Birds have some of the most sophisticated courting and mating rituals in the animal kingdom. They perform songs and dances and are subjected to tests, all of which give the females and males a chance to assess each other’s qualities and strengthen their bond. Once the courtship is over, if the female has accepted the male as a mate, then the mating and fertilization take place. Males, however, are sterile outside the mating season.All bird species have internal fertilization; the male transmits its sperm cells directly to the female’s genital opening, allowing them to reach and fertilize her mature eggs inside her ovaries. From there, the sperm is carried through the oviduct, where it sometimes stays for several weeks in a special chamber before it is needed for fertilization in the ovary. Once the eggs have been fertilized, they are carried out of the oviduct system while the embryo develops. In the womb, the embryo is equipped with a protective calcium shell, and shortly after, the egg is laid.
Internal fertilization is performed by all animals that give live birth or lay eggs with a protective shell surrounding the embryo. The process is necessary for all terrestrial animals whose fertilization does not take place in water.
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