By J. Michael Checkett
Have you ever heard that you can tell where a mallard is from by the color of its feet? As the story goes, the legs and feet of northern mallards are redder than their southern cohorts because low temperatures in higher latitudes cause more blood to flow to the birds' extremities. These mallards are also thought to be larger and hardier than mallards raised in southern parts of the species' range. Old-timers called these big, late-migrating mallards "redlegs."
In reality, the brightly colored feet and bills of mallards and other ducks are caused by changes in hormone levels during late fall and winter while the birds are pairing. The feet of both male and female mallards turn bright orange—almost red—in December and January as they go through courtship and pairing. Heavier adult mallards typically develop breeding plumage and display brightly colored feet earlier than younger, lighter birds, giving rise to the mistaken belief that "redlegs" are a different race or subpopulation of larger mallards. In summer, hormone levels in ducks decrease, and their feet and bills become drab in color again, which helps camouflage the birds while nesting and molting.
Of course, the legs and feet of waterfowl play a vital role in many other important activities, including locomotion (walking, swimming, and flying) and thermoregulation (maintaining body temperature). Features such as webbing of the feet arose over time as the birds adapted to make the most of their wetland environments. For example, researchers recently discovered that while swimming, waterfowl push both backward and downward with each stroke of their webbed feet. This provides a combination of lift and thrust, propelling the birds through the water with remarkable speed and efficiency.
The feet of water birds are all structurally similar but vary among species. The most common difference is in the amount of webbing between the birds' toes. Cormorants and boobies have totipalmate feet, where all four of the birds' toes are connected by webs. Ducks and geese have palmate feet, where only the three front toes are webbed and the hind toe (called the hallux) is small and elevated. Coots have lobate feet, where the toes have a series of webbed lobes that open when the foot is pushed backwards—much like the base of a push pole used by duck hunters to traverse the marsh. Lastly, some waterfowl such as the Australian magpie goose and the Hawaiian goose (or nene) have half-webbed semipalmate feet, an adaptation that is useful for occasional swimming and walking on soft surfaces.
The legs and feet of waterfowl also play an important role in maintaining body temperature. Ever wonder how a mallard can stand comfortably on ice? A unique heat-exchange system in the birds' legs known as counter-current circulation makes this possible. The large, flat feet of waterfowl are natural radiators, so to minimize heat loss, the arteries and veins in the birds' legs work in tandem to retain heat. Arteries supplying blood to the feet pass alongside the veins removing blood. The warm arterial blood flowing to the feet is cooled by venous blood flowing back to the body where it is warmed again. Consequently, very little of a duck's body heat is lost through its extremities. Thus, while the core body temperature of a duck standing on ice is near 100 degrees Fahrenheit, the temperature of the bird's feet may be just above freezing.
To further conserve heat in cold weather, waterfowl reduce the volume of blood flowing to their feet by constricting blood vessels in their legs. Experiments have shown that waterfowl gradually reduce blood flow to their feet as the air temperature drops to 32 degrees Fahrenheit (the freezing point). When temperatures fall below freezing, however, waterfowl again increase blood flow to their feet to prevent tissue damage. The birds also protect their feet by drawing them into their flank feathers and close to their body. To further minimize exposure in bitter cold weather, waterfowl often stand on one leg at time, tucking the other leg into their body feathers to protect it from the elements.
In a similar but reverse manner, waterfowl can release excess body heat through their feet, primarily by standing or swimming in water that is cooler than the air. This capability helps waterfowl avoid heat stress on long, hot summer days. Where legs and feet are positioned on the bodies of waterfowl also influences how the birds interact with their environment. In dabbling ducks and geese, the legs are located near the middle of the body, providing the birds with good balance for standing and walking. This offers many advantages, including the ability to feed on dry land and in very shallow water, nest in upland habitats, and spring almost vertically into flight to escape predators.
The feet of diving ducks are located near the back of their body. This makes walking difficult, but is beneficial for diving and swimming. These adaptations allow diving ducks to frequent large bodies of water and feed by diving, often at considerable depths. Their excellent diving and swimming abilities also help them escape predators. The trade-off is that diving ducks can't spring vertically into flight like dabbling ducks and must instead make a running start across the water to achieve flight speed.
A final activity where the feet of waterfowl play an important role is flight. All waterfowl use their feet as rudders while flying. And as all waterfowl hunters have seen, ducks and geese lower their feet and spread the webbing between their toes right before they land. This creates a little extra drag that helps the birds slow down. Conversely, when waterfowl want to achieve maximum flight speed and efficiency, they pull their feet into their flank feathers just like retractable landing gear on an airplane.
In most circumstances, webbed feet have been wonderful adaptations that assist waterfowl in exploiting the wetland habitats where they live. Millions of years of adaptation have helped ducks, geese, and other water birds truly put their best foot forward.