By John M. Coluccy, Ph.D.
The amazing acrobatics of green-winged teal corkscrewing over the decoys and Canada geese whiffling into a marsh have inspired and awed hunters and bird-watchers for generations. Waterfowl do not merely fly. They are masters of the air and like most birds possess a variety of specialized physical adaptations that make them true flying machines.
Wings and feathers are the most obvious adaptations waterfowl and other birds have for flying. Although the size and shape of the wing varies among species, all waterfowl have relatively long, pointed wings built for speed. Rigid primary feathers provide thrust, and uniquely shaped secondary feathers provide lift. The vanes of flight feathers have tiny hooks called barbules that hold them together like a zipper, giving feathers extra strength. Soft coverts create a smooth surface for airflow over the wing, and contour feathers streamline the body, improving flight efficiency. Together, the wing and feathers form an airfoil—similar to the wing of an airplane. This wing design creates lift when air sucks the top of the wing upward and pressure underneath the wing pushes the same direction.
Because flight is so energetically demanding (nearly 15 times that of resting), waterfowl possess several less conspicuous adaptations for flight. The skeleton of waterfowl is uniquely structured. Fusion and strutlike reinforcement of hollow bones make the skeleton both lightweight and powerful, enabling it to withstand stress and support large muscles. The breastbone includes a large keel that anchors major flight muscles.
Waterfowl have developed other ways to lighten their load in the air. To further minimize weight, heavy jaws, jaw muscles, and teeth used for grinding food have been replaced by a muscular gizzard, which is located near the birds' center of gravity. In addition, reproductive organs (testes, ovaries, and the oviduct) are greatly reduced in size, especially in females. For most of the year, the reproductive organs are tiny. But as the breeding season approaches, a female's left ovary and oviduct rapidly develop to produce eggs. Although nesting females may have a number of developing eggs in the ovary, only one fully developed egg is carried in the oviduct at any one time, thus saving weight.
For power, waterfowl have large breast muscles that are red in color because of the presence of dense fibers containing red oxygen-carrying compounds. These fibers are generously supplied with oxygen-rich blood and are designed for sustained flight. The flight muscles are strategically located near the birds' center of gravity, just below the wings, for balance.
To keep these flight muscles supplied with large quantities of oxygen-rich blood, waterfowl have a large, four-chambered heart consisting of two pumps that operate in tandem. One pump supplies oxygen-rich blood from the lungs to the tissues, while the other moves oxygen-depleted blood from the tissues to the lungs. The segregation of oxygen-rich and oxygen-poor blood makes the waterfowl circulatory system extremely efficient and well suited for the rigors of flying.
The avian respiratory tract is an ingenious system consisting of the nares (nostrils), tracheal system, lungs, and air sacs. Inhaled air is cleansed and heated as it passes through the nares on its way to the respiratory tract. Air is then passed in a one-way, two-stage flow through the lungs. A breath of inhaled air first passes into the posterior air sacs and is forced into the lungs as it is exhaled. When the next breath is inhaled into the posterior sacs, air from the first breath is forced from the lungs into anterior air sacs. The air from the first breath is expelled from the bird during the second exhalation, while the second breath moves to the lungs. With each breath, nearly all the air in the lungs is replaced, resulting in extremely efficient gas exchange. This unique system provides continuous, unidirectional airflow delivering large amounts of oxygen and helps remove potentially lethal body heat produced during flight.
Although waterfowl have found many ways to streamline, lighten, or eliminate unnecessary structures, they have not cut corners when it comes to the nervous system. The life of waterfowl is a high-speed, aerial one. Therefore, the birds possess proportionately large brains connected to keen eyes with ample processing centers that relay and coordinate visual information. Commands from the brain are transmitted quickly via short nerves to the muscles controlling the wings and tail. This combination of acute vision, quick decision-making, and high-speed nerve transmission enables a wood duck to dip and dive through the limbs of a forested wetland with relative ease.
The body plan of waterfowl is uniquely adapted from head to toe for flight. The many adaptations waterfowl possess allow them to master the sky, traversing it in ways man can only imagine or perhaps experience in a high-performance jet fighter.
Dr. John Coluccy is manager of conservation planning at DU's Great Lakes/Atlantic Regional Office in Ann Arbor, Michigan.