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.