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.
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