The egg cell, or ovum, of a bird is small at first when it lies in the ovary.  From the blood supply to the ovary, the egg cell accumulates reserve nutritive material as yolk. The egg cell becomes greatly expanded and is surrounded by a firm envelope. At one pole of the ripe ovum there is a small disc of formative living matter with a nucleus. This disc lies on top of the yolk.

Either before it leaves the ovary, or in the upper part of the oviduct, the ovum is fertilised by some spermatozoa from the male. During its passage down the oviduct, the ovum becomes surrounded by various coatings of albumen, a thin paper like membrane and porous shell of carbonate of lime.

This shell often becomes beautifully and protectively marked by pigments related to the blood and bile. Because the egg moves down the oviduct, broad end first, before the pigments are fixed, different streaks and blotches result. The pattern of these streaks and blotches are normally characteristic of the species.

In some cases such as guillemots and black headed gulls, the egg colouration is extraordinarily variable. In some cliff birds, such as guillemots and razor bills, the egg rotates on its short axis when jostled, and does not readily roll off the ledge into the sea.

If the egg cell is not fertilised, it will not develop, but it may be surrounded by the white of the egg enclosed by a shell. Most birds completely lack external sexual organs -- including the males. In typical bird-sex, males introduce sperm into female bodies by pressing their sexual openings against the female's sexual opening, kiss-like (this is often called cloacal kissing). Typically this is accomplished with the male mounted atop the female, tottering and flapping his wings to keep from slipping off. Swifts and swallows mate in midair. Fertilization doesn't necessarily take place soon after mating; domestic chickens and turkeys can produce fertile eggs seventy days after copulation.

The whole embryo bird is mapped out on the disc of formative living matter at the top end of the yolk. At first it closely resembles the embryo of a reptile. It shows the same embryonic envelopes, the protective amnion (the innermost membrane that encloses the embryo of a reptile, bird or mammal), and respiratory membrane. The embryo bird breathes by means of blood vessels spread out on this membrane, using air entering through the porous shells. The embryo feeds on the legacy of the yolk for approximately three weeks.

When the chick is fully formed, it breaks its way through the shell by knocking with its beak. It then enters the world as a very helpless nestling, or as a precocious chick that can run around almost immediately.

Egg incubation periods last 10-82 days before hatching, this period differs depending on the species. For example, the albatross egg requires 82 days of incubation before hatching will take place. Those that hatch quickly usually produce naked, helpless young, requiring long periods of paternal care and feeding before they become independent. Those with long incubation periods often produce down covered chicks that can move about soon after hatching, to find some of their own food.

Some species can fly within hours of hatching. Others (e.g. albatrosses) can take up to 6 months to become independent. Condor chicks take up to 1 year to become independent.

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