Bird reproduction

All birds reproduce by laying eggs. All birds except the megapodes incubate their eggs by sitting on them.

In birds, as well as mammals, eggs are fertilized internally. In some species, the female can store sperm for several weeks.

In many species, the reproductive system follows an annual cycle, switching on at the beginning of the breeding season, and then shrinking again when the work is done. Female birds usually only have one ovary, which produces eggs like units on a production line.

Eggs contain all the nutrients that a developing embryo needs, but they do not start to develop until all the eggs are laid, and incubation begins.

Breeding systems
Around 90 percent of all species of bird are usually monogamous: they have one partner. Polygamous birds have several partners in a single breeding season. Most species that are normally monogamous may also take part in extra meetings outside their pair bond.

Anatomy
In birds, the gonads are markedly asymmetrical. The left ovary and left oviduct in the female are functional, however; the right ovary and right oviduct degenerate early in the bird's embryonic development. Both testes or testicles, of the male are both present and functional, but the left testis is usually considerably larger than the right one.

The testes of birds are inside the body cavity, not outside in a bag (scrotum) as in mammals.