Flight is essential for birds and captivating for birdwatchers. Consider the speed of a diving falcon, the burst of a flushed grouse, the erratic pattern of a courting hummingbird, or the endurance of long-distance migrants.
Each demonstrates the power of flight, which requires muscles so massive that they can account for a third or more of a bird’s body weight. When birds first appeared on the scene, however, the typical vertebrate skeleton plan couldn’t accommodate muscles so large.
Birds had to draw on their engineering skills and make some changes. (Actually, natural selection favored traits that enhanced flight.) To understand the problem, consider a non-avian vertebrate, such as us humans, and think of the arm as a wing.
We have a large, prominent breast muscle, called the pectoralis major, that originates along the breastbone, or sternum, and inserts near the head of the upper arm bone (the humerus). When the breast muscle contracts, it brings the arm close to the body. (The motion is like a bird’s downstroke.) Bodybuilders can bulk up their breast muscle, but a flat vertebrate sternum doesn’t have enough surface area to accommodate attachment of the enlarged muscles required by birds.
To raise an arm, we use a smaller muscle, known as the deltoid, on the top of the shoulder. Place your hand on the edge of your shoulder and raise your arm. You will feel the deltoid muscle bulge slightly. The anatomy of the vertebrate shoulder doesn’t provide a site for attaching a larger arm-raising muscle, and this location is woefully inadequate for the requirements of a bird. By the way, because the pectoralis major is large and the deltoid is small, you can bring your arm down with much greater force than you can raise it.
The first avian solution was to add a vertical keel to the sternum. The keel dramatically increases the surface area for muscle attachment. As you can see in the diagram above, the horizontal sternum forms a T with the vertical keel. In the top illustration, the pectoralis major, the lower muscle on the keel, is shown in red. It inserts in the humerus and is shown contracting, pulling the wing down.
Because muscles function only by shortening, conventional wisdom holds that a muscle must be located above the wing in order to raise it. But birds lift their wings using a large muscle located beneath the wing. Attached to the keel of the sternum, the muscle, known as the supracoracoideus, connects to the top of the humerus by way of a pulley, an ingenious mechanism found nowhere else among vertebrates.
The supracoracoideus, shown in white in the top illustration, is found just above the pectoralis. Its tendon loops over the shoulder, inserting on the top of the humerus. The supracoracoideus is shown in red in the middle illustration. It’s contracting and, because of the pulley, lifting the wing.
As you can see at bottom in the diagram, the pulley is located where three bones — the coracoid, scapula, and clavicle (not shown) — come together to form the shoulder joint. The tendon slides over the grooved head of the coracoid.
The coracoids are large, stout bones that connect the sternum with the shoulder. The shoulder joint is much like the cupped palm of your hand. To visualize this, pretend the fist of your right hand is the head of the humerus, and place it into the cupped left palm. Now imagine that the tendon of the supracoracoideus passes over your left hand and inserts on the top of your right wrist, and that the tendon of the pectoralis major inserts underneath the wrist. Holding your wrist rigid, move your elbow up and down to simulate the alternating contractions of the supracoracoideus (upstroke) and pectoralis major (downstroke).
Birds have undergone many adaptive changes for flight. Among the most dramatic are the extreme enlargement of the breast muscles and the skeletal modifications that accommodate them, and the development of a unique pulley system that allows a muscle located under the wing to raise it. All of this points again to the amazing structure and function of birds. — Eldon Greij, Founding Editor
Learn while eating
To appreciate adaptations for flight, carve a rotisserie chicken.
Carefully remove all of the breast meat, exposing the vertical keel attached to the flat sternum. Notice how large the breast muscle is relative to the body size and how much the keel increases the surface area for attachment of the breast muscles.
As you trim the muscle in front of the keel, you will find the V-shaped, fused clavicles, also known as the wishbone. Just behind it are the two stout coracoid bones. Notice how they attach to the front of the sternum and the base of the wing, and visualize how the bones serve both as fulcrums for flapping wings and as pillars that keep the contractions of the flight muscles from collapsing the rib cage.
This article from Eldon Greij’s column “Amazing Birds” appeared in the January/February 2014 issue of BirdWatching.