By J. Dale James, Ph.D.
Whether it's a huge flock of lesser snow geese passing overhead, a hen pintail leading her recently hatched brood overland, or a canvasback diving for aquatic vegetation, waterfowl are fascinating to watch when they are on the move.
Years of natural selection have made waterfowl exceptionally well adapted to their environments, allowing the birds to fill diverse ecological niches. This process has resulted in great variation in the body structures of waterfowl, which affects how the birds fly, swim, and walk.
There are few spectacles in nature more impressive than the annual migrations of waterfowl across this continent, and it's the marvel of flight that allows these impressive bird movements to occur. Waterfowl wings provide the two essential elements of flight: lift and thrust. Primary feathers (the outer flight feathers) provide thrust, which is the force that propels a bird through the air and maintains forward momentum. The secondary feathers (the inner flight feathers) provide lift, the force that pushes a flying bird in an upward direction. Other special adaptations for flying that are shared by all waterfowl include a streamlined body, lightweight hollow bones, and a rigid skeleton.
The wings of each waterfowl species are designed to help the birds exploit specific habitat types. For example, dabbling ducks spend much of their time feeding and resting on small, shallow wetlands, where the birds are vulnerable to a variety of predators. Thus dabblers have long, broad wings that enable them to take off quickly and to maneuver gracefully around trees and other obstacles. In contrast, diving ducks frequent large lakes, rivers, and bays, often diving to great depths while feeding. Consequently, their wings are shorter, narrower, and swept back like those of a fighter jet. This design enables diving ducks to fly at high speeds over open water. It also allows them to compress their wings tightly against their body while diving. The trade-off is that diving ducks must run across the water to reach the speed necessary for takeoff and beat their wings more rapidly to remain airborne. This relationship between the size of a bird's wings and its body is known as wing loading. Thus, dabblers exhibit low wing loading, while divers have high wing loading.
The legs and feet of waterfowl play an important role in the birds' movements on land and in water. Designed primarily for paddling, the legs of waterfowl are set back on the body. It's that placement, along with their large webbed feet, that gives the birds their characteristic waddle when they walk. Geese and dabbling ducks often feed on land and typically make their nests there, so their legs are not positioned as far back as those of diving ducks. Among North America's waterfowl, black-bellied whistling ducks are especially well adapted for walking. Black-bellies have a longer metatarsus (the leg bone that attaches to the toe bones) and smaller feet than other waterfowl, which enables them to walk gracefully on land.
Waterfowl developed webbed feet to help them swim and dive more efficiently in wetland environments. While swimming, waterfowl push both backward and downward with their legs and feet. The webbing between their toes spreads out on the down stroke to create more surface area and push more water. The toes are then folded together or turned inward on the forward stroke to minimize water resistance. This motion provides both the lift and thrust needed to propel waterfowl quickly and efficiently through the water. Diving ducks such as redheads, scaup, and canvasbacks use their webbed feet to generate the force required to dive long distances underwater. Divers also paddle constantly while bottom feeding to offset the buoyancy of their bodies and keep from floating to the surface. Swans, geese, and dabbling ducks paddle with their feet to keep their bodies partially submerged while tipping up to feed in shallow water.
The many interesting adaptations that affect how waterfowl fly, swim, and walk developed over time to allow the birds to thrive in diverse habitats. The next time you see a duck in motion, consider the remarkable design behind the bird's movements as well as the importance of conserving the wetlands and other habitats that support the waterfowl we all enjoy.
Dr. Dale James is manager of conservation planning in DU's Southern Region.