Skeletal Adaptations of Birds for Flight

By Renn Tumlison, tumlison@hsu.edu

 In order to attain the lift necessary for flight, birds have evolved a number of modifications to their skeletal system, including pneumatic, or hollow bones, and reduction of the number of bones by loss or fusion. Hollow, air-filled bones lighten the weight of the skeleton. Skeletal adaptations lend strength to the skeleton so that the thrust (forward force) generated by the wings can lead to maximal lift, and the bird can be propelled through the air with minimal compression to the body cavity.

Hollow Bone

The hollow inside of a bird's ulna - a bone from the arm - is depicted in the photograph at left. In some species of birds, the air-filled skeleton is so lightweight that the bird's feathers weigh more than its entire skeleton! 

 Side view of a bird skeleton showing various bones that aid flight.

Chicken Front
Chicken Back

 Back view of the same bird skeleton. The labeled structures are discussed in detail below.

The thoracic vertebrae of birds (right image), and vertebrae other than those located in the neck region, are fused to help keep the trunk of the body stiff during flight. Because the bill is the only structure used to manipulate objects, such as food items, the vertebrae of the neck must be highly flexible. 

Thoracics
Synsacrum

 The bones of a bird's pelvic girdle and the lumbar, sacral, and a few caudal vertebrae are fused into a single, solid structure called the synsacrum. Additionally, through the fusion of these bones, shock incurred from the force of the beating wings is transferred to the air. This photo was taken from above of the back of a bird. In the photo, the two "scooped" areas near the top are the crests of the ilium, the line down the middle is the fused vertebrae (synsacrum), and the center of the bottom is the start of the tail .

 The pygostyle, or tailbone, supports the bird's tail feathers and is comprised of fused caudal vertebrae. Birds can maneuver a fan-shaped tail in a rudder-like fashion to slow or change direction during flight. Notice that a bird's tailbone (shown at right from the side) is much shorter than is seen in many other vertebrates, such as lizards.

Pygostyle
Furculum

The collarbone of birds is fused for stability to form the furculum, or "wishbone," which acts as a strut to brace the wings apart. The furculum in the photograph has been broken and has re-healed, which shows the strain to which the bone may be subjected.  

Avian ribs are broad, flat, and lie close enough together to prevent much compression. In addition, each rib overlaps the next due to an uncinate process (right photograph pointing toward the top right of photo). The uncinate processes provide additional strength to the rib cage encasing the bird's vital organs and further restrict movement between vertebrae. 

Uncinate
Sternum

 The strong, keeled sternum of a bird provides a large amount of surface area for attachment of the flight muscles (seen here from a side view). When people eat chicken breast and pull the meat from this bone, they usually find the back half to be flexible (because it is cartilage).

Notice the bumps on the ulna from a bird in the photograph on the top. These bumps help support attachment of the secondary flight feathers, which provide lift (seen in photo on the bottom). The photo at top creates an optical illusion. To make the bumps stand out in a photograph, the bone was held nearly parallel to the sun’s rays, so sunlight would cast a shadow. That added depth to the flat image. The light was coming from the right side, so the bright area is the bump and the dark area to the left is the shadow. Without knowing this, and presuming the light is coming from the left in the photo, it looks like a hole with the right border of the pit reflecting light.

Ulna and Feather Combined
 
 
 
 
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