Each female emperor penguin (Aptenodytes forsteri) lays one large egg. Initially, both sexes share in incubating the egg by carrying it on his or her feet and covering it with a fold of skin. After a few days of passing the egg back and forth, the female leaves to feed in the open water of the Arctic Ocean. Balancing their eggs on their feet, the male penguins shuffle about the rookery, periodically huddling together for warmth during blizzards and frigid weather. If an egg is inadvertently orphaned, a male with no egg will quickly adopt it. Two months after the female’s departure, the chick hatches. The male feeds it with a milky substance he regurgitates until the female returns. Now padded with blubber, the females take over feeding the chicks with fish they have stored in their crops. The females do not return to their mate, however, but wander from male to male until one allows her to take his chick. It is then the males’ turn to feed in open water and restore the fat layer they lost while incubating.

The elephant bird (Aepyornis maximus), an extinct flightless bird of Madagascar, also known as the giant bird or roc, laid the largest known bird eggs. Some of these eggs measured as much as 13.5 inches (34 centimeters) in length and 9.5 inches (24 centimeters) in diameter. The largest egg produced by any living bird is that of the North African ostrich (Struthio camelus). The average size is 6 to 8 inches (15 to 20.5 centimeters) in length and 4 to 6 inches (five to 15 centimeters) in diameter.

The smallest mature egg, measuring less than 0.39 inch (1 centimeter) in length, is that of the vervain hummingbird (Mellisuga minima) of Jamaica.

Generally speaking, the larger the bird, the larger the egg. However, when compared with the bird’s body size, the ostrich egg is one of the smallest eggs, while the hummingbird’s egg is one of the largest. The Kiwi bird of New Zealand lays the largest egg, relative to body size, of any living bird. Kiwis are comparable to chickens in size, but their eggs are comparable to ostrich eggs in size. The egg of a Brown Kiwi is 14 to 20 percent of the female’s body weight. Its egg weighs up to 1 pound (0.5 kilogram).