The Amazing Egg
An inside look at egg formation, structure, and development in waterfowl
For waterfowl, the cycle of life begins anew each year with the eggs that are laid and carefully nurtured by nesting birds on their breeding grounds. An egg consists of three main parts: the yolk, albumen (egg white), and shell. Everything a duckling needs for its development is contained within these three components.
The yolk consists of fat, protein, vitamins, and minerals, while the albumen is primarily protein. The shell is mostly calcium and provides structural strength. Pores found throughout the shell allow oxygen, carbon dioxide, and water to pass through it.
Depending on the species, nesting waterfowl can produce one egg every 24 to 48 hours. Each egg is fertilized and formed as it travels through the female reproductive tract (see sidebar). Ducks lay one egg per day, geese lay one egg every day and a half, and swans lay one egg every two days.
A clutch is a full set of eggs laid by a single female. In ducks, clutch sizes range from three to 12 eggs. During the first few visits, the hen only attends the nest briefly, but as the size of her clutch grows, she spends more and more time there. While on the nest, she gathers grass or other vegetation and down feathers plucked from her belly to form a bowl, which insulates and conceals the eggs.
Incubation actually starts late in the egg-laying process, when the female begins to apply body heat to the clutch by placing an area of bare skin on her belly, known as a brood patch, directly on the eggs. This transfer of heat helps the eggs reach a temperature in which the cells begin dividing to form the embryo. The hen uses her bill and feet to roll and reposition the eggs so that all of them receive an equal amount of heat. This ensures that the ducklings develop at roughly the same rate and hatch at about the same time. Incubation lasts between 21 and 31 days, depending on the species. During incubation, hens spend most of their time on the nest, taking only brief breaks to feed in nearby wetlands.
Inside the egg, the embryo’s development can be observed using a technique known as candling (see sidebar). Freshly laid eggs start out clear, but in just four days the embryo and blood vessels are visible. While the embryo develops, the egg actually becomes lighter as it loses water and as yolk is used by the embryo for growth. When hatching time approaches, the air pocket within the egg grows larger and the shell gets thinner as calcium is absorbed by the developing embryo.
Shortly before hatching, the nearly developed embryos produce clicking and peeping sounds inside the eggs. Known as “pipping,” these vocalizations help synchronize hatching among different members of the brood. The hen responds by making soft clucking sounds to her unhatched ducklings. These early communications are a crucial part of a process known as imprinting, in which the ducklings learn to recognize their mother’s voice. This ensures that the brood will follow their mother when it’s time to leave the nest.
As the hatching process begins, each duckling penetrates the inner shell membrane of the air cell with its bill, and its lungs start to function. Next the ducklings take on the arduous task of breaking out of their shells. Young waterfowl use an egg tooth—a hard, horny structure on the upper tip of the bill—to break through the outer shell membrane and shell. It typically takes about a day for all the ducklings to hatch. Once they have emerged from the eggs, the ducklings remain in the nest with their mother for another 24 hours or so until their down has dried. Then the ducklings are finally ready to leave the nest and follow their mother to the nearest wetland to feed.
This wondrous sequence of events can only take place if nesting waterfowl have access to high-quality wetlands and associated upland habitats on their breeding grounds. Since 1937, Ducks Unlimited has been dedicated to conserving the habitats that waterfowl need to nest and raise their broods. Given the many threats currently facing wetlands and grasslands across this continent, waterfowl need your support more than ever to ensure a secure future for the birds and their habitats.
Candling is a method in which bright light is used to see what’s inside an egg. Waterfowl researchers use a short piece of radiator hose and sunlight to candle eggs in the field. Using this technique, biologists can determine the incubation stage of an egg by examining the size of the embryo, shape of the yolk, development of the circulatory system, density of opaque areas, and size of the air cell within the egg. Once the incubation stage is determined, biologists can estimate when the eggs were laid, when incubation began, and when they will hatch. This information helps researchers monitor nest success and know when to return to the nest to mark hatched young.
How an Egg Is Formed
Many organs and systems help convert the raw materials consumed by female waterfowl into the various elements that become an egg. Female waterfowl have a single functional left ovary and left oviduct, located in the body cavity near the spine. The ovary contains thousands of tiny follicles. During the prelaying period, some of the follicles begin to grow rapidly and accumulate yolk to form an ovum.
When the ovum within a follicle is mature, the follicle bursts and the ovum is released into the oviduct. Following ovulation, the ovum enters the first section of the oviduct, known as the infundibulum, where it is fertilized by sperm from a male during the half hour or so it spends there. The ovum then moves to the magnum, where the albumen (egg white) is deposited around the yolk in about two and a half hours. Finally, the shell is formed as it moves through the isthmus and into the uterus, where it remains for about 20 hours until laying.
By Dr. John Coluccy, director of conservation planning, and Ed Farley, conservation intern in DU’s Great Lakes/Atlantic Region.
Originally published in Ducks Unlimited magazine.