead of laying mash is recommended. According to longtime research in this area, reduced kidney function due to excess calcium when young, even if the birds do not show any immediate signs of sickness, can lower life expectancy as well as reduce future egg-production ability.   

While egg-producing hens may need 3.5% or more dietary calcium, such levels can be injurious to growing pullets. Excess calcium can cause renal or kidney damage to young chickens and other poultry.

The growth of medullary bone in female birds is triggered by ovarian steroids and hormones, including both the female hormone estrogen and the male hormone androgen. Production of medullary tissue begins full-force about two weeks before a pullet lays her first egg. It is at this time that she should be switched to a laying feed with a higher calcium level. In commercial flocks, pullets are often switched to an intermediary feed, containing about 2.5% calcium for this time period. However, since this is rarely available or practical for home use, switching young pullets to layer rations about two weeks before the start of expected egg production is generally satisfactory. 

Bone and skeletal health of poultry are extremely important for overall growth rates when young, maximum egg production once the pullets reach sexual maturity and healthy, long life of each individual bird in the flock. Taking care of the birds’ bones will pay you dividends during the years that you have them.  

Originally published in the December 2019/January 2020 issue of Backyard Poultry and regularly vetted for accuracy.

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