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.
Dr. John Coluccy is a regional biologist and Jennifer Thieme is a conservation intern at DU’s Great Lakes/Atlantic Regional Office in Ann Arbor, Michigan.