By Tom Moorman, Ph.D.
Ducks need an abundance of carbohydrate-rich food to meet their needs during migration and winter
Energy policy has been in the news lately, with daily reports on fluctuating fuel prices, alternative energy sources, and other complex issues. Most of us probably don’t think very much about how we acquire and use energy on a daily basis—we simply stop by the corner gas station for fuel or run by the store and buy food. If our house gets chilly, we adjust the thermostat and turn on the heat. These are all ways that we acquire and use energy.
Have you ever wondered how waterfowl meet this most basic of biological requirements? Waterfowl don’t have the luxury of stopping at the gas pump or adjusting a thermostat on a cold winter night. To meet their energy needs, waterfowl must find and acquire food in the habitats where they live. Over the years, scientists have learned a great deal about the role of energy and habitat in the lives of waterfowl, and this information helps guide landscape-level habitat conservation.
Let’s consider how a female mallard’s need to acquire and store energy during the fall and winter relates to waterfowl habitat management programs. Starting out in September on the Saskatchewan prairie, our mallard hen is very lean after spending the summer nesting, raising a brood, and completing a molt of her flight feathers—all very demanding events. So prior to the fall migration, she feeds on starchy foods rich in carbohydrates that can be efficiently converted to and stored as fat. Fats are relatively compact, energy-rich molecules, and they are the fuel birds burn to stay alive and migrate. Waterfowl store fat under their skin and around their internal organs and thigh muscles.
Carbohydrates used to build these fat reserves come from a variety of sources. In wetlands, our hen may eat the seeds of moist-soil plants like wild millet or submersed aquatic vegetation like sago pondweed, or she may fly out to feed on waste grain and other crops in dry harvested fields. These foods contain about three times more useable energy than a comparable amount of protein-rich invertebrates. But she must balance her diet of waste grain with native plant seeds and invertebrates to acquire all essential nutrients.
Within a few weeks, our hen has eaten enough to rebuild her fat reserves just in time to depart Saskatchewan on the heels of an October cold front. With the help of a strong north wind, she travels 800 miles in less than a day and settles on a wetland in eastern South Dakota. Throughout the fall migration, she will make only a few such stops to refuel on carbohydrate-rich foods. Flight is the most energetically expensive activity a duck performs, so our hen mallard needs to maintain a full fuel tank of fat reserves during migration. Generally, mallards maintain fat reserves that meet their minimum daily energy needs for five to seven days, but migration and other activities can increase the rate at which these energy reserves are used.
Stored fat reserves also provide waterfowl with insurance against unpredictable habitat conditions along their migration routes. For example, in early November, our hen departs South Dakota and makes a 500-mile direct flight to Missouri’s Confluence region just north of St. Louis. In most years, food is plentiful on waterfowl management areas, duck clubs, and adjacent agricultural lands in this region, but in 2008, devastating spring floods limited food resources available to waterfowl during the fall. Fortunately, our hen was able to maintain and rebuild her fat reserves during her stay in South Dakota, giving her the energy to migrate another 300 miles to her wintering grounds in Arkansas, where food will hopefully be more abundant.
Upon her arrival in east-central Arkansas, our hen finds excellent habitat conditions in and around Bayou Meto Wildlife Management Area. Over the winter, she spends much of her time in flooded forested wetlands, where she pairs with a mate and feeds heavily on acorns (another kind of seed that is rich in energy and essential nutrients), seeds of moist-soil plants, and a small but important complement of invertebrates that provide protein for the winter molt. Good wintering habitat conditions allow our hen and her mate to make it through the winter in good physical condition and, importantly, to begin their northward migration in February with ample energy reserves. Poor wintering habitat conditions could have forced the birds to delay their spring migration. Time is of the essence for our hen and her mate because mallards that arrive early on the breeding grounds are generally more productive than those arriving later. Similarly, heavier females are also more successful nesters.
Ducks Unlimited and its partners have conducted extensive research on waterfowl energetics and habitat quality on key wintering areas, such as the Central Valley of California, Mississippi Alluvial Valley, and Gulf Coast. This research has helped determine the daily energy needs of waterfowl (see sidebar), how much food and energy are available per acre in different habitat types, how much of each habitat currently exists on specific landscapes, and how many waterfowl use these habitats and for how long. This information has been used to develop waterfowl foraging habitat models that estimate how much of each habitat type is needed to support waterfowl in key wintering areas.
DU and its partners have also worked together on priority wintering areas to estimate how much foraging capacity is available compared to what is required by waterfowl. In many areas, waterfowl rely on a combination of well-managed habitat on public land owned by state and federal agencies and on private lands managed by farmers and waterfowl hunting clubs. DU is working with these partners and others to deliver landscape-level conservation programs that will provide the foraging habitat waterfowl need to survive the winter and return to the breeding grounds in good shape.
Dr. Tom Moorman is director of conservation planning at DU’s Southern Regional Office in Ridgeland, Mississippi.
Diver Diets: Diving ducks and sea ducks have more varied diets during migration than puddle ducks. Scaup, for example, depend heavily on small, shrimp-like amphipods for building fat reserves, but they also consume snails, mussels, and other invertebrates, as well as some plant matter.