Understanding Waterfowl: Balanced Diets

By Joseph D. Lancaster, PhD, and Ryan J. Askren, PhD

Understanding the feeding habits of waterfowl has been at the core of waterfowl ecology since the earliest days of the field. By knowing what ducks and geese eat and how their diets change throughout the year, we can better manage wetlands to provide the food resources needed to support healthy and diverse populations of these birds.

In the early 1900s, scientists examined food in duck gizzards obtained from hunter-harvested birds during fall and winter but largely ignored what the birds ate during the breeding season. These early researchers examined thousands of samples from large geographies and identified numerous wetland plants that are important food sources for waterfowl. Decades later, scientists learned that gizzards retain mostly plant material, so the early studies underestimated the importance of soft-bodied invertebrates in waterfowl nutrition. Since the 1970s, scientists have developed new techniques, such as watching birds feed before collecting them, examining contents of the esophagus, and investigating food availability at collection sites. These practices have given us a clearer picture of what waterfowl eat and how their diets change throughout their annual cycle.

Food provides the energy and nutrients that waterfowl need to grow, maintain body condition, and reproduce. There are three primary components of the waterfowl diet: amino acids, carbohydrates, and minerals. Amino acids form proteins, carbohydrates serve as the primary source of energy, and minerals supply essential nutrients. Protein builds and maintains the body (including organs, muscles, and feathers) and is a large component of eggs. Consumption of carbohydrates varies throughout the year based on the birds’ activities and energy needs. Minerals, such as calcium and phosphorous, are important for bone and egg development. Each food consumed by waterfowl contains different amounts of amino acids, carbohydrates, and minerals, and waterfowl change the amounts and types of food they eat to meet seasonal demands.

Winter is an energetically costly period for waterfowl due to their need to avoid predation, engage in courtship, and thermoregulate during bouts of ice and snow, which can limit access to food. Waterfowl feeding strategies and the foods consumed during winter are specialized and diverse but consist largely of carbohydrate-rich plant materials such as natural seeds, agricultural grains, and roots and rhizomes. When consumed, carbohydrates are broken down into sugars, which the birds use as energy to meet daily needs. Excess energy is converted into fatty acids and stored as lipids (fat) for migration and sustenance when food is scarce.

During mid- to late winter, most female ducks undergo a complete molt—or feather replacement—except for their wing feathers. This molt occurs over six to seven weeks and requires 30 to 100 percent more protein than normal. Ducks get this extra protein by increasing their consumption of aquatic invertebrates. Although agricultural grains contain large amounts of carbohydrates, they lack necessary amino acids and minerals, which must be acquired by eating natural seeds and invertebrates. Thus, supporting the nutritional needs of wintering waterfowl requires abundant wetlands that contain a variety of food types.

During the breeding season, nearly all ducks shift to a diet dominated by aquatic invertebrates to meet increased protein and calcium requirements for egg production. Even American wigeon, which eat plants almost exclusively for most of the year, shift to a largely invertebrate diet for a short time during the breeding season. The extent, timing, and duration of this shift to invertebrates is highly variable among species. For example, northern pintails consume large quantities of invertebrates during the egg-laying period but quickly return to seeds once all the eggs have been laid. Other duck species continue to consume a diet with moderate amounts of invertebrates throughout the breeding season. In general, early-arriving species, such as mallards and northern pintails, increase their invertebrate intake during spring migration as they refuel at stopover sites. Later-nesting ducks, such as gadwalls, delay the transition to invertebrates until they arrive on the breeding grounds.

Unlike excess energy, which can be compactly stored as lipids, proteins cannot be condensed and are typically stored as muscle. Flying with excess muscle mass increases energy expenditure, which is why most waterfowl obtain the necessary proteins for egg production at or near breeding sites. This poses a limitation for geese and some sea ducks that begin nesting upon arrival on arctic breeding areas before large quantities of nutritious forage become available. In these cases, geese obtain protein from plant shoots and roots on spring staging areas before departing for northern breeding areas. Nesting geese then rely on lipid stores and protein from their own muscles for egg production and energy, often losing more than 30 percent of their body mass. After the nesting period is over, the combination of nutrient-rich plant growth and nearly continuous daylight allows geese to forage intensively and restore their body condition.

While waterfowl have evolved specialized bill structures and foraging strategies that allow them to exploit certain food resources with little competition from other species, all ducks and geese are capable of dietary flexibility in response to short- or long-term changes in food abundance and distribution. In the short term, diet shifts can occur when normal foods become scarce or inaccessible. For example, Canada geese will sometimes consume shad in ice-free rivers during extremely cold periods. These shifts may also occur in response to a short-lived superabundance of foods, such as when sea ducks gorge on roe during the herring spawn. Over the long term, waterfowl diets may shift as a result of landscape changes. This has been observed in mallards and several goose species that have expanded their winter distributions to exploit rich sources of waste grains on agricultural landscapes.

Decades of dietary research conducted throughout the annual cycle of waterfowl have provided a wealth of information about the feeding habits of ducks and geese and their dependence on diverse wetland habitats. This science has been instrumental in guiding wetland and waterfowl conservation and management in breeding and nonbreeding areas of North America. Continued innovations in research, such as studies quantifying the energetic value and amino acid composition of waterfowl foods, promise to further advance our understanding of waterfowl diets as well as our ability to improve management and conservation efforts to benefit multiple species of ducks and geese.

Dr. Joe Lancaster is biological team leader of the Gulf Coast Joint Venture, and Dr. Ryan Askren is director at the Five Oaks Ag Research and Education Center in partnership with the University of Arkansas at Monticello.

Counting Calories

During winter, food is the most limiting resource for waterfowl. Conservation planners use bioenergetic models to estimate habitat objectives to support the foraging demands of regional waterfowl populations. In a simplified way, bioenergetic modeling is like planning a child’s birthday party. If 15 children will attend the party, and each child will eat two slices of pizza on average, and each pizza has eight slices, you’ll need to order four pizzas. Likewise, bioenergetic models estimate foraging demand, expressed in calories, based on expected waterfowl numbers, the duration of the birds’ stay in the region, and the average amount of energy the birds will require each day.

Energy supply, expressed as calories per acre, is calculated from the availability of foods in foraging habitats and the average amount of energy available per unit of food. Comparing estimates of energy supply and demand allows managers to determine whether a regional landscape has sufficient resources to support waterfowl populations at target levels, and to thereby more effectively target where additional habitat conservation and management efforts are needed.