The Amazing Egg

An inside look at egg formation, structure, and development in waterfowl

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Photo © Mike Checkett

By John M. Coluccy, Ph.D., and Ed Farley

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. 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 attends the nest only 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 positioning herself so that an area of bare skin on her belly, known as a brood patch, rests directly on the eggs. This transfer of heat helps the eggs reach a temperature at 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. 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 ready to leave the nest and follow their mother to the nearest wetland to feed.

This wondrous sequence of events can take place only 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.

Dr. John Coluccy is director of conservation planning and Ed Farley is a conservation intern in DU's Great Lakes/Atlantic Region.