by Bruce Batt Ph.D.
Nothing in nature is more critical to waterfowl reproduction than the eggs that are laid each spring. Each egg has all the vital components needed to produce a new life in the form of a developing embryo, a duckling, and ultimately, an adult duck capable of producing its own eggs. Besides self-maintenance and survival, virtually everything else that waterfowl do in the months leading up to spring is focused in some way on nesting and the eggs that are produced.
Eggs have three major components—the shell, the yolk, and the albumen. Besides giving structural strength, the shell is the source of most of the calcium that the embryo absorbs during development to form cartilage and bone. The yolk is mostly various forms of fat, and the albumen (the "white") is mostly protein. The micronutrients necessary for development are found among all three components.
The set of eggs that a hen lays is known as a clutch. For a few species, the clutch may actually weigh as much as the hen. This is quite remarkable when you consider that the female produces all the elements in the eggs from the body reserves she carried from wintering and migration habitats as well as what she obtained from local breeding habitat. This again highlights the critical importance of quality habitats during all seasons. It also explains why this package of goodies is so attractive to predators, as virtually nothing in nature is more nutritious than an egg.
But what goes on inside the nest and the egg between laying and hatch? Once nesting has started, ducks lay one egg each day, usually in the early morning when nocturnal predators are bedding down for the day and daytime predators haven’t started moving around yet. When the first eggs are laid, the hen attends the nest only for a few minutes. As the clutch grows, she spends more and more time on the nest, pulling in grass and twigs to form the nest bowl. At about the mid-laying period, the hen starts to pluck down feathers from her body and mix them with the other nesting material to form a very effective blanket that insulates and conceals the nest.
Later in the laying period, the female starts to apply her body heat by making close contact between the skin on her breast and the eggs. This is the beginning of the incubation period, during which egg temperature is raised to a level that causes the embryonic cells to divide and the embryo to begin its magical transition into a duckling. For mallards, incubation takes just over three weeks, and during this time, the hen is on the nest for 90 percent or more of the time.
When egg temperature drops below the optimal level, embryonic development will slow and even stop. During the early laying period, egg temperature remains below the level needed for development, as it is very important that all ducklings develop and hatch at about the same time so the hen can lead her brood away in a cohesive group. During very hot days, the eggs can reach lethal temperatures, and at these times, the hen will stand over the nest and shade it from the sun while not applying any heat from her body.
Because the eggs at the center of the nest get more heat than those at the edge, the hen does a variety of things to equalize the heat each egg receives. She frequently rotates the direction she faces, moves her feet up and down in a paddling motion, and pulls her bill through the eggs. These movements cause the eggs to be rolled over and shifted into, and out of, the center of the nest so that all receive about the same total amount of heat over the course of the incubation period.