Life in the Egg

A look at the incubation process of the egg

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.

This system is not perfect. Remember that some eggs were incubated to some extent before the clutch was complete. These eggs would be further developed than those deposited at the end of the laying period. And despite the hen’s diligence, some eggs may still develop slower, or faster, than others.

Near hatch, a couple of intriguing things occur to adjust for these possibilities. For one, the embryos begin to communicate with each other by emitting clicking sounds that cause the slower embryos to quicken their metabolism and the development of their tissues. The hen also starts to give quiet clucking sounds that may help synchronize hatching and, most importantly, are the first communication between the hen and her soon-to-emerge brood. This is known as imprinting and assures that the hen is able to maintain a cohesive brood that responds to her leads as to whom to follow, where to feed, and when to avoid predators.

The hatching period lasts several hours. Some ducklings may be hatched and dried out before the last ones have hatched. All the ducklings must have enough nutrients to sustain them between the nest and the first brood pond, a period of time that could last up to a full day in bad weather. The necessary energy and nutrients are provided by the remaining yolk, which is absorbed into the gut and used until that first meal is available.

In the end, most ducklings will be dried out and ready to leave the nest all together when the hen gives the right signal. This adventure usually begins in early morning, as the hen and her brood head for the pond she has chosen for the ducklings to seek their first opportunity to feed and get water. The hen does not feed the ducklings. She only takes them to the places she has found where they can feed themselves. While the ducklings are still very small, she may protect them from inclement weather by brooding them under her body, but they soon outgrow this possibility. Not surprisingly, the nesting period and the first hours and days after hatch are when the highest mortality rates occur for all waterfowl.

This intricate sequence of events is repeated every nesting season. Each species may work things out a little differently, but all these steps are critical and can only be successful when the habitats the birds use provide all the needed elements.

Dr. Bruce Batt recently retired after serving for more than 15 years as chief biologist at DU's national headquarters in Memphis.

Second Chances

Most prairie ducks will renest if their first nest is lost to predation or some other cause. Mallards have been known to make as many as five nesting attempts during one breeding season. Pintails, on the other hand, may renest only once, and often they do not renest at all. However, biologists have discovered that ducklings from later nests have a reduced probability of surviving until the following nesting season. This could stem from several factors, such as a shortage of the full complement of nutrients from the hen to assure the highest quality eggs, ducklings hatching past the optimal time for food availability in brood habitat, or too little time for the ducklings to grow adequately before fall migration.