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Where eggs come from

One Brown-headed Cowbird egg and five Eastern Phoebe eggs, June 2007, by Galawebdesign.
One Brown-headed Cowbird egg and five Eastern Phoebe eggs, June 2007, by Galawebdesign.

The incredible egg. Essential to the species. Relished fried or poached. And the perfect package. But how did it come about?

Like reptiles, birds broke away from the need to lay eggs in a watery environment by bringing a watery environment to the eggs. The process by which each egg is produced is a marvel. Early on, the developing embryo, perched on top of the yolk, sends out layers of cells that are not part of the chick but are essential to its development. One of the membranes, called the amnion, surrounds the embryo and contains the watery environment. The other two are highly vascular. One surrounds the yolk and brings nourishment to the embryo. The other underlies the shell and functions as both a lung and a kidney by facilitating gas exchange through the shell and holding nitrogenous waste.

The story of the egg begins in the ovary. The females of almost all birds have only one functional ovary, oviduct, and uterus. The right side atrophies during early development.

Why only one? The reproductive structures are large, and space is at a premium, and there is room for only one side to function at a time, anyway. Plus, jettisoning the right side reduces weight, an important consideration for flight.

Elephant bird Aepyornis maximus skeleton and egg. Quaternary of Madagascar by Monnier, 1913.
Elephant bird Aepyornis maximus skeleton and egg. Quaternary of Madagascar by Monnier, 1913.

Resembling a bunch of grapes, the ovary consists of a cluster of follicles, each of which contains a potential ovum. Under the influence of hormones from the anterior pituitary gland, several follicles begin to enlarge just prior to the breeding season and the ova start to mature. In chickens, the single-cell ovum is about an inch in diameter when ovulated and is referred to as the yolk.

Once ovulated, the ovum is swept into the oviduct by currents created by cilia lining the infundibulum, or opening of the oviduct, and begins its trip downward. After fertilization occurs in the upper oviduct, glands in the lining secrete albumen (egg white), which surrounds and protects the yolk. Thick albumen on opposite sides of the yolk form the chalazae, twisted bands that help keep the yolk centered. You may notice the chalazae when viewing a fresh chicken egg in a frying pan.

In the isthmus, farther down the oviduct, two shell membranes are formed that encapsulate the yolk and albumen. Water is then absorbed, causing the developing egg to swell and become turgid.

When the egg reaches the shell gland in the uterus, a form of calcium carbonate called calcite is deposited, and the hard shell is made. Open pores scattered throughout the shell allow air to enter for gas exchange (respiration) with the vascular membrane below the shell.

Many birds can complete the process from ovulation to egg laying in 24 hours, while others take several days. Whatever the time period, the developing egg spends most of it in the shell gland of the uterus.

For years, scientists were puzzled as to how birds could muster enough calcium to create eggshells for a typical clutch. Blood calcium levels don’t come close to providing enough, and many birds lay from 6 to 12 eggs. Finally, it was learned that shortly before the nesting season, birds’ hollow long bones become filled with a special form of bone that is mobilized during the laying season and deposited in the eggshell. In a nice turn of events, the eggshell gets thinner as incubation progresses, as the developing chick absorbs calcium from the shell to help form its own skeleton.

Producing eggs is a great physiological strain, especially when the number of eggs is high or the eggs are large. Altricial birds, whose youngsters are hatched naked and blind, lay eggs with smaller yolks than precocial birds, whose hatched youngsters are larger, fully feathered, and ready to leave the nest right away. Also, the eggs of precocial birds require longer incubation periods because the youngsters must develop more completely.

While most songbirds, woodpeckers, and ducks lay an egg a day, swans, herons, cranes, owls, and other birds that lay relatively larger eggs usually require a two-day interval.

Others, including some eagles, penguins, and cassowaries, require three- to five-day intervals. Mound-building birds known as megapodes allow rotting vegetation and the sun to incubate their eggs and have the most precocious youngsters of all. They require from four to eight days to synthesize the large yolks.

Today’s unchallenged champions of relative egg size are the kiwis, whose single eggs weigh about a fourth of a female’s body weight. The largest of all time, however, were produced by the elephant birds of Madagascar, which weighed up to 1,000 pounds. Their eggs measured about 13.5 x 9.5 inches and had a volume of about two gallons, the equivalent of 130-160 chicken eggs or about 50,000 hummingbird eggs.

The incredible egg requires females to synthesize yolk and mobilize calcium faster than thought possible and, for some, to lay many eggs or to lay eggs that are unusually large. While difficult, it’s just another example of the amazing biology of birds.


This article from Eldon Greij’s column “Amazing Birds” appeared in the May/June 2015 issue of BirdWatching


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