The down surrounding this gadwall nest is clearly visible.

For female waterfowl, hatching a nest requires a big investment of time and energy

In waterfowl, it is the females who are responsible for incubation. 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.

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 by periodically turning and manipulating the eggs within the nest and by regulating the amount of time spent on the nest.

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, 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. Older females also tend to spend more time on the nest than less experienced females. And females in poor condition spend less time on the nest than those in good condition. Lower nest attentiveness 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.

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 because they are able to store larger amounts of endogenous reserves. Because, they fast for long periods, burning endogenous reserves to support metabolism, they often lose substantial amounts of weight.

Female snow geese begin nesting before most food sources are available in the arctic and 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. During late springs female snow geese may burn their reserves waiting for the spring thaw and will forgo breeding altogether resulting in what biologists call a “bust year.”

Small-bodied waterfowl on the other hand 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.

Females incubate their eggs in relative silence. Then, one or two days before hatching, clicking and peeping sounds are emitted from the eggs. I’ll talk more about hatch next week.

Check Out:

Life in the Egg