By John M. Coluccy, Ph.D., & Jennifer Thieme

Incubation provides the heat needed for embryos to develop inside the eggs of ducks and geese. Because the required temperature range of developing embryos is narrow, parents must commit to a rigorous incubation schedule. As a result, nesting waterfowl must weigh their efforts to meet the needs of developing embryos against such risks as predation, debilitation, and even starvation.

Because the eggs in waterfowl nests hatch at about the same time, incubation was initially thought to begin once the last egg had been laid. But researchers have found that waterfowl spend increasing amounts of time on the nest as laying progresses. Incubating for short periods during egg laying ensures that embryos remain viable. Once the clutch is complete, the majority of the female's time is then devoted to incubation. Three incubation strategies have been observed in waterfowl-shared incubation, female-only incubation, and brood parasitism-but female-only incubation is most common.

At the beginning of incubation, the female plucks down from her belly and arranges it within the nest for insulation. The resulting bare spot on her belly is called a "brood patch." A supplemental set of blood vessels develops in this area and allows warm blood to flow near the surface of the skin, passing heat directly to the eggs. Females adjust the temperature of the eggs by altering how often the brood patch comes into contact with them and by regulating the amount of time spent on the nest.

Female waterfowl periodically turn and manipulate eggs within the nest to maintain close contact with the brood patch and to promote even heat distribution. Eggs in the center of the clutch are closer to the female's brood patch and consequently warmer than eggs on the perimeter. To reduce temperature differences among eggs, females shift them within the nest so that each one can be directly exposed to the brood patch.

During incubation, all female waterfowl take breaks. These recesses are infrequent and brief because each time a female leaves the nest, her eggs are at risk of dropping below a safe temperature, overheating, or being discovered by a predator. On average, females leave the nest three times per day, and each recess lasts about an hour. Hens spend most of their recesses feeding and preening. When leaving the nest by choice, females use feathers and other nest materials to cover the eggs like a down blanket to provide insulation and hide them from predators.

The amount of time spent on the nest is influenced by weather, body size, age, condition, and predation. In general, females spend more time on the nest during cold, rainy, and snowy weather, and less time on the nest during warmer, milder weather. Large-bodied waterfowl species spend more time on the nest than smaller waterfowl. Older females also tend to spend more time on the nest than younger, less experienced females. And females in poor condition spend less time on the nest than those in good condition. Lower nest attentiveness by young or less healthy females prolongs the incubation period and increases exposure of eggs to predators, resulting in fewer successful hatches.

The incubation period for waterfowl lasts from 21 to 31 days, and females spend from 73 to more than 99 percent of each day on the nest. In the extreme case of emperor geese, females spend as much as 99.5 percent of each day on the nest. This leaves a mere seven minutes a day for other activities such as feeding.

To meet the energetic demands of incubation, waterfowl rely on fat and protein stored during spring migration and early in the breeding season. These energy stores are known as endogenous reserves. Large-bodied species such as geese generally spend more time on the nest during incubation than smaller waterfowl, which leaves less time available for feeding. Therefore, they fast for long periods, burning endogenous reserves to support metabolism, and they lose substantial amounts of weight. Female snow geese meet an estimated 78 percent of their energy requirements during incubation from endogenous reserves. As a result, they lose up to one-third of their body mass, leaving most birds emaciated by the time the eggs hatch. In some instances, female snow geese actually deplete endogenous reserves and must abandon their nest to survive. A few actually starve to death on the nest.

Small-bodied waterfowl are limited by their ability to store significant endogenous reserves. They rely less on these stores during incubation because of their short fasting endurance. For example, endogenous reserves account for only 17 percent of the energy used by nesting blue-winged teal, and females lose only 15 percent of their body mass because they spend more time off the nest feeding. Sources of abundant, high-quality food are critical to ducks during incubation because feeding time is restricted and they rely little on endogenous reserves.

For approximately four weeks, females incubate their eggs in relative silence. Then, one or two days before hatching, clicking and peeping sounds are emitted from the eggs. Small cracks appear on the surface of each egg as the hatchlings strike the shell using their egg tooth-a tooth-like accessory on the bill that falls off shortly after hatching. Vocalizations by the young help synchronize hatching and assist with imprinting because they stimulate vocalization by the female. The hatchlings finally emerge from the eggs after about 3 to 24 hours and dry in a few additional hours.

After the brood hatches, the female frequently preens oil onto her breast, belly, and flanks, which is spread to the young. The female usually spends one night on the nest brooding the hatchlings prior to leaving the nest, and she will become increasingly vocal as the time approaches for exodus. This behavior encourages her young to emerge from the nest and follow her closely as they explore their new surroundings.