By John M. Coluccy, Ph.D.
Conservation planning for migrating and wintering waterfowl is based on the fundamental premise that food energy is the primary limiting factor for the birds during the nonbreeding period. Food abundance is presumed to directly or indirectly impact the physiological condition, survival, and even reproduction of waterfowl, which ultimately influences the health of their populations. Why? Waterfowl that encounter food shortages on migration
and wintering areas may be at greater risk of predation, disease and harvest. In addition, poor habitat conditions
may prevent some female waterfowl from acquiring adequate body fat to meet the energetic demands of egg laying and incubation. Females arriving on nesting areas in poor condition may also lay fewer eggs, suffer reduced nest success, or forgo nesting
altogether. In essence, food deficits during migration and winter can result in fewer females returning to breeding areas and reduced reproductive success among those females that attempt to breed.
Thus a primary goal of Ducks Unlimited and its partners in the North American Waterfowl Management Plan
is to ensure that waterfowl have adequate food resources on key migration and wintering areas across this continent. How can we accomplish this ambitious goal? The most widely accepted and employed approach is through the use of bioenergetics models. Waterfowl habitat
managers use these models to estimate how much habitat is required to support waterfowl and to assess the "carrying capacity" of the current habitat base on high-priority landscapes.
A primary goal of Ducks Unlimited and its partners in the North American Waterfowl Management Plan is to ensure that waterfowl have adequate food resources on key migration and wintering areas across this continent.
Bioenergetics models are similar to supply-and-demand models used in economics, except energy is the common currency rather than dollars. Energy demand is dictated by the birds, whereas energy supply is determined by habitat. Bioenergetics models utilize a wealth of data, including the amount of various habitat types
in the area of interest, how much food energy is available to waterfowl in these habitats, the number and diversity of waterfowl that use these habitats and for how long, and daily energy requirements of individual birds.
The first step in determining regional energy demand for waterfowl is estimating the abundance, distribution, and migration chronology (timing) of waterfowl as they move from breeding areas to staging and wintering areas. Seasonal waterfowl surveys are conducted in these areas to measure migration chronology and species composition, abundance, and distribution. This information is used to calculate the total number of "use days" by each waterfowl species
. Waterfowl use days are multiplied by the daily energy requirements of individual birds to determine the total energy demand for each species in the area. Total waterfowl energy demand for all species is then calculated by adding together these species-specific energy demands.
Calculating Daily Energy Requirements in Waterfowl Waterfowl biologists estimate the daily energy requirement (DER) for individual ducks and geese by calculating the amount of energy the birds use while at rest and how much energy they expend during other activities like flying, swimming, feeding, and preening. For example, a bird in flight expends nearly 13 times more energy than one at rest. Biologists apply data from waterfowl observation studies to create daily "time-activity budgets" for waterfowl based on how much time individual birds spend resting and doing other activities, as well as the energetic costs of these activities. DER estimates among waterfowl range from 130 kilocalories a day for green-winged teal to 1,984 kilocalories a day for trumpeter swans.
Once waterfowl energy demands have been calculated, the next step is to determine how much energy is available to waterfowl in each habitat type on the landscape. First, the total acreage of various waterfowl habitats
must be quantified. In the past, this would have required tedious, land-based habitat surveys. Today it is accomplished via high-powered computers with special software used to map and analyze layers of geographic information (see "Quantifying Waterfowl Habitat
The second step in quantifying the waterfowl energy supply is determining how much food is available in various habitat types. Foraging habitats used by waterfowl ranging from agricultural fields to bottomland hardwood forests are sampled using a variety of techniques. Estimates are made of the biomass of seeds, submersed aquatic vegetation, roots, and tubers, as well as invertebrates and other animal foods. Data from recent research on waterfowl feeding habits are used to ensure that only important waterfowl foods are considered in these estimates.
The final step is to determine the energy values in the various foods waterfowl consume. This is usually accomplished by conducting experimental feeding trials on captive waterfowl. Test foods of a known quantity are fed to waterfowl held in metabolic chambers, where researchers capture all the birds' waste during a post-consumption fasting period. A method known as "bomb calorimetry" is then used to determine the amount of energy in the test food and collected waste. Subtracting the energy in the waste from the gross energy of the raw food provides an estimate of the amount of energy waterfowl are able to extract from each type of food. Together, the product of habitat acreage, food availability, and food energy provide an estimate of the total energy supply available to waterfowl in each habitat type.
By comparing waterfowl energy supply-and-demand data on key migration and wintering areas, DU and its partners can determine whether adequate foraging habitat is available to support desired waterfowl populations. Areas with foraging habitat deficits are targeted for additional habitat restoration and enhancement work. In places where adequate or even surplus foraging habitat exists, protecting the current habitat base is a top priority. Through these planning efforts, we can help ensure that waterfowl always have enough food during migration and in winter to survive and return to their breeding grounds in prime condition.
Quantifying Waterfowl Habitat Geographic Information Systems (GIS), supported by innovative computer hardware and software, and remote-sensing techniques have made it possible for waterfowl habitat managers to assess the extent and distribution of waterfowl habitat across vast areas. Ducks Unlimited and several partners are currently in the process of updating the National Wetlands Inventory, which will classify and digitally map all wetlands across the United States. This effort will also determine how many acres of each wetland type remain in key waterfowl breeding, migration, and wintering areas. In addition, GIS technology and remote-sensing techniques enable DU and its partners to monitor trends in wetland habitat quantity and quality on key waterfowl landscapes across the nation.
Dr. John Coluccy is director of conservation planning at DU's Great Lakes/Atlantic Regional Office in Ann Arbor, Michigan.