Reproductive System of Birds
In male birds the two testes lie within the body cavity fairly close to the kidneys. From each of these there is a minute duct leading to the cloaca. Adult female birds, unlike other vertebrates, have only one functional ovary and oviduct – the left-hand one – in which a number of ovum develop but only a few ripen during the breeding season.
Large birds of prey such as this Griffon Vulture rely on rising warm air currents to help save energy. Since little warm air rises over the sea migrating raptors make for the shortest sea-crossings.
When it ripens the ovum bursts its follicle and is caught up by the funnel-like entrance to the oviduct down which it moves. During copulation, the hindmost part of the cloaca is turned outwards, and held tightly to that of the male. The male reproductive cells are then ejected directly into the female cloaca and move up the oviduct to fertilize each descending ovum. Gradually albumen is deposited around the yolk and, finally the shell membrane and the shell itself are added to the egg in the uterus.
During the greater part of the year the sexual organs are very small but with the coming of the breeding season they enlarge considerably. In the House Sparrow the sexual organs may weigh hundreds of times more in the breeding season than during the rest of the year.