The female bird's ovaries also enlarge during breeding season to produce the ovum. Female birds usually only have one functional ovary, the left one. In birds, an ovum is fertilized in the female bird's oviduct by a sperm cell from the male bird. Once fertilized, the ovum becomes the nucleus of the egg. The egg, that has its own food source, the yolk, will be laid by the female into her nest, incubated, and then the baby bird will hatch.