Differences in wing size and shape determine the flight capabilities of waterfowl
By J. Michael Checkett
From the raw speed of canvasbacks to the limb-dodging acrobatics of wood ducks, the flight capabilities of waterfowl have long inspired awe among hunters. As with all birds, a duck’s flight characteristics are determined by the shape of its wings and the way it uses them. But biologists generally divide bird wings into four categories: slotted high-lift wings, elliptical wings, high aspect-ratio wings, and high-speed wings. Waterfowl are in the high-speed category, but there are differences among species that have this wing type.
All waterfowl species are suited to particular habitats, and it is easy to see how these varied environments, which present unique flight challenges, influenced wing evolution. Most puddle ducks frequent small marshes, sloughs, and flooded bottomlands. As a result, puddle ducks need large wings for fast takeoffs and twisting flight to dodge trees and other obstructions. But diving ducks frequent open lakes, rivers, and coastal bays, so they need wings built for flying at high speed over open water. Consequently, the wings of diving ducks need to be flatter than puddle duck wings, moderately long and narrow, and swept back like the wings of a fighter jet.
Looking at how each part of a wing functions offers insight into the flight dynamics of waterfowl (see diagram on page 29). Primary flight feathers are rigid and provide thrust while flapping. Secondary flight feathers are shaped to give lift for gliding. The vanes of flight feathers have tiny hooks called barbules that zip together, giving the feathers the strength needed to form an airfoil. Wing coverts are softer than flight feathers and create a smooth surface for the air to flow over the wing, providing lift and keeping the bird flying efficiently. This feather arrangement gives a high-resistance down stroke and a low-resistance upstroke.
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Flying is essentially a balance between two sets of forces: lift and weight, and thrust and drag. As a bird flaps its wings, most of the lift and propulsion is generated on the down stroke of the wings. But lift is also obtained from the secondary flight feathers as the wings are raised.
The size of the wing determines how much lift is generated. A species’ body size and weight influence the size of wing needed for flight and dictate “wing loading,” which is the ratio of wing area to body weight. The larger the wing surface in proportion to the bird’s weight, the more easily the bird can become airborne. Wing loading determines the size limits of flying birds and puts severe constraints on nearly all large birds. It is easier for small birds to have wings big enough to support their weight than it is for large birds. For a Canada goose to have the same wing loading as a cardinal, the goose’s wings would need to be nearly 10 times larger than they are. Wings of such gigantic proportions would be both physiologically and mechanically impossible. Because large waterfowl such as Canada geese and swans have high wing loading, these birds must make extended runs across the water to reach the speed necessary for takeoff.