By John M. Coluccy, Ph.D., and Jessica B. Ruthenberg
It seems that we are constantly bombarded with messages about improving our fitness. Just pick up a magazine or turn on the TV and you are sure to encounter an ad or infomercial touting the latest fat-burning product. By now all of us are keenly aware of the health risks associated with carrying a few extra pounds in the wrong places, but for waterfowl, at certain times of year, fat is fit.
The ability of waterfowl to quickly store fat during key periods of their annual cycle is one of the most impressive examples of adaptive obesity in the animal world. Fat is the perfect waterfowl fuel—it's light, compact, requires no maintenance, and supplies twice as much energy as protein. Waterfowl store fat under their skin and around their internal organs and thighs, where it can later be used to fuel migration and meet the energetic demands of winter and reproduction.
For waterfowl and other migratory birds, preparation for migration entails a serious weight gain program with the distinct goal of increasing fat reserves. This program involves both behavioral and physiological changes. A dramatic change in appetite and food consumption, known as hyperphagia, takes place in the weeks leading up to departure and persists throughout the migration period. Accompanying this feeding frenzy is an increase in the efficiency of fat production and storage.
Fat stores accumulated prior to and during migration come in handy because flight is among the most energetically costly activities that waterfowl undertake. Ducks burn 12 times more energy in flight than at rest. The energetic costs of flying are proportional to the size of the bird—the bigger the bird, the more energy required. An average-sized hen mallard will burn approximately 1.8 million calories during a 1,500-mile journey from Saskatchewan to southern Louisiana. That loss of calories equates to burning 194 grams of fat, or roughly 18 percent of her body mass.
Most waterfowl make several stops during migration to replenish burned fat, rest, and carry out other essential activities such as courtship. Access to high-quality habitat with an abundance of high-energy food is therefore essential. The quantity and quality of food available on stopover habitats impacts how quickly waterfowl replenish lost fat reserves and continue their migration. For example, if our hen mallard is able to consume 60 grams of moist-soil plant seeds per day, she would have to spend six and a half days feeding to replenish fat reserves burned during a 10-hour flight, while also meeting her daily energy requirements. However, if moist-soil seeds are more plentiful and she consumes 100 grams of seeds each day, she could replenish her fat reserves in only four days.
Accumulated fat reserves also provide waterfowl with a buffer against unpredictable weather events encountered during migration and winter. In bitter cold weather, fat from high-energy sources such as corn and rice provides the fuel that waterfowl use to produce heat and maintain their body temperature (thermoregulation). Stored fat also allows ducks and geese to hunker down and survive when food is unavailable due to snow and ice cover or poor habitat conditions.
For many female waterfowl, stored fat also plays a significant role in reproduction. For example, Arctic-nesting geese typically arrive on breeding areas when there is little or no food available. Thus females must rely on stored fat (as well as protein and calcium) acquired on migration and wintering areas to produce eggs and successfully incubate their clutch. Female lesser snow geese are at their heaviest when they arrive on the breeding grounds. By the end of incubation, however, they are lighter than at any other time of year, having lost more than 40 percent of their original body mass and burned almost 90 percent of their fat reserves. After hatching their clutch, female snow geese busy themselves restoring lost fat stores while tending to their brood. Sadly, some of the birds are unable to recover from the rigors of incubation and succumb to starvation.
An abundance of high-quality foraging habitat is essential for waterfowl to build the fat reserves they need to survive during migration and winter and return to their breeding grounds in good shape. Poor habitat conditions may delay the birds' departure as they attempt to acquire adequate body fat to meet the energetic demands of migration and reproduction. Tardy arrival on breeding areas has serious consequences for waterfowl because late-arriving females lay fewer eggs, have reduced nest success, and in some cases forgo nesting altogether. For these reasons, Ducks Unlimited and its partners are working hard to conserve waterfowl habitat on key migration and wintering areas to help ensure that the birds have a "round-trip ticket" to return to their breeding grounds on time each year.
Dr. John Coluccy is director of conservation planning and Jessica Ruthenberg is a conservation specialist at DU's Great Lakes/Atlantic office in Ann Arbor, Michigan.