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
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,