by John M. Coluccy, Ph.D. and Elizabeth St. James
As dawn crawls over the marsh, seemingly every corner of the wetland springs to life with waterfowl activity. A small pod of northern shovelers swims in unison, probing the water's surface with their spatula-shaped bills. Meanwhile, a pair of trumpeter swans tip up in the shallow water and a group of lesser scaup dive and resurface farther from shore. How can it be that all of these waterfowl species can coexist in the same wetland?
Waterfowl are an extremely diverse group of birds capable of exploiting a variety of aquatic (freshwater and marine) and terrestrial habitats. These diverse environments provide a veritable smorgasbord of food, including roots, tubers, submersed and emergent vegetation, seeds, and small animals. Over time, waterfowl have developed numerous adaptations to exploit these habitats and their abundant food resources. These adaptations allow different waterfowl species to intermingle in the same wetlands without competing for the same food resources.
Trumpeter swans, Canada geese, mallards, and green-winged teal all tip up or dabble (see "Finding Food") to forage on submersed aquatic plants. However, their varying neck lengths allow them to access foods at different water depths. For example, the extremely long neck of trumpeter swans allow them to access food resources up to 30 inches deep, while the much shorter neck of green-winged teal limits them to feeding in only a few inches of water.
FINDING FOOD Waterfowl employ a variety of "feeding modes" to satisfy their daily energy needs. Perhaps the most common, recognizable feeding mode is tipping up or dabbling. Ducks from the tribe Anatini are collectively known as dabbling ducks, but swans and geese will also tip up to feed. Pochards, stifftails, and sea ducks are commonly referred to as divers because they seek out food below the water's surface. Grazing is another common feeding mode employed by species such as the Canada goose and American wigeon. A few species like the lesser snow goose are known as grubbers because they uproot grasses and sedges to eat roots and tubers. And finally, northern shovelers and similar species like the pink-eared duck of Australia are known as strainers, because they slurp up water and jet it through lamellae to extract food items.
Dabbling ducks, geese, and swans are at home in both wetland and upland environments. The feet of these waterfowl are positioned well under the body to provide support and allow foraging on land. The trade-off is that their swimming capabilities are somewhat reduced. In comparison, the feet of pochards (diving ducks), stifftails (ruddy ducks), and sea ducks are positioned farther back on the body closer to the tail. Their legs, which are also stouter and feature a lobed hind toe, propel them through the water with greater power. The trade-off for divers, however, is clumsiness on land, which almost completely precludes them from foraging in upland habitats.
Divers are also less buoyant than dabbling ducks, geese, and swans, because their bodies are denser and more compact, which helps them stay underwater for prolonged time periods. To further reduce their buoyancy, divers compress their feathers against their bodies before diving, reducing the volume of air trapped in their downy feathers.
Moreover, the wings of divers are much more compact, which allows the birds to squeeze them tightly against their bodies, reducing drag as they dive. Once underwater, diving ducks use their wings and feet to propel them in quest of food. Collectively, these adaptations allow diving waterfowl to forage in deepwater habitats off limits to dabblers.
Perhaps the most fascinating and diverse feeding adaptation waterfowl possess is their bill. This body part is primarily designed to help locate and consume food resources. In general, bills are round-tipped and soft around the edges so waterfowl can locate food by touch, much like children using their fingertips to locate hidden pennies in a sand box. The bill is lined with lamellae (see "Filter Feeders") and consists of two mandibles: upper and lower. The upper mandible is fixed to the skull, while the lower mandible can move up and down freely, functioning much like our own jaws. The tip of the upper mandible also has a nail that is used for prying or moving food and other items.
FILTER FEEDERS Lamellae are small, fine comblike structures found in rows along the inside of the bill. When waterfowl are feeding, sediment and water enter the bill. Lamellae filter out inedible material, while trapping invertebrates, seeds, and other food items. Most dabbling ducks have 50 to 70 lamellae on their upper and lower mandibles. Northern shovelers are filter feeders that have very well-developed lamellae, which help them extract tiny food items from the water. Each northern shoveler has about 400 lamallae—180 on their upper mandible and 220 on their lower mandible. While lamellae are prominent in filter-feeding waterfowl, they are almost nonexistent in species such as mergansers and scoters, which feed on snails, shellfish, and fish.
Despite their common features, waterfowl bills are very diverse and highly specialized. Similar to precision instruments, the bills of waterfowl have evolved in an array of sizes and shapes to exploit a variety of food items. Mallards are considered the generalists of the waterfowl world, foraging on seeds, waste grain, invertebrates, vegetation, and just about anything they can swallow. Compared to other species the shape of the mallard bill is broad and relatively unspecialized, allowing them to forage on a wide variety of food items.
In contrast to mallards, many other species have highly specialized bills. Among the most specialized are the bills of mergansers. All three North American species (common, hooded, and red-breasted) have long, narrow, serrated bills, which are uniquely adapted for grasping small fish. The wood duck bill is very short and narrow, which facilitates picking up acorns and cutting bulbs from wetland plants. Sea ducks like scoters and eiders dine on shellfish and mussels. Their strong, stout bills assist them in prying and opening shells to obtain the meaty treats inside.
The Jimmy Durante of ducks, the northern shoveler, has perhaps the most unique bill of all waterfowl. It's wide, shovel-like bill with well-developed lamellae functions as a large scoop and sieve for skimming invertebrates and seeds from the water's surface. It is not uncommon to see groups of shovelers foraging together like pelicans.
Waterfowl species that grub or rip up tubers and roots, such as canvasbacks and snow geese, have stout, wedge-shaped bills ideal for prying plant material loose from beneath the mud. Grubbing waterfowl species also have strong, muscular necks, which helps them uproot the buried plant parts that they savor.
The bills of Canada geese are uniquely adapted for grasping and snipping new shoots of grasses, leaves, and stems. While considered a dabbling duck, American wigeon also graze; the shape of their bill is very similar to that of a Canada goose. Thus it's not uncommon to see both of these aquatic grazers dining on lush grasses in pastures.
Through the ages, waterfowl have developed an assortment of adaptations and feeding modes to exploit the wealth of food resources available in diverse wetland and upland habitats. Regrettably, many of these habitats continue to be lost or degraded at an alarming rate. Fortunately, Ducks Unlimited and its partners are devoted to protecting and restoring the diversity of this continent's habitats that North American waterfowl require.
Dr. John Coluccy is director of conservation planning and Elizabeth St. James is a conservation specialist at DU's Great Lakes/Atlantic office in Ann Arbor, Michigan.