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Secrets of flight are hidden in front of your eyes

Between bones and beating hearts, birds have specialized anatomy catered to flying

Posted: April 6, 2012 - 12:01am

Watching a bald eagle drawing lazy circles overhead, I realize I’m fascinated because an eagle is large enough to observe how it flies. As it soars, I can see it slightly curve one wing to level off, tuck its wings a fraction to drop lower, all the minute adjustments it makes to ride the air.


Have you ever stopped to wonder how alien birds really are, under that coating of feathers? How honed their bodies are, literally down to their very bones, for the ability to fly?


Taking to the air relies on much more than a pretty pair of wings. Flapping flight is the most extreme of sports. It requires a strong but lightweight frame, high amounts of oxygen, and a heart capable of handling intense physical demands. Although wings and feathers are essential for birds to fly, some of their critical equipment is hidden beneath their skins.


The first necessity is a light, yet sturdy, skeleton. The foundation is the structure of the bones themselves. Unlike the more solid bones of mammals, the major bones in a flying bird’s body, such as the large bones of the legs and wings, are hollow. Numerous small struts of bone crisscross the width of the hollows to reinforce the bone, making it incredibly strong for its size.


If you look at the remains of a roast chicken after dinner, you can see that birds share some recognizable bones with mammals, with a few distinctly avian twists. The familiar “wishbone,” or furcula, is a fused-together version of our mammalian “collar bones.” When a bird flaps, the furcula flexes open and back like a spring. A bird’s ribs attach to the spine in back and the “breastbone,” or sternum, in front like yours do. To protect the internal organs from being squeezed during flapping, however, partway along a bird’s ribs are spokes that extend to the rib behind. This provides a reinforced basket around the squishy bits.


Fused bones in parts of a bird’s skeleton reduce the number of bones, lightening the weight while providing a strong platform for muscles. The tail is short with several fused vertebrae at the end that support the tail feathers, much lighter than a long, bony tail would be. In its wing, a bird has two separate wrist bones (humans have eight), while the rest have fused with the “hand” bones. This provides the rigid base necessary to support the long primary flight feathers. In an eagle, the primaries are the finger-like feathers at the ends of the wing.


The shape of a bird situates its center of gravity below the wings. From the sternum rises a broad, thin sweep of bone called the keel, shaped somewhat like the hull of an old-fashioned sailing ship. Although there are up to 50 muscles involved in flight, two important major muscle groups attach to the keel. One set pulls the wings down. Surprisingly, the other set lifts the wings. The muscles extend up through an opening in the shoulder bones, attaching to the top of the humerus, the upper “arm” bone. When the muscle in the bird’s chest contracts, it works like a pulley system to cause an upstroke of the wing.


When you start breathing hard during a sport, it’s because increasing your physical activity requires increasing your oxygen intake. Flying needs a great deal of oxygen. Birds have developed an incredibly efficient respiratory system that leaves even an Iron Man triathlete behind.


Like other mammals, you have lungs like balloons that inflate and deflate by using the diaphragm muscle below your lungs to expand or contract your chest cavity. Air flows in and out of your lungs, a two-way air flow. When you exhale, a good amount of air remains in your lungs (don’t worry, it’s supposed to).


A bird’s respiratory system is completely different than a mammal’s. In addition to lungs, a bird has a system of air sacs spread throughout the body, including inside some of the larger bones. Instead of a diaphragm muscle, the bird moves its sternum to enlarge its chest cavity to inhale. Air flows in a single direction, allowing a bird to replace almost all the air it inhales, which means more oxygen.


It takes two cycles of inhaling and exhaling for a breath of air to move through a bird’s system. When a bird inhales, air doesn’t go into its lungs. Instead, the air follows the bronchial tube past the lungs directly to the air sacs located in the bird’s belly. When the bird exhales, the air moves from the air sacs into the lungs. Instead of balloon-like structures, bird lungs are spongy and compact, tucked up along the ribs on either side of the spine. The second inhale moves air from the lungs into another series of air sacs in the bird’s chest. The second exhale pushes air from these air sacs out of the bird’s body.


The heart is yet another internal difference between birds and mammals. Because of the high energy demands of flight, a bird needs a powerful heart to circulate blood carrying oxygen and energy throughout its body. Birds have hearts anywhere from 50 to 100 percent larger than mammals of the same size. A bird at rest may have a heart rate half that of a mammal the same size, but the bird’s larger heart will move the same volume of blood.


A bird’s heart needs to be able to handle high levels of activity, sometimes for a long time, especially for birds that migrate for days at a time. Not only does a bird’s heart have more muscle fibers than a mammal’s, the fibers are thinner and the cells contain more mitochondria (the parts of a cell that produce energy by using oxygen). The thinner fibers allow for faster absorption of oxygen. More fibers mean a stronger pump. Better oxygen absorption means the heart can function longer during intense activity.


Birds far outclass mammals as athletes due to their specialized bones, breathing, and heartbeat. This is even more amazing when you think of it in the context of the thousands of miles birds travel each year migrating to and from Alaska and their wintering grounds. When you spot your first sparrow or hummingbird of the spring, enjoy marveling that something so small is so finely tuned to flit through the air.


 


• Beth Peluso is a freelance writer and illustrator and avid birder. She enjoys spying on wildlife across Alaska.

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